solid waste

CHAPTER 8

SOLID WASTEEugene A. Glysson, Ph.D., P.E.*

Solid waste management continues to progress from conventional collection techniques and disposal meth-ods to an integrated approach focused on source reduction and recycling. As land becomes more limited andregulations increase, the environmental engineer also directs attention to development and application of ad-vanced disposal technologies.

Solid wastes are those materials, other than liquids or gases, that are deemed by their owner to no longerpossess value and are discarded. They are generated by almost every activity, and the amount varies bysource, season, geography, and time.

Historically, solid waste disposal consisted of open dumping but now is carried out in double-lined land-fills with collection of and controls for gases and/or leachate. Other disposal means include composting andvarious incineration processes, which also may be used for codisposal of wastewater treatment sludges.These disposal means typically require controls for created pollutants, such as leachate and odor from com-post operations and chemical and particulate emission from incinerator combustion.

Recovery and reuse are practiced widely. Source or central facility separation is used for a variety ofproducts including paper, glass, plastics, ferrous metals, and nonferrous metals. Also, refuse-derived fuelsmay be used for energy production, and yard wastes may be composted to produce a humus soil conditioner.

SOLID WASTE—SOURCE AND EFFECT

The individual or organization discarding solid waste becomes the waste generator. The concept of wastehaving no value is defined by the generator, since the waste may represent some value to others through re-cycling or reclamation.

The amount of solid waste generated varies by season, geography, and time. The amount of solid wastegenerated from various sources under average conditions is discussed in this section. Waste characteristicsare discussed in another section.

Source

Solid waste generation can be subdivided into residential and nonresidential, depending on its source. Resi-dential wastes are generally considered to be household-type wastes, whereas nonresidential includes com-mercial, light industrial, and other wastes.

8.1

*Contributors to this chapter are William C. Anderson, PE.; Richard C. Bailie, Ph.D., P.E.; Jay A. Campbell, P.E.; Eliot Ep-stein. Ph.D.; Kenneth E. Hartz. Ph.D., PE.; Herbert I. Hollander, PE.; John C. Jenkins, P.E.; Bruce R. Natale; Robert S. Scott, PE.;Charles O. Velzy, P.E.

Source: STANDARD HANDBOOK OF ENVIRONMENTAL ENGINEERING

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright © 2004 The McGraw-Hill Companies. All rights reserved.

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Residential Waste Generation. Wastes generated by residential households are usually calculated inpounds (kilograms) per capita per day. This measurement is suitable for gross estimates for use in sizing dis-posal facilities and resource recovery operations, but is not appropriate for the design of collection systems(1).

Collection systems are more appropriately designed utilizing the annual average weight (pounds) perhousehold (or stop) per week (PPHW). Households are an easily observed unit along a collection route. TheU.S. Environmental Protection Agency (2) reports a range of from 46.2 to 71.0 lb per household per weekfrom nine cities with curbside pickup in the 1970s. The average was 57.3 lb per household per week, whichmight be considered a typical value. In 1981 two studies (1) showed generation rates to be between 48.7 to52.0 lb per household per week. One state department of natural resources recommends 52.0 lb per house-hold per week for residential refuse collection (3). One pound per household per week (PPHW) equals 0.454kg per household per week.

Estimates for household waste generation should be based on actual measurement. This means actuallycounting the residences on residential routes and weighing the refuse collected. Care must be taken to in-clude the entire spectrum of residential premises and the various seasons of the year. Attempts have beenmade to correlate residential solid waste generation to several measurable factors including populationserved, households served, value of property, size of living area, and household income. Statistical analysishas led to the conclusion that population served is the most significant variable.

Measurement of residential refuse picked up at the curbside indicates that weekly refuse generation canbe expressed by the following equation:

G = a + bP

where G = generation of household refuse in mass per week per household a, b = constants determined by waste measurement survey

P = average persons per household in sample area (see Figure 8.1.)

Typical values of a and b as derived by field measurement are as follows:

Field Study I a = 45.0, b = 3.3 Field Study II a = 44.4, b = 2.8

Residential waste generation is not uniform throughout the year. An EPA report (2) shows average week-ly rates per household for each month in 11 cities scattered throughout the United States. These data havebeen reduced to a monthly multiplier (Table 8.1) for use with the annual average weekly generation rate.Table 8.1 gives monthly multiplier and the maximum and minimum multipliers based on the data from these11 cities. These data are also shown graphically in Figure 8.2.

Nonresidential Waste Generation. Refuse generation from various other sources has been evaluated byvarious agencies. These waste-generation rates are shown in Table 8.2.

Effect

Solid waste has a prevailing characteristic that sets it aside from the liquid and gaseous wastes produced bysociety. The characteristic is that it remains highly visible in the environment in which we live. Liquidwastes are quickly relegated to a sewer and are out of sight, and gases disperse into the atmosphere. Solidwastes, however, are stored and transported in and through societies’ living space and have great potentialfor adversely affecting the quality of the environment.

The environmental effect of solid waste management begins with on-site storage. This aspect of manage-ment has a profound impact on the local environment, since improperly stored refuse may attract insects and

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rodents, present a fire hazard, be an unattractive nuisance, and produce odors, litter, and other unsightly con-ditions. On-site storage involves proper containerization in order to minimize these possible adverse effects.Various containers are available: galvanized steel and cans, plastic cans, plastic and paper bags for residen-tial use, and steel bulk containers for commercial and other wastes. For residential use, single-use plasticand paper bags have, in general, been shown to be most suitable from all aspects, while plastic cans withgood covers are next. Metal cans have the least capability to cope with all the conditions of proper on-siteresidential refuse storage. The frequency of refuse collection should include consideration for reducing oreliminating as many of the adverse effects of on-site storage as possible.

SOLID WASTE 8.3

FIGURE 8.1 Household refuse collection (1).

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8.4

TABLE 8.1 Multiplier for Annual Average Solid Waste Generation (2)

Month Average Maximum Minimum

January 0.876 0.983 0.786 February 0.871 1.028 0.726 March 0.972 1.123 0.872 April 1.050 1.162 0.956 May 1.125 1.256 0.986 June 1.107 1.268 0.979 July 1.085 1.163 0.991 August 1.024 1.195 0.931 September 1.010 1.083 0.922 October 0.994 1.105 0.890 November 0.985 1.049 0.886 December 0.901 1.098 0.769

FIGURE 8.2 Monthly variations in solid waste generation (2).

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Refuse collection involves the selection of vehicles and their routing through the community to most ef-ficiently collect the solid wastes generated. Administrative decisions must be made as to requiring the pub-lic to set their containerized refuse at the curbside for pickup or to require the collectors to pick up thewastes from the backyard or other points of storage.

The principal environmental effects of collection relate to the care exercised by the collector in avoidingspilling refuse from the containers and picking up loose material. Empty containers left at the curbside aftercollection can be unsightly and present a potential hazard to traffic if they roll out into the street.

The environmental effect of various processing and disposal methods will be addressed in later sections.

SOLID WASTE 8.5

TABLE 8.2 Unit Waste Factors for Various Generators

Category or generator Unit waste factor

Commercial (SIC 50–99) 5.75 lb/employee/day (4)Industrial (SIC 19–49) 10.6 lb/employee/day (4)Transportation equipment (SIC 37) 20.5 lb/employee/day (5)Nonelectrical machinery (SIC 35) 25.5 lb/employee/day (5)Electrical machinery (SIC 36) 23.5 lb/employee/day (5)Hospitals 2 to 4.5 lb/staff/day (6)

Patient care 8.6 lb/bed/day (6)Food service 2.7 lb/bed/day (6)Rehabilitation care 6.4 lb/bed/day (6)

Prisons 4.5 lb/inmate/day (7)Universities with student housing 1.0 lb/student/day (7)Colleges without student housing 0.6 lb/student/day (7)Office buildings 1.5 lb/employee/day (7)Multiple housing units 2.7 lb/resident/day (7)Wood industry 151.0 lb/employee/day (4)Demolition/construction debris 1.2 lb/capita/day (4)Street sweepings 0.3 lb/capita/day (4)Agricultural 13.0 lb/capita/day (4)Campgrounds 1.3 lb/camper/day (5)Family picnicground 1.0 lb/picnicker (5)Group picnicground 1.16 lb/picnicker (5)Organization camp 1.81 lb/occupant/day (5)Resort area

Rented cabin (with kitchen) 1.46 lb/occupant/day (5)Lodge room (no kitchen) 0.59 lb/occupant/day (5)Restaurant 0.71 lb/meal served (5)Residence 2.31 lb/occupant/day (5)

Ski area Overnight lodge (all facilities) 1.87 lb/visitor-day (5)Day lodge (all facilities) 2.92 lb/visitor-day (5)

Observation site 0.05 lb/incoming axle (5)Visitor center 0.02 lb/visitor (5)Swimming beach 0.04 lb/visitor (5)Concession stand 0.14 lb/patron (5)Administrative residence 1.37 lb/occupant/day (5)

Note:1 lb = 0.454 kg.

SOLID WASTE



SOLID WASTE CHARACTERIZATION

Any system employed to process waste must have the inherent flexibility to cope with its varying character.There are myriad influences of varying intensity that continually alter the quantity, composition, and physi-cal and chemical character of the material. The intensity of the influences can vary within the community,from community to community, from year to year, from season to season, and even from day to day.

There are three primary purposes for waste characterization. First, the data become the basis for planningeconomic analysis, design, and subsequent management and operation of a disposal system or materials–en-ergy resource recovery facilities. These data must consider the varying nature of the material to beprocessed. Second, waste characterization for rehabilitation or retrofit of a facility redefines the quantity andtype of waste for disposal. For this purpose, waste recharacterization is concerned with marked changes inlegislation or in the economy in general that may have some effect on the waste generated. Third, plant opti-mization, emissions monitoring, or malfunction analysis of a waste-to-energy facility can be expedited bythe characterization of the wastes being processed. Therefore, a sampling characterization program can de-lineate the major constituents of the waste—such parameters as moisture and ash content—which have con-sequential impact on the energy value of the materials, and other parameters that influence combustion andthe character of gas emissions.

Although these precepts have been widely recognized and much effort has been expended in waste char-acterization at various locations in the United States and abroad, there have been no standard methods, pro-cedures, or programs established. Each investigator has resorted to his or her own devices, ingenuity, re-sourcefulness, and expediency to satisfy the current need for information and data. Consequently,correlations of the data obtained by many investigators (federal and state agencies, municipal administra-tions, involved industries, trade associations, plant operators, consulting engineers, academic researchers,and even citizen groups) using an array of techniques and procedures for differing assortments of con-stituents still prompt misgivings regarding the confidence level in the information reported.

The primary concern in waste characterization is the selection of the sample, its size, and the number ofsamples necessary to provide confidence that the sample and data are representative of the large mass of ma-terial. The secondary concern is in the actual analysis technique(s).

These concerns surface repeatedly when attempting to finalize mutually beneficial commercial agree-ments between producers and users of secondary materials and fuels derived from municipal waste. TheAmerican Society of Testing and Materials (ASTM) E-38 Committee on Resource Recovery addressedthese concerns with consensus guidelines and standards that can be the basis of agreements minimizingmany of the uncertainties.

Some investigators endeavored to characterize “their own household discards” with the expectation thatthey would be typical of the community. Others sorted and characterized crane bucket loads of material ran-domly drawn from an incinerator waste-receiving bin, and still others grabbed samples from newly dumpedloads of material on the floor of a processing facility or landfill. In establishing the credibility of the datagenerated (reproducibility of the data being the objective), the size of each grab sample, the number of sam-ples, and the location taken from the mass of material were areas of uncertainty.

Statistical analyses have been made and reported by several researchers regarding the efficacy of drawingmany small size samples [200 to 300 lb (90 to 140 kg)], to determine waste composition and subsequentchemical analysis (8–13). Although there is apparent recognition that as-discarded, heterogeneous materialsare coarse and fine in size, dense, compressible, loose, bagged, boxed, do not have granular characteristics,do not flow, do not blend but will segregate, the “cone-and-quartering” technique is frequently employed toobtain “the representative sample” for analysis. This sample selection technique is highly dependent on crewjudgment; therefore, it can easily and inadvertently become biased. Although the cone-and-quartering tech-nique is an expedient procedure to obtain a sample of reasonable size for closer analysis, it is best employedwhen the materials to be sampled are reasonably uniform in size and density.

8.6 CHAPTER EIGHT

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This section is confined to describing two procedures for sampling and characterizing as-discarded mu-nicipal solid waste. These procedures are

� Truckload sampling � Spot sampling

Sampling and chemical analysis procedures and standards for prepared refuse-derived fuels (RDF) andthe separated materials for recycling is available in the American Standards and Testing Materials (ASTM)Standards, Water and Environmental Technology, Sec. 11, Vol. 11.04.

Many of the standards and procedures described can also be applied to commercial and industrial officeand shipping waste as differentiated from production-line waste.

The ASTM publication STP 832, “Thesaurus on Resource Recovery Terminology,” can also be a usefuldocument.

Solid Waste Constituents

Municipal solid waste (MSW) consists of both materials and products. Materials in MSW include paper andpaperboard, yard trimmings, glass, metal, plastic, wood, and food wastes. With exception of food wastes andyard trimmings, each material category is made up of many products. In 1995, MSW generation in the Unit-ed States totaled 208 million tons; Table 8.3 provides a breakdown by material categories and associatedweight (92, 93).

A portion of each material category was recycled, including being composted. Recovery rates for someproducts within a material category are higher than the overall recovery rate for the material category, be-cause some products are not recovered at all. For example, aluminum cans are recovered at rates above 60%,

SOLID WASTE 8.7

TABLE 8.3 Generation and Recovery of Materials in MSW (93)

Weight Percent of Weight Recovery asgenerated, total weight recovered, percent of

Material million tons generated million tons generation

Paper and paperboard 81.5 39.2 32.6 40.0Glass 12.8 6.2 3.1 24.5Metals

Ferrous metals 11.6 5.6 4.2 36.5Aluminum 3.0 1.4 1.0 34.5Other nonferrous metals 1.3 0.6 0.9 69.4

Plastics 19.0 9.1 1.0 5.2Rubber and leather 6.0 2.9 0.5 8.9Textiles 7.4 3.6 0.9 12.2Wood 14.9 7.2 1.4 9.6Other materials 3.6 1.7 0.8 23.1Other wastes

Food wastes 14.0 6.7 0.6 4.1Yard trimmings 29.8 14.3 9.0 30.3Miscellaneous inorganics 3.2 1.5 Negligible NegligibleTotal other wastes 46.9 22.5 9.6 20.4

Total municipal solid waste 208.0 100.0 56.2 27.0

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but the overall recovery rate for aluminum is only 35%. Likewise, even though corrugated containers are re-covered at rates above 64%, the overall recovery rates for paper and paperboard is 40%.

Products in MSW are grouped into three main categories: (1) durable goods, such as appliances; (2) non-durable goods, such as newspapers; and (3) containers and packaging. Other wastes include food wastes andyard trimmings. These product categories generally contain each type of MSW material. Table 8.4 presentsa summary by product categories and associated weight from the 1995 characterization study.

8.8 CHAPTER EIGHT

TABLE 8.4 Generation and Recovery of Products in MSW by Material (93)

Weight Percent of Weight Recovery asgenerated, total weight recovered, percent of

Material million tons generated million tons generation

Durable goodsFerrous metals 8.7 4.2 2.7 30.7Aluminum 0.8 0.4 Negligible NegligibleOther nonferrous metals 1.3 0.6 0.9 69.4Total metals 10.8 5.2 3.6 33.1Glass 1.3 0.6 Negligible NegligiblePlastics 6.2 3.0 0.2 3.8Rubber and leather 5.2 2.5 0.5 10.3Wood 4.2 2.0 Negligible NegligibleTextiles 2.3 1.1 0.1 50Other materials 1.1 0.5 0.8 77.8Total durable goods 41.9 20.1 5.3 17.0

Nondurable goodsPaper and paperboard 43.5 20.9 12.7 29.3Plastics 5.1 2.5 Negligible <1Rubber and leather 0.8 0.4 Negligible NegligibleTextiles 5.0 2.4 0.8 15.8Other materials 2.7 1.3 Negligible NegligibleTotal nondurable goods 57.0 27.4 13.5 23.7

Containers and packagingSteel 2.8 1.3 1.6 54.6Aluminum 2.0 1.0 1.0 51.6Total metals 4.8 2.3 2.6 53.4Glass 11.5 5.5 3.1 27.3Paper and paperboard 38.1 18.3 19.9 52.3Plastics 7.7 3.7 0.7 9.7Wood 10.6 5.1 1.4 13.5Other material 0.1 >0.1 Negligible NegligibleTotal containers and 72.9 35.0 27.8 38.1

packagingOther wastes

Food wastes 14.0 6.7 0.6 4.1Yard trimming 29.8 14.3 9.0 30.3Miscellaneous inorganics 3.2 1.5 Negligible NegligibleTotal other wastes 46.9 22.5 9.6 20.4

Total municipal solid waste 208.0 100.0 56.2 27.0

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For planning and design projects, characterization of solid waste constituents must be tailored to the areaor generator served. Two important reasons for sorting and sampling MSW are:

� To determine the constituent mix from the standpoint of recovered secondary materials for reuse or recy-cling

� To determine the character of the waste for use as a fuel or just incineration

To address these objectives, the following are deemed to be a practical array of constituents of interestusually found in municipal solid wastes.

Combustible Noncombustible

Newsprint FerrousOther paper Aluminum Diapers NonferrousTextiles and garments GlassPlastics, film BrickPlastics, rigid OBW (oversize bulky wastes)Food wastesWoodYard wastes (grass clippings)Sweepings (floor of sorting area)

While sorting the gross sample or increment of waste into these constituents and determining the weightpercentage, random fractions of the combustible constituents are accumulated for subsequent labora-tory analysis to determine basic fuel characteristics such as moisture, calorific (heating) value, and ash con-tent.

Sampling Methodology

In endeavoring to define the character of municipal solid waste, it is necessary to obtain representative sam-ples. The number and size of samples depends upon the variability in actual physical size and other proper-ties of the constituents, as well as the confidence level desired.

Municipal solid waste is typically made up of a broad spectrum of materials, some abundant and somesparse, as well as a cross section of physical size from mattresses to dirt. Since the bulk of waste is quitecoarse, a relatively large sample increment may be necessary for high-confidence-level characterization. Inaddition to the bulk size of some wastes, there are other constituents that are quite small and sparse in themix of waste. In order to not lose the concentration of sparse constituents, a large sample size is usually nec-essary to reasonably assure accountability of these constituents. If all constituents were relatively uniform insize, such as found in a bushel of mixed nuts, a modest sample size could more readily represent the totaland conventional statistical sampling theory would apply.

The objective of a sampling program is to determine the character of the waste by sorting it into repre-sentative constituents of interest and comparing the analysis with data obtained elsewhere by other investi-gators.

Recognizing the variabilities of municipal solid waste, particular attention must be directed to avoidingbias when obtaining the samples. The program timing should avoid possible extraordinary external influ-

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ences on the character of the wastes such as pre- and post-holidays, or prolonged adverse weather condi-tions. The following sampling procedures are based on obtaining sufficient repetitive, truly random samples,thereby obtaining replicable high-confidence-level data.

Regardless of the sampling method, it is necessary to establish areas of the community from which thesample will be drawn, as well as when and how this shall be done. The community planner, in conjunctionwith the individual(s) responsible for waste collection and disposal, should delineate the geographic areasfrom which the waste character might tend to be different. These areas may include tenements, condomini-ums, townhouses, single-family lots, campus areas, permanent residential, transient resort, and other socioe-conomic factors.

If not already a local collection practice, arrange for once-per-week collection for the selected area(s)during the sampling period, thereby avoiding the day of the week variability or bias that might occur in thewaste character.

Truckload Sampling

Each day, randomly select a loaded collection truck from one of the designated areas for waste sorting andcharacterization. An attempt should be made to limit the weight of the truckloads to 3 or 4 tons (2.7 or 3.6metric tons)—a manageable quantity for sorting in one day. The procedure should be followed each day un-til all of the designated geographic (planning) areas have been sampled and characterized. The communityshould be divided into at least five geographic areas. A reasonable time interval should elapse, perhaps twoor three days, following heavy weather to minimize the bias that is bound to occur (14).

Planning the Sorting Program. The sorting area should be indoors and sufficiently large to permit a col-lection vehicle to deposit its collected load and still have available adequate space for personnel to maneuvereasily with the sorting drums, laboratory sampling drums, and access to a 500-lb (225-kg) platform scale. Atypical sorting facility arrangement is shown in Figure 8.3. The area should be about 30 ft (9 m) wide by 60ft (18 m) long, well-lit, and ventilated. The floor should be of a smooth, easily cleanable surface. Provisionsshould be made for a standby container or collection truck to dispose of the materials after they have beensorted, weighed, and sampled.

Prior to the start of the program, the waste collectors should be contacted and advised that one of theirtrucks will be randomly selected each day to deposit their load in the sorting room after having beenweighed. The waste collectors may also be requested to have their drivers fill out a refuse collection vehiclesurvey data sheet (Figure 8.4), describing in detail the route of the truck, truck number, fuel tank capacity,and truck tare weight. Drivers of municipal collection vehicles are usually the most cooperative. The grossand net weights of each truck delivering to the sorting area should be documented. (The actual weight of therefuse unloaded from the truck is subsequently compared with the total weight of the constituents sorted.The net truck load weight should turn out to be somewhat higher—the difference being the weight of mois-ture that may have evaporated during the sorting activity.)

The information on the survey sheet will provide the weight of the waste to be characterized and also theopportunity to develop a waste profile correlating the quantity and waste character with the residential com-munity served, based on population and the socioeconomic and age base. This correlation can be valuable inplanning for other activities and facilities serving the community.

Another method of determining the per capita waste generation rate that may be influenced by the char-acter of the community is a curbside weighing program. This program is relatively simple and can be initiat-ed and terminated quickly. Further discussion of this method is found at the end of this section.

The following is a list of equipment recommended to conduct this sorting program:

� A portable platform (dial) scale with a maximum capacity of 500 lb (225 kg) with ¼-lb (0.1-kg) gradua-tions

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� Twenty 32-gal (120-L) heavy-duty plastic trash containers equipped with (detachable) casters for easymobility

� A pair of long-sleeved coveralls for each crew member� Four snow shovels� Two rakes � Two heavy-duty pushbrooms � Twelve pairs of heavy-duty puncture-resistant gloves � Heavy work boots for each crew member

SOLID WASTE 8.11

FIGURE 8.3 Truckload sorting facility arrangement.

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8.12 CHAPTER EIGHT

FIGURE 8.4 Refuse collection vehicle survey data.

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� One hundred, 4-mil-thick 32-gal (120-L) plastic bags for constituent samples� A 12 ft × 4 ft × 3 ft (3.5 m × 1.2 m × 1 m) sorting table with 1-in (2.5-cm) square screening� Fifty heavy cardboard boxes for sample storage� Tape, identification tags, and marker for laboratory samples� Two handheld quick-release magnets for metal sorting

The laborers necessary for this sorting and sampling program should consist of 10 workers, including thecrew chief.

The facility layout for the program should include a portable scale placed as close as possible to a col-lection truck hopper, container or 30 yd3 (20 m3) roll-off receptacle for the deposition of materials that havebeen sorted and weighed. The sorting area should easily accommodate the largest load to be characterized.Laboratory sample boxes should be placed adjacent to the scale for ease of sample deposition and packag-ing.

Sorting Methodology. After being weighed, the collection truck would deposit its load in the center of thesorting floor. The sorting crew members circle the deposited load while pulling one or two of the plastictrash drums mounted on casters. Each member of the team is assigned a specific item on the list of con-stituents to remove from the pile.

The most effective technique first addresses those constituents in the greatest abundance. When thedrums have been filled with sorted material, the crew member wheels them to the platform scale and returnswith empty drums to repeat the procedure.

The crew chief and scale attendant weigh each filled sorting drum and record its net weight on a specialdata sheet. A sample sorting weight chart is shown in Figure 8.5. (Tare weights of the empty drums shouldhave been recorded previously.) The scale attendant then selects pieces from the contents in the sortingdrums considered most representative of that particular constituent and deposits these pieces into the des-ignated laboratory sample bag that has been assigned for that constituent. (Laboratory analysis sampling isdiscussed further in the following section.) The sorting drum, after having been weighed and sampled, isemptied into the receptacle or rear loading packer for discard. The emptied drum is then returned to thescale area for reuse in sorting. The materials considered oversized and bulky, such as tires, mattresses, toi-lets, and rugs, are pulled out for separate classification as oversized bulky wastes (OBW) and pho-tographed.

Initially, all metals are placed into the same sorting drum. Subsequent differentiation of ferrous and non-ferrous is accomplished by emptying the contents of the drums onto a clean section of the floor and passinga handheld quick-release magnet over it to draw off the ferrous fraction. This weight should be recorded asdescribed above.

After the mound of trash is reduced to approximately one-fourth of its original size, a filtration of fairlysmall top-size trash particles [less than 6 in (15 cm)] will have become apparent. Sorting trash with dimin-ishing top sizes is especially laborious and greatly adds to sorting time. Therefore, a sorting table can beused for the remaining portion (Figure 8.6). Two or three of the crew members and at least four drums areassembled around the table. One of the members, using a snow shovel, scoops some of the remaining wasteto be sorted onto the table. The remaining crew members then sort from the table. This “assembly line” tech-nique is less taxing and improves sorting time and accuracy. All particles sifting through the 1-in (2.5 cm)square screening of the table are weighed and labeled as sweepings for further analysis.

The daily total of net weights from the sorting drums should be compared with the net MSW truckloaddelivered. The possible loss in weight can be attributed to the moisture loss during exposure and handlingfrom the day-long sorting activity. Although this difference in weight may be small (0.3 to 3.0%) it can bedistributed among the constituents based on the assumed tendency for that particular constituent to pickupor lose moisture, thereby providing a moisture loss “adjusted” value.

SOLID WASTE 8.13

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8.14 CHAPTER EIGHT

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All jugs, jars, and bottles that appear to contain material are arbitrarily deemed to be “hazardous” andshould be accumulated unopened for disposal. Typically, there are so few “hazardous” containers that an ac-counting of them has been considered of no consequence.

Compositing Samples. The procedure to obtain representative constituent samples for laboratory analysisis to randomly select approximately 3 ft3 (0.08 m3) of each specific material from each truck load. By accu-mulating small samples from each sorting drum, a daily composite sample of each of the 10 combustiblecategories is obtained for laboratory analysis for a total of 50 samples for a five-day sampling program. Po-tential bias is minimized since constituent materials of varying sizes are selected randomly during the dayfrom each sorting drum.

At the end of each day, the 10 bags of laboratory samples should be securely sealed. Each sealed bag isthen resealed in another 4-mil plastic bag, labeled, weighed, and dated. The weight is logged onto a laborato-ry specimens chart (Figure 8.7). This double-sealed sample is then placed into a corrugated box, sealed withplastic tape, labeled, and stored. The boxed laboratory samples should be logged into the laboratory withinone week after completion of the sampling program.

The program should address each waste constituent separately. Subsequently, the laboratory parametricanalyses for each constituent are combined to form composite analyses to generally characterize the wasteas a whole. For the data to have credibility, the elements of prime concern include the number, size, and rep-resentativeness of the laboratory samples.

It is only really necessary to have laboratory analyses conducted on constituents numbered 1 through 10,the designated combustible categories, to yield the data considered to be of practical significance. Con-stituent categories numbered 11 through 16 are considered to be essentially noncombustible, contributinglittle other than ash (residue) when consumed in a furnace. Nevertheless, it should be recognized that somesurface moisture and some combustible material are present in these items such as, labels, decals, and coat-ings (paints). Containers may contain some organic residues and “some” oxidation of the container materialitself would take place. However, the weight percent of the lot is usually small relative to the whole. Theweight fraction of the combustible portion would be so small that the complexities of specific laboratoryanalyses may not be justified.

Data Summaries

The field and laboratory data obtained and the correlations prepared can result in many charts and tables.These can be synthesized into summary tables and charts, such as those illustrated herein.

Constituent Makeup. The constituent makeup of the waste for each day of the five-day program and theircomposite averages are shown in Tables 8.5 and 8.6. These data reflect the “as-sorted,” as well as the “ad-justed” (moisture-distributed), weights discussed previously.

So that correlations and extrapolations might readily be made, the data can also be presented to reflectthe constituent mix on a “yard-waste-free” basis. The principal yard waste usually encountered is grass clip-pings. Presenting the data in this manner also provides an indication of the waste constituent makeup thatmight be encountered during the months of little yard activity. This information is particularly significant inview of the high percentage of yard waste that can be encountered in the waste and its very high moisturecontent (especially in townhouse and suburban areas of the community). The impact of yard waste on thecharacter of the refuse is dramatic. Table 8.5 reveals that it can be 40% of the total weight or 48% of thecombustible portion, and can average 70% moisture.

Similarly, data summaries can be structured with and without the noncombustibles (constituents 11

8.16 CHAPTER EIGHT

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SOLID WASTE 8.17

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14).

SOLID WASTE



through 16), so as to readily reflect the makeup of the organic and combustible fraction of waste—as if sub-jected to a highly selective and efficient form of front-end or source-separation program. A data display asgiven in Table 8.4, can be especially informative. This illustration indicates that for the five-day composite,the noncombustible constituents were only 11.3% of the total waste (as discarded), and 18.7% when report-ed on a yard-waste-free basis.

The weight percentages illustrated for ferrous, aluminum, and glass appear to be lower than the valuesusually reported elsewhere, even on a yard-waste-free basis (19). Perhaps the values are closer to reality orreflect the impact of the recent passage of a regional “bottle bill.”

Several other items draw focus; the very low percentages of food waste (less than one-third the valuesusually reported), the low percentages of aluminum, nonferrous, and particularly glass (less than half what isusually reported), and the high percentage of plastics (more than one-fourth greater than usually reported).So much of the textiles, garments, and footwear are composed (at least in part) of synthetic materials thatcategorizing them expediently can be taxing. The weight percent for this constituent was generally higher(twice) than expected.

This sampling procedure reduces the size of the catch-all usually described as “miscellaneous.” The con-stituent designated as sweepings is the catch-all for the described procedure. The quantity encountered iscomfortably small.

As mentioned previously, by programmed intent, collection vehicles should be randomly selected forwaste load characterization based on knowledge of the general residential areas they serve. In this manner,the data can reflect the variation in waste character based on socioeconomic and age considerations. Inter-estingly, the tabulations in Table 8.5 did not reveal marked differences in constituent concentrations. Therewas only modest variation from locale to locale in the weight percent of the constituent mix during this char-acterization program.

Laboratory Analysis. An analysis should be made only for the parameters of fundamental interest. For thisprogram, the parameters selected were moisture, ash (inerts), calorific value, sulfur, total chlorine, and wa-ter-soluble chlorides. The analytical procedures and methods should closely adhere to the consensus stan-dards developed for municipal waste constituents as described in the ASTM Standards, Water and Environ-mental Technology, Sec. 11, Vol. 11.04.

Moisture. The daily and composite moisture data for the 10 “combustible” constituents are displayed inTable 8.7. The high moisture value(s) for the waste can be attributed principally to the relatively high weightpercent of the (high-moisture-content) yard waste constituent. The high moisture values for diaperlike mate-rials and food wastes were as expected. However, the high moisture (22%) for plastic film was not anticipat-ed and may be attributed to the extensive use of plastic bags as the containment for the large quantity of verywet grass clippings. Therefore, this high moisture for plastic film should be considered as essentially surface(free) moisture rather than inherent moisture.

These values could be recast and tabulated on the basis of the total weight of all 16 constituents, whichwould indicate 42.1% moisture versus 47.4%. Recasting the data on a yard-waste-free basis, the five-daycomposite values would be 28% moisture for the combustible portion and 22.8% moisture based on the re-maining 15 constituents. This wide range in moisture values illustrates the care that must be taken in themanner of reporting moisture data to avoid creating an erroneous impression regarding the character of thewaste.

Ash. A similar tabulation for the inherent ash (dry) in the combustible materials is displayed in Table 8.8.The tabulated high ash content for sweepings should be of little concern since the quantity of sweepings istypically small. The ash content in plastic film and in yard waste is higher than expected. However, the datareported should not be considered as absolute values for each of the constituents; e.g., for plastic film theremust have been moisture adhering to the film surface, as well as dirt, grass clippings, and other small parti-cles whose weight is quite high relative to the very light weight of the plastic film itself. Overall, the five-

8.18 CHAPTER EIGHT

SOLID WASTE



8.19

TAB

LE 8

.5D

aily

Ref

use

Con

stit

uent

Wei

ght D

istr

ibut

ion

(14)

RA

W R

EF

US

E C

HA

RA

CT

ER

IZA

TIO

N S

AM

PL

ING

AN

D A

NA

LYS

ES

RE

FU

SE

SO

RT

ING

PR

OG

RA

MA

UG

US

T 2

7 T

HR

OU

GH

31

TO

TA

L N

ET

WE

IGH

T: 6

5,80

0 L

BT

RU

CK

S: D

H 4

52, I

N-4

9, N

A-2

0, W

E-6

S, B

-234

Aug

ust 2

7A

ugus

t 28

Aug

ust 2

9A

ugus

t 30

Aug

ust 3

1

Wei

ght,

Wei

ght,

Wei

ght,

Wei

ght,

Wei

ght,

lb a

sW

eigh

t,lb

as

Wei

ght,

lb a

sW

eigh

t,lb

as

Wei

ght,

lb a

sW

eigh

t,C

onst

itue

ntso

rted

%so

rted

%so

rted

%so

rted

%so

rted

%

1.N

ewsp

rint

822

6.84

1.96

18.

621.

250

8.64

853

7.94

402

7.96

2.O

ther

pap

er2,

967

24.7

5,78

725

.53,

373

23.3

2.09

819

.61,

447

28.6

3.D

iape

rs15

61.

3027

51.

2015

21.

0510

81.

014

0.09

4.Te

xtil

es/g

arm

ents

219

1.82

1,09

14.

8079

75.

5142

53.

9580

1.59

5. P

last

ics,

fil

m27

72.

3186

63.

8054

53.

7725

92.

4085

1.69

6. P

last

ics,

rig

id1%

1.63

528

2.31

368

2.54

256

2.37

861.

707.

Foo

d w

aste

496

4.13

787

3.45

149

1.03

554

5.15

170.

358.

Woo

d66

0.56

340

1.50

143

0.99

143

1.32

320.

64

9.Y

ard

was

te5,

444

45.3

8,20

336

.15,

232

36.2

4,86

645

.42,

034

40.2

10.

Sw

eepi

ngs

420

3.50

257

1.12

341

2.36

190

1.76

115

2.29

11.

Ferr

ous

240

2.00

690

3.02

579

4.00

364

3.39

140

2.78

12.

Alu

min

um16

0.14

370.

1689

0.63

270.

25

13.

Non

ferr

ous

350.

2910

80.

4797

0.68

690.

64—

8—14

.G

lass

455

3.79

673

2.95

494

3.42

276

2.56

285

5.64

15.

Bri

ck19

0.16

152

0.67

724

5.02

300.

28—

8—16

.O

BW

182

1.53

984

4.33

125

0.86

212

1.98

327

6.47

____

____

___

____

___

___

____

___

___

____

___

___

____

___

___

Tota

l12

,010

100.

022

,739

100.

014

,458

100.

010

,730

100.

05,

054

100.

0T

ruck

net

wei

ght

12,2

0022

,900

14,5

0011

,000

5,20

0D

iffe

renc

e19

01.

5616

10.

7042

0.29

270

2.46

146

2.18

Not

e:1

lb =

0.4

5 kg

.

SOLID WASTE



8.20

TAB

LE 8

.6C

onst

itue

nt W

eigh

t Dis

trib

utio

n—5-

Day

Com

posi

te (

14)

RA

W R

EF

US

E C

HA

RA

CT

ER

IZA

TIO

N S

AM

PL

ING

AN

D A

NA

LYS

ES

RE

FU

SE

SO

RT

ING

PR

OG

RA

MA

UG

US

T 2

7 T

HR

OU

GH

31

CO

MP

OS

ITE

OF

5 D

AY

S C

OL

LE

CT

ION

TO

TA

L N

ET

WE

IGH

T: 6

5,80

0 L

B

Adj

uste

d fo

r m

oist

ure

loss

dur

ing

sam

plin

gE

xclu

ding

yar

d w

aste

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

___

Wei

ght,

Wei

ght,

Wei

ght,

Wei

ght.

Wei

ght,

lbW

eigh

t,lb

ad-

Wei

ght,

lb a

d-W

eigh

t,lb

ad-

Wei

ght,

lb a

d-W

eigh

t,C

onst

itue

ntso

rted

%ju

sted

%ju

sted

%ju

sted

%ju

sted

%

1.N

ewsp

rint

5,28

88.

145,

322

8.09

5,32

29.

125,

322

13.4

5.32

216

.42.

Oth

er p

aper

15,6

7224

.115

,726

23.9

15,7

2626

.915

,726

39.5

15,7

2648

.53.

Dia

pers

695

1.07

835

1.27

835

1.43

835

2.09

835

2.58

4.Te

xtil

es/g

arm

ents

2.61

24.

022,

644)

4.01

2.64

04.

522.

640

6.62

2,64

08.

14

5.P

last

ics,

fil

m2,

032

3.13

2.08

93.

172.

089

3.58

2.08

95.

242.

089

6.44

6.P

last

ics,

rig

id1,

434

2.21

1,44

92.

201,

449

2.48

1,44

93.

641.

449

4.47

7.Fo

od w

aste

2,00

33.

082.

194

3.33

2.19

43.

762.

194

5.50

2,19

46.

778.

Woo

d72

41.

1175

51.

1575

51.

2975

51.

8975

52.

33

9.Y

ard

was

te25

,779

39.7

25,9

4039

.425

,940

44.4

—8

—8

—8

8—10

.S

wee

ping

s1,

323

2.04

1,42

12.

161,

421

2.43

1,42

13.

561,

421

4.38

11.

Ferr

ous

2,01

33.

102,

013

3.06

2.01

35.

0512

.A

lum

inum

169

0.26

169

0.26

169

0.42

13.

Non

ferr

ous

309

0.48

309

0.47

309

0.78

14.

Gla

ss2,

183

3.36

2,18

33.

322,

183

5.48

15.

Bri

ck92

51.

4292

51.

4192

52.

3216

.O

BW

1,83

02.

821,

830

2.78

1,83

04.

59__

____

____

___

___

____

___

___

____

___

___

____

___

___

____

_

Tota

l64

,991

100.

065

,800

a10

0.0

58,3

72b

100.

039

,860

100.

0032

,431

c10

0.0

Dif

fere

nce

809

1.2

7,42

811

.3d

25,9

4039

.474

28

a Gro

ss w

eigh

t.b%

of

gros

s w

eigh

t.c49

% o

f gr

oss

wei

ght.

dN

onco

mbu

stib

le c

onst

itue

nts—

11 th

roug

h 16

.N

ote:

1 lb

= 0

.45

kg.

SOLID WASTE



8.21

TAB

LE 8

.7C

ombu

stib

le C

onst

itue

nts

Moi

stur

e D

istr

ibut

ion—

Dai

ly a

nd C

ompo

site

(14

)

RA

W R

EF

US

E C

HA

RA

CT

ER

IZA

TIO

N S

AM

PL

ING

AN

D A

NA

LYS

ES

AU

GU

ST

27

thro

ugh

31

Adj

uste

dM

oist

ure

cont

ribu

tion

, lb

5-D

ay to

tal

Com

posi

teto

tal

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

moi

stur

e,av

erag

eC

onst

itue

ntw

eigh

t, lb

Day

1D

ay 2

Day

3D

ay 4

Day

Sto

tal l

bm

oist

ure,

%

1.N

ewsp

rint

5,32

210

830

728

713

349

.888

516

.62.

Oth

er p

aper

15,7

2678

41,

851

736

399

296

4,06

625

.93.

Dia

pers

836

125

180

92.5

103

13.8

514

61.6

4.Te

xtil

es/g

arm

ents

2,64

012

.325

812

657

.46.

2946

017

.45.

Pla

stic

s, f

ilm

2,08

975

.717

012

770

.327

.547

022

.5

6.P

last

ics,

rig

id1,

449

22.2

20.4

9.9

18.7

3.97

755.

187.

Food

was

te2,

194

441

695

118

532

46.9

1,83

383

.68.

Woo

d75

55.

1959

.318

.425

.75.

7311

415

.19.

Yar

d w

aste

25,9

403,

803

6,14

33,

600

3,70

21,

339

18,5

8771

.610

.Sw

eepi

ngs

1,42

123

012

717

985

.447

.466

947

.1__

____

____

____

____

____

____

___

____

____

___

____

____

___

Tota

l58

,372

5,60

69,

811

5,29

45,

126

1,83

627

,673

47.4

Wei

ght c

olle

cted

, lb

11,2

5320

,256

12,3

9210

,022

4,44

858

,372

Moi

stur

e (y

ard-

was

te-f

ree

49.8

48.4

42.7

51.1

41.3

47.4

28ba

sis)

, %

Not

e:1

lb =

0.4

54 k

g.

SOLID WASTE



8.22

TAB

LE 8

.8A

sh in

Com

bust

ible

Con

stit

uent

s—D

aily

and

Com

posi

te (

14)

RA

W R

EF

US

E C

HA

RA

CT

ER

IZA

TIO

N S

AM

PL

ING

AN

D A

NA

LYS

ES

AU

GU

ST

27

thro

ugh

31

Adj

uste

dA

sh c

ontr

ibut

ion,

lb (

Dry

bas

is)

5-D

ay to

tal

Com

posi

teto

tal

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

ash,

aver

age

Con

stit

uent

wei

ght,

lbD

ay 1

Day

2D

ay 3

Day

4D

ay S

tota

l lb

ash,

%

1.N

ewsp

rint

4437

13.6

25.8

11.8

8.55

2.36

62.1

1.40

2.O

ther

pap

er11

660

135

156

138

111

61.4

060

15.

153.

Dia

pers

322

1.51

3.78

1.02

0.99

0.34

7.64

2.38

4.Te

xtil

es/g

arm

ents

2180

4.85

33.8

23.5

7.82

4.80

74.8

3.43

5.P

last

ics,

fil

m16

1920

.510

454

.7.

10.6

9.66

200

12.4

6.P

last

ics,

rig

id13

7410

.956

.610

.87.

791.

5187

.66.

387.

Food

was

te36

16.

7710

.03.

985.

600.

7027

.17.

518.

Woo

d64

12.

2518

.41.

513.

610.

4026

.24.

099.

Yar

d w

aste

7353

312

378

299

210

149

1348

18.3

10.S

wee

ping

s75

255

.651

.345

.755

.244

.125

233

.5__

___

____

____

___

____

____

___

____

___

____

____

___

Tota

l30

699

563

838

590

421

274

2686

8.75

Dry

wei

ght c

olle

cted

, lb

5648

1044

770

9948

9626

1230

699

Ash

(dr

y ba

sis)

, %9.

978.

028.

318.

6010

.58.

75(w

t. as

h/w

t. re

fuse

co

llec

ted)

Not

e:1

lb =

0.4

54 k

g.

SOLID WASTE



day composite ash content of the combustible constituents, 8.75% (dry basis) can be considered low. Theash content of the composite including the noncombustibles is 15.4% on an as-received basis, and 22%when reported as yard-waste-free.

Calorific Value. The higher heating value (HHV) on a moisture- and ash-free (MAF) basis is reported inTable 8.9 for each constituent. The five-day composite MAF heating value for the combustible constituentsis 9673 Btu/lb (22,500 kJ/kg) and on a yard-waste-free basis (9826 Btu/lb) (22,850 kJ/kg). This value issomewhat higher than what is generally reported. The MAF heating value is a convenient base from whichcalculated transformations can readily be made to provide values reflecting specific entrained and inherentmoisture and ash values encountered (14).

Summary tabulations of the basic thermochemical parametric data based on an as-received and yard-waste-free basis can be displayed as in Table 8.10.

Sulfur and Chlorine. In view of the intense interest and concern regarding potential acid gas formationduring combustion, replicate analyses should be made for sulfur and chlorides on samples obtained of syn-thetic materials in addition to the normal determinations for these parameters on the other constituents. Ta-bles 8.11 and 8.12 illustrate summaries of such data obtained during the reference characterization pro-gram.

In an effort to determine the chlorine derivatives that may have a harmful effect in a combustion system,it is generally postulated that the chlorine atoms that are insoluble in water are those of particular concern(15). The premise for this assumption is that the temperatures usually encountered in a furnace will vaporizethe insoluble forms of chlorine and will therefore readily combine with other constituents and becomechemically aggressive.

Laboratory investigation of chlorine-bearing constituents usually focuses on identifying the organic orinsoluble chlorine(s). This can be accomplished by deducting the values for the water-soluble chlorides(H2O-soluble Cl–) from the determined total chlorine values. The resulting arithmetic difference will be theorganic chlorine. These values are illustrated in Table 8.11. However, a qualification is necessary when as-sessing these data. Not all organic chlorines are insoluble in water and not all inorganic chlorides are solu-ble in water. Therefore, the values in Table 8.11 can only be considered indicative, not absolute. The datausually reported in the literature are the total chlorine values rather than the arithmetic differences just dis-cussed.

The full significance and impact of the above is apparent when examining the data in Table 8.11 reportedfor textiles and garments, which were sampled only on day 3. The total chlorine is reported to be 3.78% andthe water-soluble chloride is 3.73%. The calculated difference of 0.05% is the organic chlorine. Althoughsome of the as-determined values reported may seem high, it must be recognized that it is the difference inthese values that should be used in assessing the degree of possible chemical aggressiveness. Inspection ofthe low as-determined values in Table 8.11 reveals that the arithmetic differences (organic chlorine) aremuch greater than what appear at first glance. This is reflected in the last column of tabulated data reportedon an MAF basis.

The sulfur and chlorine values for all 10 combustible constituents were determined only for the laborato-ry samples obtained on day 1. These are reported in Table 8.12 on an as-received basis and reflect the com-posite weight averages, all of which are considered to be relatively low.

Parametric Data. For design, confirmation, or operational purposes, specific laboratory analyses aregenerally necessary, especially for unusual types and/or quantities of materials encountered. However, forpreliminary planning purposes, overview laboratory parametric data may be adequate in lieu of conductinganalyses of specific constituents. In addition to the specific characterization data available in the aforemen-tioned tables, typical densities of waste components are found on Table 8.13, and fuel proximate analysis oftypical components in discarded solid wastes is found on Table 8.13. This information may provide usefuldata for the investigator or planner.

SOLID WASTE 8.23

SOLID WASTE



8.24

TAB

LE 8

.9B

tu in

Com

bust

ible

Con

stit

uent

s (M

AF

)—D

aily

and

Com

posi

te (

14)

RA

W R

EF

US

E C

HA

RA

CT

ER

IZA

TIO

N S

AM

PL

ING

AN

D A

NA

LYS

ES

AU

GU

ST

27

thro

ugh

31

Tota

l MA

FB

tu c

ontr

ibut

ion

(MA

F),

Btu

× 5

35-

Day

tota

lC

ompo

site

Adj

uste

d__

____

____

____

____

____

____

____

____

____

____

____

____

____

____

__B

tu,

aver

age

Con

stit

uent

wei

ght,

lbD

ay 1

Day

2D

ay 3

Day

4D

ay S

Btu

× 1

05M

AF,

Btu

/lb

1.N

ewsp

rint

4375

61.7

4914

2.21

281

.958

63.0

4630

.869

379.

834

8682

2.O

ther

pap

er11

059

174.

280

311.

721

232.

761

136.

250

91.8

0094

6.81

285

623.

Dia

pers

314

5.83

69.

648

5.27

84.

503

1.09

326

.358

8394

4.Te

xtil

es/g

arm

ents

2105

18.9

4076

.345

59.6

8332

.231

6.85

919

4.05

892

195.

Pla

stic

s, f

iber

1419

29.2

9311

8.15

859

.398

34.9

569.

432

251.

237

1770

5

6.P

last

ics,

rig

id12

8630

.383

78.3

6064

.140

42.2

7215

.950

231.

105

1797

17.

Food

was

te33

49.

029

10.2

483.

032

6.93

00.

674

29.9

1389

568.

Woo

d61

55.

471

24.9

5913

.041

10.9

582.

844

57.2

7393

139.

Yar

d w

aste

6005

122.

031

156.

517

120.

006

96.8

4451

.959

546.

993

9109

10.S

wee

ping

s50

014

.575

10.1

5211

.394

5.92

53.

869

45.9

1591

83__

___

____

____

____

___

____

____

____

____

____

___

____

___

___

Tota

l28

012

471.

587

938.

320

650.

691

433.

915

214.

985

2709

.498

9673

MA

F o

r co

mbu

stib

les

wei

ght,

lb50

8496

0965

0944

7523

3828

012

HH

V (

yard

-was

te-f

ree

basi

s)—

Btu

/lb

9276

9765

9997

9696

9195

9673

9826

Not

e:1

lb 0

.454

kg;

1I B

tu/l

b 0.

43 k

J/kg

.

SOLID WASTE



8.25

TAB

LE 8

.10

Com

para

tive

Sum

mar

y—M

oist

ure,

Ash

, HH

V (

14)

RA

W W

AS

TE

CH

AR

AC

TE

RIZ

AT

ION

SA

MP

LIN

G A

ND

AN

ALY

SIS

AU

GU

ST

27

thro

ugh

31

5-D

ay c

ompo

site

(Y

ard-

was

te-f

ree)

Aug

ust 2

7 as

-rec

eive

d,A

s-re

ceiv

ed,

Com

bust

ible

all c

onst

itue

nts

all c

onst

itue

nts

All

con

stit

uent

sco

nsti

tuen

ts

Moi

stur

e, %

45.9

42.1

22.8

28.0

Ash

(as

rec

eive

d), %

12.4

15.4

22.0

4.13

Ash

(dr

y ba

sis)

, %22

.926

.528

.55.

73H

HV

(as

rec

eive

d), B

tu/l

b38

6641

1854

2566

68H

HV

(dr

y ba

sis)

Btu

/lb

7146

7112

7027

9263

HH

V (

MA

F),

Btu

/lb

9276

9689

9826

9826

Tota

l chl

orin

e*0.

171

H2O

chl

orid

e0.

082

�C

hlor

ine

(org

anic

)0.

089

Sul

fur*

0.05

8

*See

Tab

les

8.11

and

8.1

2 fo

r m

ore

spec

ific

dat

a on

chl

orin

e an

d su

lfur

.N

ote:

1 B

tu/l

b =

0.4

3 kJ

/kg.

SOLID WASTE



8.26

TAB

LE 8

.11

Sul

fur

and

Chl

orin

e* in

Tex

tile

and

Pla

stic

Con

stit

uent

s (1

4)

RA

W R

EF

US

E C

HA

RA

CT

ER

IZA

TIO

N S

AM

PL

ING

AN

D A

NA

LYS

ES

AU

GU

ST

27

thro

ugh

31

As

rece

ived

Dry

bas

isM

oist

ure

and

ash

free

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

_

Tota

l Cl

H2O

-m

inus

H2O

-To

tal C

l,S

olub

le C

l– ,To

tal C

l,H

2O-

Tota

l Cl,

H2O

-Sol

uble

,C

onst

itue

ntS

,%S

olub

le C

l–%

S, %

%%

S, %

Sol

uble

Cl

%%

%

Dat

e 8-

27Te

xtil

es/g

arm

ents

0.00

70.

026

2.33

<0.

007

0.02

82.

47<

0.00

70.

029

2.53

2.50

Pla

stic

s, f

ilm

0.00

60.

122

0.20

1<

0.00

80.

165

0.27

2<

0.00

90.

182

0.30

10.

119

Pla

stic

s, r

igid

0.38

0.14

31.

150.

430.

161

1.29

0.46

0.17

11.

371.

20

Dat

e 8-

28Te

xtil

es/g

arm

ents

0.03

00.

146

0.29

50.

039

0.19

00.

385

0.04

10.

198

0.40

10.

203

Pla

stic

s, f

ilm

0.01

0.19

70.

282

<0.

010.

244

0.35

0<

0.01

0.28

60.

410

0.12

4P

last

ics,

rig

id0.

010.

099

0.29

0<

0.01

0.10

30.

303

<0.

010.

116

0.34

10.

225

Dat

e 8-

29Te

xtil

es/g

arm

ents

<0.

013.

733.

78<

0.01

4.43

4.49

<0.

014.

594.

650.

06P

last

ics,

fil

m<

0.01

0.26

50.

277

<0.

010.

345

0.36

0<

0.01

0.39

70.

414

0.01

7P

last

ics,

rig

id<

0.01

0.29

90.

374

<0.

010.

307

0.38

4<

0.01

0.31

70.

396

0.07

9

Dat

e 8-

30Te

xtil

es/g

arm

ents

0.06

10.

095

0.09

80.

070

0.10

90.

113

0.07

10.

111

0.11

50.

004

Pla

stic

s, f

ilm

<0.

010.

101

0.3%

<0.

010.

135

0.53

0<

0.01

0.14

20.

558

0.41

6P

last

ics,

rig

id<

0.01

0.15

40.

477

<0.

010.

166

0.51

4<

0.01

0.17

20.

531

0.35

9

Dat

e 8-

31Te

xtil

es/g

arm

ents

0.02

20.

058

0.32

20.

024

0.06

30.

348

0.02

60.

067

0.37

10.

304

Pla

stic

s, f

ilm

0.06

00.

329

0.36

40.

083

0.45

70.

506

0.09

60.

530

0.58

60.

056

Pla

stic

s, r

igid

0.07

40.

032

0.08

60.

077

0.03

40.

090

0.07

80.

035

0.09

20.

057

*Tot

al c

hlor

ine

min

us w

ater

-sol

uble

chl

orid

es (

H2O

-sol

uble

Cl– )

= o

rgan

ic c

hlor

ine.

SOLID WASTE



8.27

TAB

LE 8

.12

Sul

fur

and

Chl

orin

e* in

Com

bust

ible

Con

stit

uent

s (1

4)

RA

W W

AS

TE

CH

AR

AC

TE

RIZ

AT

ION

SA

MP

LIN

G A

ND

AN

ALY

SIS

Lab

orat

ory

Sam

ples

Col

lect

ed A

ugus

t 27

As

rece

ived

Tota

lA

djus

ted

Sul

fur

cont

ribu

tion

H2O

-C

l, m

inus

tota

l__

____

____

____

____

_so

lubl

eH

2O-s

olub

leH

2O-s

olub

leC

onst

itue

nt†

wei

ght,

lblb

%To

tal C

l, lb

Cl– ,

lbTo

tal C

l, %

Cl– ,

lbC

l– , %

1.N

ewsp

rint

829

0.82

90.

010

0.30

70.

298

0.03

70.

036

0.00

12.

Oth

er p

aper

2.98

12.

981

0.10

5.75

32.

683

0.19

30.

090

0.10

33.

Dia

pers

191

0.03

80.

020.

193

0.18

70.

101

0.09

80.

003

4.Te

xtil

e/ga

rmen

ts22

2—

<0.

007

5.17

30.

058

2.33

0.02

62.

304

5.P

last

ic, f

ilm

292

—<

0.00

60.

587

0.35

60.

201

0.12

20.

079

6.P

last

ic, r

igid

202

0.76

80.

382.

323

0.28

91.

150.

143

1.00

77.

Food

was

te54

00.

054

0.01

0.95

60.

853

0.17

70.

158

0.01

98.

Woo

d70

0.04

20.

060.

0413

0.02

70.

059

0.03

90.

029.

Yar

d w

aste

5.48

02.

192

0.04

4.17

34.

603

0.08

60.

084

0.00

210

.Sw

eepi

ngs

447

0.13

410.

030.

796

0.64

80.

178

0.14

50.

033

____

____

____

____

____

____

___

____

____

___

___

____

_

Tota

l wei

ghts

11.2

537.

0381

20.8

2310

.002

0.08

9W

eigh

ted

aver

age,

%0.

063

0.18

50.

096%

11.F

erro

us24

012

.Alu

min

um16

13.N

onfe

rrou

s35

14.G

lass

455

15.B

rick

1916

.OB

W18

2__

____

____

____

____

____

___

____

__

Tota

l12

.200

7.03

8120

.842

310

.002

Wei

ghte

d av

erag

e, %

0.05

80.

171

0.08

20.

089

*Tot

al c

hlor

ine

min

us w

ater

-sol

uble

chl

orid

es (

H2O

-sol

uble

C1– )

= o

rgan

ic c

hlor

ine.

† Sul

fur

and

chlo

rine

det

erm

inat

ions

for

“al

l” c

ombu

stib

le c

onst

itue

nts

(1 th

roug

h 10

) co

nduc

ted

only

on la

bora

tory

sam

ples

obt

aine

d on

Aug

ust 2

7.N

ote:

1 lb

= 0

.454

kg.

SOLID WASTE



8.28 CHAPTER EIGHT

TABLE 8.13 Densities of Waste Components

Component Density

Waste densities, lb/yd3

Loose waste 100–200After dumping from compactor truck 350–400In compactor truck 500–700In landfill 500–900Shredded waste 600–900Baled in paper baler 800–1200

Bulk densities, lb/ft3

Cardboard 1.87Aluminum 2.36Plastics 2.37Miscellaneous paper 3.81Garden waste 4.45Newspaper 6.19Rubber 14.90Glass 18.45Food 23.04

True densities, lb/ft3

Wood 37Cardboard 43Paper 44–72Glass 156Aluminum 168Steel 480Polypropylene 56Polyethylene 59Polystyrene 65ABS 64Acrylic 74Polyvinylchloride (PVC) 78

Resource recovery plant products, lb/ft3

dRDF 39Aluminum scrap 15Ferrous scrap 25Crushed glass 85

Source: Prepared by Cal Recovery Systems, Richmond, Calif., forASTM E-38.

Note: 1 lb/yd3 = 1.685 kg/m3 1 lb/ft3 = 0.0624 kg/m3.

SOLID WASTE



TABLE 8.14 Fuel Proximate Analysis for Typical Components (21)

Higher heating

Proximate analysis (as-received), weight %value, Btu/lb

_____________________________________________________________

MoistureVolatile Fixed Non- As- and ash

Moisture matter carbon combustible S received free

Paper, mixed 10.24 75.94 8.44 5.38 0.20 6,800 8,055Newsprint 5.97 81.12 11.48 1.43 0.16 7,974 8,600Brown paper 5.83 83.92 9.24 1.01 0.11 7,256 7,800Trade magazines 4.11 66.39 7.03 22.47 0.09 5,254 7,150Corrugated boxes 5.20 77.47 12.27 5.06 0.21 7,043 7,850Plastic-coated paper 4.71 84.20 8.45 2.64 0.08 7,341 7,940Waxed milk cartons 3.45 90.92 4.46 1.17 0.10 11,327 11,890Paper food cartons 6.11 75.59 11.80 6.50 0.16 7,258 8,250Junk mail 4.56 73.32 9.03 13.09 0.09 6,088 7,400Office trash 4.10 79.80 7.90 8.20 <.1 6,950 7,860

Vegetable food waste 78.29 17.10 3.55 1.06 0.20 1,795 8,700Citrus rinds and seeds 78.70 16.55 4.01 0.74 0.12 1,707 8,300Meat scraps (cooked) 38.74 56.34 1.81 3.11 0.19 7,623 13,110Fried fats 0.00 97.64 2.36 0.00 0.07 16,466 16,466Mixed food waste 72.00 20.26 3.26 4.48 0.52 2,370 10,100

Hardwood (pallets, crates) 11.60 74.80 14.00 0.60 <.1 7,500 8,520Green logs 50.00 42.25 7.25 0.50 0.08 2,102 4,250Rotten timbers 26.80 55.01 16.13 2.06 1.2 4,710 6,560Demolition softwood 7.70 77.62 13.93 0.75 <.1 7,300 7,995Waste hardwood 12.00 75.05 12.41 0.54 <.1 6,430 7,340Furniture wood 6.00 80.92 11.74 1.34 <.1 7,350 7,940Evergreen shrubs 69.00 25.18 5.01 0.81 0.19 2,708 8,960Balsam spruce 74.35 20.70 4.13 0.82 0.20 2,447 9,850Flowering plants 53.94 35.64 8.08 2.34 0.26 3,697 8,460Lawn grass 75.24 18.64 4.50 1.62 0.42 2,058 8,900Ripe leaves 9.97 66.92 19.29 3.82 0.16 7,984 9,270Wood and bark 20.00 67.89 11.31 0.80 0.05 6,900 8,700Brush 40.00 8— 8— 5.00 0.05 4,745 8,600Mixed greens 62.00 26.74 6.32 4.94 0.05 2,690 8,135

Upholstery 6.9 75.96 14.52 2.62 <.1 6,960 7,690Tires, whole 1.02 64.92 27.51 6.55 1.5 13,800 14,900Leather 10.00 68.46 12.49 9.10 0.40 7,960 9,850Leather shoe 7.46 57.12 14.26 21.16 1.00 7,243 10,150Shoe heel and sole 1.15 67.03 2.08 29.74 1.34 10,899 15,790Rubber 1.20 83.98 4.94 9.88 2.00 11,200 12,600

Mixed plastics 2.0 8— 8— 10.00 8— 14,100 16,000Plastic film 3.20 8— 8— 8— 0.07 —8 14,870Polyethylene 0.20 98.54 0.07 1.19 0.03 18,687 20,000Polystyrene 0.20 98.67 0.68 0.45 0.02 16,419 16,510Polyurethane 0.20 87.12 8.30 4.38 0.02 11,203 11,730Polyvinyl chloride 0.20 86.89 10.85 2.06 0.14 9,754 10,000

Linoleum 2.10 64.50 6.60 26.80 0.40 8,150 11,450Rags 10.00 84.34 3.46 2.20 0.13 6,900 7,844Textiles 15.31 8— 8— 8— 0.20 —8 8,300Oils, paints 0 8— 8— 16.30 8— 13,400 16,000Vacuum-cleaner dirt 5.47 55.68 8.51 30.34 1.15 6,386 9,960Household dirt 3.20 20.54 6.26 70.00 0.01 3,670 13,650Street sweepings 20.00 54.00 6.00 20.00 0.20 4,800 8,000

Note: 1 Btu/lb = 0.43 kJ/kg.

SOLID WASTE



Spot Sampling

The second method of sampling is randomly withdrawing many small-weight increments of waste from themass of waste material collected daily. Since municipal waste is subject to many influences and is highlyvariable in character, many increments are necessary so that average values for quantity and character arerepresentative of the whole. As a practical matter, a program withdrawing many 200- to 300-lb (90- to 135-kg) increments of waste for sorting can be reasonably indicative of the character of the waste.

A procedure that has been found to be expedient employs the use of a crane with grapple bucket, such asthose found at an incinerator, resource recovery plant, or a mobile articulated unit at a transfer station orlandfill (16, 17). The crane operator dumps a small [approximately 1 yd3 (0.75 m3)] randomly selectedbucket load onto a clean surface area preferably covered with a plastic tarpaulin to better manage the finerparticles in the waste sample. [In an incinerator plant, the sampling and sorting would take place at the fur-nace charging level (parapet) at one end of the receiving pit.] An attempt should be made by the crane oper-ator to mix the mass of waste by bucket action prior to extracting the sample for sorting. Although this pro-cedure may improve the mix of materials, it does tend to sift out the granular material and fines. There isalso the tendency to avoid the larger objects encountered.

Another spot-sampling procedure involves obtaining waste increments using a front-end-loader vehiclefrom a series of waste collection trucks (16×18). A truck, selected at random for sampling, is unloaded onthe designated floor area. Determining where from the dumped load to select the sample for sorting is thechallenge. Although the material was loaded in the truck at random and some mixing does take place whenthe load is dumped, additional mixing should be attempted by the front-end-loader operator prior to extract-ing a bucket load for deposit onto the cleaned floor area for sorting and sampling. However, every effortshould be made to avoid segregation of the fines and oversize materials. The procedure performed on 10 to15 randomly selected vehicles each day for two or more weeks can provide more than 120 sets of data fromsorting 12 to 20 tons (10 to 18 metric tons) of waste. This repetitive procedure tends to level the variancesand temper bias. Although the percentages for each may be relatively small, the analysis data should bequalified in regard to the probable losses of granular material and fines, oversized objects, and loss of sur-face moisture to avoid creating erroneous impressions regarding the character of the waste investigated. Toomany times significant quantities are included in a “miscellaneous” category as a catch-all expediency toclose the material balance calculation.

The procedures described in the truckload sampling methodology for obtaining samples for laboratoryanalysis also apply to spot sampling.

As mentioned, many investigations and attempts have been made to use statistical analysis in determiningthe quantity and character of the wastes discarded by the community so as to project the quantity and quali-ty of the materials and energy that may be recovered. Depending on the need and degree of accuracy neces-sary, some of the procedures are quite useful especially for refuse-derived products. Several of these are in-cluded in the reference list of this chapter.

A fundamental discussion on statistical sampling analysis is covered in the book Refuse Derived FuelProcessing (13).

A statistical analysis was conducted of the truckload sampling method and the spot sampling method. Al-though the extremes in the as-received heating value of the truckload sampling and analysis program rangedfrom approximately 3860 to 4890 Btu/lb (9000 to 11,400 kJ/kg), the comparative statistical analysis re-vealed that the mean HHV was 4650 Btu/lb (10,800 kJ/kg) with a standard deviation of 242; therefore therewas a 5.2% coefficient of variation and a relative standard error of 300 or 6.45%. This relates that 95% ofthe time the as-received heating value would be expected to range from 4350 to 4950 Btu/lb (10,100 to11,500 kJ/kg) (13).

Similarly, the spot sampling program conducted on municipal waste revealed a mean HHV of 4900Btu/lb (11,400 kJ/kg) with a standard deviation of 1075, and therefore, a 21.9% coefficient of variation anda relative standard error of 400 Btu/lb (930 kJ/kg) or 8.16%. This related that 95% of the time the as-re-ceived heating value would be expected to range from 4500 to 5300 Btu/lb (10,500 to 12,300 kJ/kg) (13).

8.30 CHAPTER EIGHT

SOLID WASTE



Yard waste can have a major influence on the fuel character of municipal waste. As discussed previous-ly, yard waste can be a high weight percent of the total waste and because of its typically high moistureadds little, if any, calorific value. Therefore, any analysis must account for the influence of yard waste onthe data.

Since the wide range in yard waste quantity has such a great impact on the overall composition of munic-ipal waste, displaying the comparative data on a yard-waste-free basis does level the compositions, as dis-played in Table 8.15. However, each of these communities had approximately 25% more paper waste and aslittle as 50% of the metals reported as the national average. Although there was no New Jersey bottle bill,there were aggressive materials recycling programs in Gloucester and Cape May counties. The glass andbrick fractions seem high, although within the national average.

The food waste quantities appear to be uniformly low compared to the reported national average, and thevery high paper fraction encountered during the spring 1980 sorting program at Central Wayne County,Michigan, seems to be suspect and warrants further study. The influence of the higher paper fraction on theheating value appears to be far less than the lower quantities of synthetic materials.

The close agreement of the moisture and ash-free heating values for each of the entities listed illustratesthat moisture and ash are the major influences on the energy character of municipal waste.

This type of comparative analysis can assist planners, designers, managers, and facility operators to un-derstand, prepare for, and manage their disposal and materials–energy resource recovery programs more ef-fectively.

Curbside Weighing Program

This is a direct method of obtaining basic waste weight generation data for demographic correlations, suchas residential density, habitat, socioeconomic level, and other influences of interest. This weighing programis best suited when the household trash is set out at the curb for pickup by the collection truck. The time ofday must be coordinated with the truck schedule. The best time of year to conduct a program of this nature isthe early fall, when school is back in session and prior to leaf fall. Generation rates at this time generally ap-proximate the annual average. Obviously, such operations should not be conducted during severe weather orduring the week following disruption of regularly scheduled collection due to a major holiday, severe weath-er, or other conditions (20).

During the curbside weighing period, the specific collection truckload involved should also be weighedand the households contributing to these loads cataloged. There should be a close correlation between the to-tal of curbside weights and the net weight of the truckload. Nonresidential waste collection in the same truckshould be avoided during this period. If this is not possible, the generators of these wastes should be identi-fied by location and nature of business for later canvassing and cataloging. These data can give an approxi-mation of nonresidential waste generation for similar businesses.

A curbside program can provide basic data on number and nature of items set out, total container weightsfor each household, and the number of persons contributing. These data could be related to population andhousehold makeup for the entire collection area serviced.

Careful planning and preparation for the curbside program is required in the interest of economy andquality of data. Planning for the program is based on a preliminary estimate of the waste stream, routeschedules, and maps of existing collection operations. Preparation of a route overlay to a planning and zon-ing map indicating individual properties can be useful.

Selection of weighing locations should be based on a defined random method to preclude introduction ofbias by the team members. For the selected location(s), all trash containers set out along a street or alley in apredefined area should be weighed and notations made of unusual materials or containers.

A two-person team can cover about 10 households per hour. With proper planning, the number of teamsrequired to cover an area in the available time frame can be established. It may be necessary to leapfrog thecollection crew and return later to obtain tare weight for all reusable containers and the occupancy data. At

SOLID WASTE 8.31

SOLID WASTE



8.32

TAB

LE 8

.15

Com

para

tive

Com

posi

tion

s of

Mun

icip

al S

olid

Was

te*, †

, ‡

Glo

uces

ter

Cap

e M

ayB

ranf

ord,

Cen

tral

Way

ne C

ount

y, M

ich.

Cou

nty,

N.J

.C

ount

y, N

.J.

Con

n.A

vera

ge U

.S.

____

____

____

____

____

____

___

____

____

___

____

____

____

____

____

____

Aug

ust 1

979,

Apr

il 1

980,

Oct

ober

198

1S

epte

mbe

rO

ctob

er 1

982

EPA

,W

aste

con

stit

uent

Wei

ght %

Wei

ght %

Wei

ght %

1982

Wei

ght %

Wei

ght %

Wei

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her

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e, B

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r he

atin

g va

lue,

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9270

9190

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MA

F

*Fro

m tr

uckl

oad

sam

plin

g an

d ya

rd-w

aste

-fre

e an

alys

es.

† Lar

ge s

ubur

ban

com

mun

ity—

Cen

tral

Way

ne C

ount

y, M

ichi

gan;

Sm

all s

ubur

ban

com

mun

ity—

Tow

n of

Bra

nfor

d, C

onne

ctic

ut; R

ural

cou

nty—

Glo

uces

ter

Cou

nty,

New

Jer

sey;

Res

ort C

ount

y—C

ape

May

Cou

nty,

New

Jer

sey.

‡ Dat

a gr

oupe

d to

sim

plif

y co

mpa

riso

n w

ith

nati

onal

ave

rage

s; n

umbe

rs h

ave

been

rou

nded

for

sim

plic

ity.

SOLID WASTE



least 30 residences should be sampled for each area. A subarea may be defined by neighborhood, physical,social or economic criteria, or may correspond to daily collection zones.

The equipment required in the curbside method consists of

� The team, identifiable through hardhats and name tags� A portable handheld scale with hook, 100-lb (45.5-kg) capacity� A grappling harness with hooks to pick up containers and a plastic pan 2 × 3 × 6 in (0.6 m × 0.9 m × 15

cm) to handle assorted small items or those that cannot be hooked� A 6-ft-high forked rod to support the scale for heavier loads� Gloves, sturdy shoes, and clipboard

Team members should be courteous in explaining their activity when questioned by citizens. Inquiringresidents, when informed of the team’s purpose, will usually volunteer occupancy information. Neighborswill frequently provide the information for houses where information is difficult to obtain.

Data on the number of households served and that on intermingled commercial establishments can berecorded by a cooperative collection truck driver or a team member riding in the truck.

At the very least, this basic method will reveal the weight character of residential waste generation. Thesetypes of data are illustrated in Figure 8.8 for a one week curbside weighing program of a small suburbancommunity in Michigan in September, 1981. The plot indicates that the spread of data for the householdswith many members is much less than that with few members; that the rate of waste generation per capita isprone to be less for the larger households; e.g., periodicals and grocery bags discarded are not directly pro-portional to the number of individuals. This display is contrary to the expedient assumption that the wastegeneration rate per capita is essentially the same regardless of size of household. This information could beuseful for correlations with other demographic and planning data.

Characterization Program Perspective

The following items should be considered in the planning and conduct of a solid waste characterization pro-gram:

� Waste composition investigations conducted in a similar manner facilitate comparative analysis, eventhough undertaken at different times and circumstances. The comparative composition data assembled inTable 8.15 are intended to display the character of wastes on the same basis (yard-waste-free) using es-sentially the same sampling and sorting procedure for four types of communities and obtained within arelatively short time span. The United States average data are also provided for comparative purposes.

� The significant influence on the quantity and constituent mix of municipal waste is the type and makeupof the community: urban, industrial, rural, university, resort, and socioeconomic and age level, etc.

� The variation in per capita waste generation rate for a particular community is influenced in the near termby the weather and the season; over the longer term by the economy and changes in consumer packaging.

� The per capita waste generation rate is influenced by the number of persons in the household.� Additional characterization studies may be desirable to better define the seasonal changes in waste gener-

ation rate and waste composition.� The character of municipal waste will continue to be variable in spite of imposed government require-

ments for source separation of select waste materials.� A characterization program can be time consuming and costly. Therefore, it should be well planned and

the actual sampling and sorting activity closely monitored to assure that the data obtained are of the cal-iber desired. Every effort should be made to avoid external influences and bias. These might be intro-duced by weather, holidays, collection upsets, or by resorting to makeshift facilities for sorting and expe-dient procedures for personnel, all of which can cause bias.

SOLID WASTE 8.33

SOLID WASTE



8.34 CHAPTER EIGHT

FIGURE 8.8 Residential waste generation (20).

SOLID WASTE



� Once-per-week collection from residences should be arranged during the program to avoid the bias thatmay result from differences in midweek or weekend waste quantity and character.

� A justified materials weight balance should be made for each sample increment characterized, regardlessof size. The weight of the entire sample before sorting should correspond with the sum of the weights ofthe constituents sorted.

� On an MAF basis, the HHV of municipal solid wastes generally varies through the relatively narrowrange of 9000 to 10,000 Btu/lb (3870 to 4300 kJ/kg). However, as with all solid fuels, moisture and ashcontent are the major influences on the actual energy character of municipal waste. Since it would beconvenient to have a “nominal” reference fuel analysis, the ASME Research Committee on Municipaland Industrial Waste is recommending the analyses shown in Table 8.16 as a basis of discussion. Thephysical makeup of the waste influences the material handling and combustion feeding and burning sys-tems, as well as the practicality of materials resource recovery.

� A waste characterization program should be designed to provide the level of confidence required for theparticular use of the data; for overall planning, general extrapolations of published data may be all that isnecessary; for design of a new or retrofit facilities requiring contractual commitments and financing, de-finitive information on waste quantities and character may be required.

� The discrete sizes and size distribution of the constituent components and the concentration of the con-stituents within the total mass of waste will influence the size and number of sample increments neces-sary for high-confidence characterization. The larger the size of the components, the larger the sampleincrement required. With smaller particle sizes, e.g., less than tin (2.5 cm), and the greater its concentra-tion, smaller samples and fewer sample increments would be required for the data to be representative.ASTM Standards D-2013, D-2234, and EDS-8 provide sampling principles and procedures applicablefor small-particle-size materials. ASTM procedures should be used for sampling and analyses whereverpossible since these should provide reproducible data.

SOLID WASTE 8.35

TABLE 8.16 Municipal Solid Waste—Nominal Reference Fuel Analyses*

Ultimate analysis, % Proximate, %

Moisture 20–40 27 Moisture 27Carbon 20–30 24 Volatile matter 41Hydrogen 3–5 9 Fixed carbon 7Oxygen 15–25 19 Ash† 25Nitrogen 0.30–1.00 0.6

____

Sulfur 0.05–0.20 0.1 100%

Chlorine 0.10–0.50 0.3 Ash 20–30 25

100%Higher heating value

As-received, Btu/lb, AR 3500–5500 4500Moisture and ash free, Btu/lb, MAF 9100–9700 9400

*1985 ASME Research Committee on Municipal and Industrial Waste Boiler as Calorimeter Subcommittee.†Ash for municipal solid waste is the noncombustible portion of the residues made up of mineral matter, metals, stones, glass,

ceramics, etc. Note: 1 Btu/lb = 0.43 kJ/kg.

SOLID WASTE



COLLECTION AND TRANSFER OPERATIONS

The most costly element of refuse service is collection and transfer. Seventy-five to eighty percent of thesolid waste budget is spent on collection and transfer costs. Reducing these costs can best be accomplishedby understanding refuse service processes and equipment.

Collection is the part of solid waste management involving the collection route. Collection begins wherethe customer’s waste is placed for pickup and ends when the collection vehicle leaves the collection route forthe off-route haul to a disposal or processing site. A transfer station, i.e., a terminal where smaller collectionvehicles empty their loads for continued haul by a larger vehicle, may be part of the collection process.

This section describes the institutional arrangements, collection practices, equipment, crew selection,routing, and transfer elements of solid waste management. The discussion incorporates aids to vehicle androute selection and transfer station economics.

Collection Practices

Solid waste collection is one of the most visible services provided to residents of a community. Whether thesystem is public or private, each citizen has contact with refuse collection. The high visibility of solid wastecollection demands effective administration and efficient service mechanisms.

Service Arrangements. Solid waste collection and transfer can be undertaken through public, private, ormixed public-private service. Public arrangements place control in the hands of a governmental unit, whileprivate arrangements place control under a private company or individual. Deciding whether public or pri-vate service is more appropriate within a particular collection area requires a perceptive understanding ofthe community and clearly articulated collection aims and priorities. Service recipient, service provider, ser-vice arranger, and service type are particularly important elements in choosing public or private service.

Public Arrangements. Public solid waste collection is conducted by municipalities that retain control ofadministration and/or operations. Generally, authority resides in a major department of the municipality.Public arrangements take the form of either municipal collection or contract collection. Larger municipali-ties may use both types of collection, using competition to control costs and improve productivity.

Under municipal collection, waste is collected by public employees using publicly owned equipment.The operations are conducted under the direct supervision of a municipal department such as the Depart-ment of Public Works. Normally, single-family residences within a community are covered by the service.Frequently, small industrial and commercial establishments, schools, hospitals, and other institutions, andsmall multifamily residential units are collected as well. The municipality retains full control of operations,maintenance, and fee collection.

Where contract collection is used, waste is collected by a private firm under contract to a municipality.The contractor owns the equipment, furnishes the employees, and manages operations. The public agencystipulates the service level, collection frequency, and other program elements such as container characteris-tics or hours of operation. The municipality retains responsibility for receiving complaints, billing cus-tomers, and controlling the activities of the private firm within the municipal jurisdiction. Generally, con-tracts are awarded through competitive bidding for three years or more to provide time to amortizeequipment and to provide incentive to the private firm. Contracts usually provide for cost adjustments atleast annually.

Private Arrangements. Private arrangements are used where collection services are not provided by agovernmental unit. Primary solid waste collection responsibility and control rests with one or more privatefirms. The distinguishing feature of private arrangements is that collection firms are paid directly by thecustomer. The principal forms of private arrangement are franchise collection, private collection, and self-service.

8.36 CHAPTER EIGHT

SOLID WASTE



Franchise collection consists of a governmental unit giving a private firm an exclusive license to serve aparticular area, generally encompassing more than 10,000 people. Franchises are seldom awarded for lessthan three years and may or may not be awarded by competitive bidding. The municipality enforces the li-censing arrangement and may receive customer complaints. Franchising minimally burdens a municipalitybut creates a monopoly with attendant potential for high prices due to lack of competition.

Private collection offers little public regulation of collection practice. Private firms do not have exclusiveterritories. The level of service is arranged between the customer and the collector, with customers permit-ted to change firms when they wish. Competition is often fierce and a single area may be served by severalcollection firms. The various firms seldom coordinate collection days or practices, so containers may beseen along streets several days a week. Private collection can be highly inefficient and very expensive. It isthe least satisfactory and highest cost system of refuse collection.

Self-service is the practice of letting the generator take waste directly to a transfer station or disposalarea. It is most often used in rural areas.

Public versus Private Arrangements. Each type of institutional arrangement offers specific advantagesand disadvantages. Table 8.17 details the advantages, disadvantages, and conditions favoring particular in-stitutional arrangements.

In a 1975 survey of over 2000 cities with more than 2500 people, Savas (23) found that about 1600 citiesused either municipal, private, or self-service exclusively. Of these, 41.6% used municipal service and57.8% used contract collection. Savas’ data on service arrangements is presented in Table 8.18.

Larger cities are more likely to provide municipal service. Geographically, municipal service predomi-nates in the South while northern and north-central cities favor private collection. Franchise collection ismost commonly encountered in the West.

Savas also compares mean costs and offers the following comparison of annual cost per household usingthe lowest cost as unity:

Type of service arrangement Relative cost

Municipal collection 1.15 Contract collection 1.00 Franchise collection 1.07 Private collection 1.61

Municipal service is slightly more costly than private service, but can offer somewhat higher public service(2). The reasons for higher municipal costs are, in part, due to budgeting and administrative procedures. Thecapacity for a municipality to mandate collection service is useful in controlling litter and maintaining pub-lic health.Public arrangements should be encouraged because the municipality has control of refuse collection exceptfor major commercial customers. Collection can be mandated and separate recycling can be more simplyimplemented. In public arrangements relying on private contractors, contract specifications must be generalenough to attract bidders, but restrictive enough to discourage incompetent firms. Contracts should be forthree years or more with performance bonds a requirement.

Collection System Administration. Administration is the managerial control of operation. Administrativeconcerns include record keeping, maintenance, standby equipment, and costs.

Record Keeping. Record keeping is a vital part of management in any solid waste collection or transfersystem. Without records, productivity measurements, evaluations, cost studies, and preventive maintenanceare difficult to perform. Records that should be routinely kept by the collection department are listed inTable 8.19.

SOLID WASTE 8.37

SOLID WASTE



8.38

TAB

LE 8

.17

Adv

anta

ges

and

Dis

adva

ntag

es o

f P

ubli

c an

d P

riva

te C

olle

ctio

n S

ervi

ces

(22)

Con

diti

ons

that

Alt

erna

tive

Pote

ntia

l adv

anta

ges

Pote

ntia

l dis

adva

ntag

esfa

vor

alte

rnat

ive

Pub

lic

Mun

icip

alTa

x-fr

eeM

onop

olis

tic

Past

his

tory

of

unsa

tisf

acto

ry c

on-

Non

prof

itL

imit

ed in

cent

ive

to im

prov

e ef

fi-

trac

tual

ope

rati

ons

for

publ

ic s

er-

Eco

nom

ies

of s

cale

cien

cyvi

ces

Mun

icip

alit

y ha

s ad

min

istr

ativ

eFi

nanc

ing

and

oper

atio

ns o

ften

Pub

lic

pred

ispo

siti

on to

war

d go

v-co

ntro

lin

flue

nced

by

poli

tica

l con

stra

ints

ernn

ent o

pera

tion

of

publ

ic s

er-

Can

inst

itut

e se

para

te c

olle

ctio

nFr

eque

ntly

fin

ance

d fr

om g

ener

alvi

ces

for

recy

clin

gta

x fu

nd a

nd s

ubje

ct to

1-y

ear

Qua

lity

of

serv

ice

prov

ided

mor

eC

an in

stit

ute

man

dato

ry c

olle

ctio

nbu

dget

ing

proc

ess

impo

rtan

t cri

teri

on th

an e

cono

m-

Man

agem

ent a

nd p

olic

ies

cont

inu-

Sol

id w

aste

man

agem

ent o

ften

ics

ous,

res

ulti

ng in

exp

erie

nced

per

-lo

w-p

rior

ity

item

in b

udge

tso

nnel

and

per

mit

ting

long

-ran

geL

abor

pre

ssur

es m

ay r

esul

t in

in-

plan

ning

effi

cien

t lab

or p

ract

ices

and

Rec

ords

can

be

kept

ove

r a

long

stri

kes

or in

flat

ed la

bor

cost

sti

me

Res

tric

tive

budg

et p

olic

ies

may

affe

ct e

quip

men

t rep

lace

men

t and

mai

nten

ance

Con

trac

tC

ompe

titiv

e bi

ddin

g fo

rD

ange

r of

col

lusi

on in

bid

ding

Fle

xibi

lity

to m

ake

chan

ge in

op-

cont

ract

(s)

help

s ke

ep p

rice

sP

ubli

c ag

ency

mus

t reg

ulat

e co

n-er

atio

ns th

at w

ould

res

ult i

n la

bor

dow

ntr

acto

rssa

ving

s an

d ot

her

cost

red

ucti

ons

Mun

icip

alit

y re

tain

s ad

min

istr

a-A

vail

abil

ity

of q

uali

fied

pri

vate

tive

cont

rol

cont

ract

ors

Can

inst

itut

e se

para

te c

olle

ctio

nP

ubli

c pr

edis

pose

d to

war

d pr

ivat

efo

r re

cycl

ing

sect

or in

volv

emen

t in

publ

ic s

er-

Can

inst

itut

e m

anda

tory

col

lect

ion

vice

sN

ewly

inco

rpor

ated

com

mun

itie

s,or

com

mun

itie

s w

here

pop

ulat

ion

grow

th is

out

paci

ng a

bili

ty o

fco

mm

unit

y to

pro

vide

pub

lic

ser-

vice

s

SOLID WASTE



8.39

Mun

icip

al s

yste

m a

nd p

riva

teC

ompe

titi

on h

elps

kee

p pr

ice

Cou

ld b

e ad

min

istr

ativ

ely

orM

unic

ipal

ity

is e

xpan

ding

thro

ugh

firm

s un

der

cont

ract

dow

nju

risd

icti

onal

ly c

ompl

exan

nexa

tion

or

mer

ger

wit

h ot

her

Alt

erna

tive

avai

labl

e if

eit

her

sec-

juri

sdic

tion

sto

r ca

nnot

del

iver

ser

vice

Cha

ngin

g fr

om s

epar

ate

garb

age

Mun

icip

alit

y ha

d ad

min

istr

ativ

ean

d tr

ash

coll

ecti

on to

com

bine

dco

ntro

l can

inst

itut

e se

para

te c

ol-

coll

ecti

onle

ctio

n fo

r re

cycl

ing

Pri

vate

arr

ange

men

tsP

riva

te c

olle

ctio

nC

ompe

titi

on m

ay r

educ

e co

sts

No

publ

ic a

dmin

istr

ativ

e co

ntro

lM

unic

ipal

ity

not i

nter

este

d in

Sel

f-fi

nanc

ing

Dan

ger

of c

ollu

sion

am

ong

haul

-re

fuse

col

lect

ion

ers

Cut

thro

at c

ompe

titi

on c

an r

esul

tin

bus

ines

s fa

ilur

es a

nd s

ervi

cein

terr

upti

ons

Ove

rlap

ping

rou

tes,

was

te o

f fu

elC

anno

t ins

titu

te c

ityw

ide

sepa

rate

coll

ecti

on f

or r

ecyc

ling

Dif

ficu

lt to

enf

orce

man

dato

ryco

llec

tion

ord

inan

ces

Fran

chis

eS

elf-

fina

ncin

gN

o pu

blic

adm

inis

trat

ive

cont

rol

Mun

icip

alit

y w

ants

litt

le to

do

Mon

opol

isti

c, c

an le

ad to

hig

hw

ith

refu

se c

olle

ctio

npr

ices

Can

not i

nsti

tute

sep

arat

e co

llec

-ti

on f

or r

ecyc

ling

Dif

ficu

lt to

enf

orce

man

dato

ryco

llec

tion

ord

inan

ces

SOLID WASTE



8.40

TAB

LE 8

.18

Ser

vice

Arr

ange

men

ts f

or th

e C

olle

ctio

n of

Mix

ed R

esid

enti

al R

efus

e, b

y C

ity

Siz

e, R

egio

n, a

nd F

orm

of

Gov

ernm

ent (

23)

Mun

icip

alC

ontr

act

Fran

chis

eP

riva

teS

elf-

serv

ice

Oth

er__

____

____

___

____

____

____

___

____

____

___

____

____

____

___

____

____

___

____

____

____

_

Tota

llb

%lb

%lb

%lb

%lb

%lb

%

Tota

l25

3176

830

.342

016

.616

66.

578

230

.937

614

.919

0.8

Popu

lati

on g

roup

2531

>25

0,00

037

2773

.04

10.8

00

410

.81

2.7

12.

750

,000

–249

,999

268

149

55.6

259.

322

8.2

4115

.328

10.4

31.

110

,000

–49,

999

706

242

34.3

152

21.5

598.

417

024

.181

11.5

20.

32,

500–

9,99

915

2035

023

.023

915

.785

5.6

567

37.3

266

17.5

130.

9G

eogr

aphi

c re

gion

2531

Nor

thea

st98

118

619

.021

321

.722

2.2

382

38.9

176

17.9

20.

2N

orth

cen

tral

715

143

20.0

111

15.5

162.

233

046

.210

715

.08

1.1

Sou

th46

934

172

.728

6.0

347.

233

7.0

275.

86

1.3

Wes

t36

698

26.8

6918

.993

25.4

3710

.166

18.0

30.

8Fo

rm o

f go

vern

men

t17

99M

ayor

–cou

ncil

876

374

42.7

214

24.4

424.

817

820

.364

7.3

40.

4C

ounc

il–m

anag

er72

431

944

.110

915

.110

314

.210

013

.887

12.0

60.

8O

ther

199

5427

.132

16.1

52.

560

30.2

4522

.63

1.5

Not

e:T

his

tabl

e sh

ows

the

dist

ribu

tion

of

arra

ngem

ents

, not

the

dist

ribu

tion

of

citi

es. T

here

is a

tota

l of

2531

arr

ange

men

ts in

the

2052

cit

ies.

SOLID WASTE



Maintenance. Refuse vehicles are complicated mechanical devices requiring periodic maintenance. Amechanic capable of repairing mechanical and hydraulic systems together with the necessary tools andequipment should be a part of any solid waste collection system. For small operations, such service may beprovided by a private mechanic.

Standby Equipment. Refuse collection is hard service with frequent equipment breakdown. The prudentcollection manager will provide sufficient standby equipment to allow the daily collection to be completedas scheduled. Generally, one standby vehicle should be available for every five vehicles used daily, but notless than one spare should be available.

Cost Factors. The cost of solid waste collection varies considerably from system to system. A study of314 systems in the United States by Stevens (26) found wages to be the single largest component of cost. Di-rect labor cost averages 60 to 65% of the total collection cost. Other factors affecting municipal collectioncosts include stop density, service level, management practices, waste generation, and equipment size andtype.

Crew Productivity. Crew productivity is a measure of efficiency. Several factors influence crew produc-tivity including route, service level, collection equipment, and personal characteristics of crew members.Table 8.20 outlines factors affecting crew productivity.

Crew productivity may be measured in several ways. Among these are:

� Households collected per week per crew� Weight per year per crew or crew member� Volume per year per crew or crew member

Data on productivity, measured in several cities across the nation in the mid-1970s, was used to produceTable 8.21. The table provides maximum, mean, and minimum productivity for various crew and truck con-

SOLID WASTE 8.41

TABLE 8.19 Routine Collection Service Records

Route books and route maps (updated as required) Vehicle and body records

Purchase dataMaintenance and repair recordFuel consumption recordAccident recordsOperating hoursOn-route hoursOff-route hoursTime to disposal and return Crew records

Weight or volume collected per dayHouseholds collected per dayOther stops collected per dayTruck assignmentTime on routeTime off route

Load recordsWeightNumber of trips to disposal per dayPercentage of full capacityNumber and type of units collected (periodic)

SOLID WASTE



figurations. The cost of providing service, although not as specific as the other approaches, is sometimesused as a measure of productivity. Table 8.22 compares costs for several crew and truck configurations.

Service Level

The level of service is set by the frequency of routine collection and the type of service offered to the cus-tomer. Collection frequency and scheduling are principally matters of health, aesthetics, and economics.Service type requires a decision on resident responsibilities versus crew productivity, specifically whetherresidential collection is to be in the backyard or at curbside.

Residential Collection. Residential service levels vary with each municipality, depending on citizen ex-pectations and budgetary constraints. Once-weekly service is the predominant collection interval in theUnited States. Twice-weekly collection is normal throughout the southern and the southeastern UnitedStates, where warm climates create nuisances associated with refuse stored for longer periods. In inner-cityareas where storage is limited, collections more frequently than twice-weekly may be needed.

The more frequently wastes are collected, the more costly is the collection service. Table 8.23 outlines theadvantages and disadvantages of various frequencies of residential refuse collection.

Type of Service. Whether crews pick up waste at the curb or in the backyard is an important aspect of thelevel of solid waste collection. Crew and truck sizes and service cost depend on the type of service offered.

Curbside collection requires the resident to place waste containers at curbside or alleyside on collectionday. Curbside service is less expensive than backyard service, and about 60% of the collection systems inthe United States had curbside service in the mid-1970s.

Backyard collection requires the collection crew to pick up waste where the resident stores it. In somecases, crews enter fenced areas or garages to pick up containers. Backyard service is usually in one of fourforms:

� Set out and set back. Crews carry containers to the curb and return empty containers to the storage place.� Set out. Crews carry containers from the storage place to the curb; residents return empty containers to

the storage area.� Tote barrel. Crews empty containers at the storage place into a tote barrel, then empty the tote barrel into

the collection vehicle.� Satellite vehicle. A crew member drives a small vehicle to the storage area, empties containers into the

vehicle container, then empties the smaller vehicle into the main collection truck.

Curbside versus Backyard Service. The economics of fuel consumption and service time are particularlyimportant in comparing curbside and backyard service. Table 8.24 summarizes advantages and disadvan-tages of curbside and backyard service.

8.42 CHAPTER EIGHT

TABLE 8.20 Factors Affecting Crew Productivity

Condition Factors

Routing, service level Routing design, density of stops, collection location, types of containers, traffic congestion

Vehicle Loading location, loading height, vehicle capacity, compaction density, crew size, packing cycle

Personnel Age, attitudes, health

SOLID WASTE



8.43

TAB

LE 8

.21

Com

para

tive

Pro

duct

ivit

y Fa

ctor

s—S

olid

Was

te C

olle

ctio

n C

rew

s (2

)

Wei

ght,

Wei

ght,

Vol

ume,

m3

met

ric

tons

Vol

ume,

m3

met

ric

tons

Cre

w s

ize,

Col

lect

ing

Sto

ps p

er(y

d3 ) p

er(t

ons)

per

(yd3 )

per

(ton

s) p

erpe

rson

sti

me,

h/d

aycr

ew-d

aycr

ew-y

ear

crew

-yea

rpe

rson

-yea

rpe

rson

-yea

r

Tota

l of

all s

yste

ms

Ave

rage

2.5

4.52

552

7,04

22,

313

3,31

01,

075

(9,2

10)

(2,5

50)

(4,3

30)

(1,1

85)

Max

imum

46.

81,

229

12,8

915,

098

7,35

52,

123

(16,

860)

(5,6

20)

(9,6

20)

(2,3

40)

Min

imum

13.

018

22,

783

825

1,31

934

0(3

,640

)(9

10)

(1,7

25)

(430

)A

ll m

unic

ipal

sys

tem

sA

vera

ge2.

94.

360

56,

380

2,64

52,

347

780

(8,3

45)

(2,2

65)

(3,0

70)

(860

)M

axim

um4

5.6

1,15

58,

732

3,31

13,

991

1,18

4(1

1,42

0)(3

,650

)(5

,220

)(1

,305

)M

inim

um2

3.0

280

5,27

51,

306

1,31

934

0(6

,900

)(1

,440

)(1

,725

)(4

30)

All

mun

icip

al r

ear

load

ers

Ave

rage

3.2

4.0

660

6,40

72,

059

2,02

665

8(8

,380

)(2

,270

)(2

,650

)(7

25)

Max

imum

45.

61,

155

8,73

23,

311

2,35

91,

102

(11,

420)

(3,6

50)

(3,0

85)

(1,2

15)

Min

imum

33.

045

55,

275

1,30

61,

319

340

(6,9

00)

(1,4

40)

(1,7

25)

(430

)M

unic

ipal

sid

e lo

ader

sA

vera

ge1.

54.

559

97,

103

2,10

93,

991

1,18

4(9

,290

)(2

,325

)(5

,220

)(1

,305

)M

axim

um2

4.5

——

——

—M

inim

um1

4.4

——

——

—

(con

tinu

es)

SOLID WASTE



8.44

TAB

LE 8

.21

Com

para

tive

Pro

duct

ivit

y Fa

ctor

s—S

olid

Was

te C

olle

ctio

n C

rew

s (2

)(c

onti

nued

)

Wei

ght,

Wei

ght,

Vol

ume,

m3

met

ric

tons

Vol

ume,

m3

met

ric

tons

Cre

w s

ize,

Col

lect

ing

Sto

ps p

er(y

d3 ) p

er(t

ons)

per

(yd3 )

per

(ton

s) p

erpe

rson

sti

me,

h/d

aycr

ew-d

aycr

ew-y

ear

crew

-yea

rpe

rson

-yea

rpe

rson

-yea

r

Mun

icip

al f

ront

load

ers

25.

432

95,

528

1,98

72,

764

933

(7,2

30)

(2,1

90)

(3,6

15)

(1,0

95)

Mun

icip

al b

acky

ard

43.

027

95,

275

1,56

51,

319

390

(6,9

00)

(1,7

25)

(1,7

25)

(430

)M

unic

ipal

cur

bsid

eA

vera

ge2.

74.

566

06,

568

2,13

62,

519

844

(8,5

90)

(2,3

55)

(3,2

95)

(930

)M

axim

um3

5.6

1,15

58,

732

3,31

13,

991

1,18

4(1

1,42

0)(3

,650

)(5

,220

)(1

,305

)M

inim

um2

3.0

330

5,49

71,

306

1,83

143

5(7

,190

)(1

,440

)(2

,395

)(4

80)

Mun

icip

al th

ree-

pers

on c

rew

sA

vera

ge3.

04.

261

56,

691

2,18

22,

095

722

(8,7

51)

(2,4

05)

(2,7

40)

(796

)M

axim

um3

5.6

1,15

58,

732

3,31

12,

360

1,10

2(1

1,42

0)(3

,650

)(3

,086

)(1

,215

)M

inim

um3

3.0

280

5,49

01,

306

1,83

543

5(7

,180

)(1

,440

)(2

,400

)(4

80)

Mun

icip

al tw

o-pe

rson

cre

ws

Ave

rage

2.0

5.0

464

6,31

62,

045

3,37

51,

089

(8,2

60)

(2,2

55)

(4,4

15)

(1,2

00)

Max

imum

25.

460

07,

103

2,10

93,

991

1,18

4(9

,290

)(2

,325

)(5

,220

)(1

,305

)M

inim

um2

4.5

330

5,52

81,

987

2,76

499

3(7

,230

)(2

,190

)(3

,615

)(1

,095

)

SOLID WASTE



8.45

All

pri

vate

sys

tem

sA

vera

ge2.

144.

857

37,

703

2,57

24,

270

1,37

0(1

0,07

5)(2

,835

)(5

,585

)(1

,510

)M

axim

um3

6.8

1,30

012

,891

5,09

42,

355

2,12

2(1

6,86

0)(5

,620

)(9

,620

)(2

,340

)M

inim

um1

3.8

180

2,78

382

52,

531

721

(3,6

40)

(910

)(3

,310

)(7

95)

Pri

vate

rea

r lo

ader

sA

vera

ge2.

54.

654

87,

726

2,49

03,

597

1,13

4(1

0,10

5)(2

,745

)(4

,705

)(1

,250

)M

axim

um3

5.5

652

10,6

503,

152

5,32

51,

574

(13,

930)

(3,4

75)

(6,9

65)

(1,7

35)

Min

imum

24.

029

05,

371

1,59

22,

531

721

(7,0

25)

(1,7

55)

(3,3

10)

(795

)P

riva

te s

ide

load

ers

Ave

rage

1.3

5.1

606

7,53

12,

681

5,16

91,

692

(9,8

50)

(2,9

55)

(6,7

60)

(1,8

65)

Max

imum

26.

81,

300

12,8

915,

098

7,35

52,

123

(16,

860)

(5,6

20)

(9,6

20)

(2,3

40)

Min

imum

14.

718

02,

783

825

2,78

382

5(3

,640

)(9

10)

(3,6

40)

(910

)P

riva

te b

acky

ard

Ave

rage

2.0

3.9

403

4,55

31,

315

2,65

777

5(5

,955

)(1

,450

)(3

,475

)(8

55)

Max

imum

34.

062

46,

327

1,80

02,

783

825

(8,2

75)

(1,9

85)

(3,6

40)

(910

)M

inim

um1

3.8

180

2,78

382

52,

531

721

(3,6

40)

(910

)(3

,310

)(7

95)

(con

tinu

es)

SOLID WASTE



8.46

TAB

LE 8

.21

Com

para

tive

Pro

duct

ivit

y Fa

ctor

s—S

olid

Was

te C

olle

ctio

n C

rew

s (2

)(c

onti

nued

)

Wei

ght,

Wei

ght,

Vol

ume,

m3

met

ric

tons

Vol

ume,

m3

met

ric

tons

Cre

w s

ize,

Col

lect

ing

Sto

ps p

er(y

d3 ) p

er(t

ons)

per

(yd3 )

per

(ton

s) p

erpe

rson

sti

me,

h/d

aycr

ew-d

aycr

ew-y

ear

crew

-yea

rpe

rson

-yea

rpe

rson

-yea

r

Pri

vate

cur

bsid

eA

vera

ge2.

25.

264

18,

965

3,07

54,

916

1,61

0(1

1,72

5)(3

,390

)(6

,430

)(1

,775

)M

axim

um3

6.8

1,30

012

,891

5,09

87,

355

2,12

3(1

6,86

0)(5

,620

)(9

,620

)(2

,340

)M

inim

um1

4.0

290

5,37

1 1,

592

2,85

2 1,

093

(7,0

25)

(1,7

55)

(3,7

30)

(1,2

05)

Pri

vate

thre

e-pe

rson

cre

ws

Ave

rage

3 4.

4 62

4 7,

440

2,60

8 2,

691

930

(9,7

30)

(2,8

75)

(3,5

20)

(1,0

25)

Max

imum

3 4.

8 62

4 8,

556

3,41

5 2,

852

1,13

9(1

1,19

0)

(3,7

65)

(3,7

30)

(1,2

55)

Min

imum

3 4.

0 62

4 6,

327

1,80

0 2,

531

721

(8,2

75)

(1,9

58)

(3,3

10)

(795

)P

riva

te tw

o-pe

rson

cre

ws

24.

065

210

,651

3,15

25,

328

1,57

4(1

3,93

0)(3

,475

)(6

,965

)(1

,735

)P

riva

te o

ne-p

erso

n cr

ews

Ave

rage

1 4.

3 26

0 5,

069

1,47

4 5,

069

1,47

4(6

,630

) (1

,625

) (6

,630

) (1

,625

)M

axim

um1

4.7

338

7,35

5 2,

123

7,35

5 2,

123

(9,6

20)

(2,3

40)

(9,6

20)

(2,3

40)

Min

imum

1 3.

8 18

0 2,

783

825

2,78

3 82

5(3

,640

) (9

10)

(3,6

40)

(910

)

SOLID WASTE



8.47

TAB

LE 8

.22

Ave

rage

Cos

t Rel

atio

nshi

ps f

or V

ario

us C

olle

ctio

n P

ract

ices

(2)

Cos

tA

vera

ge n

umbe

r__

____

____

____

____

____

____

____

____

____

____

____

____

____

____

__

stop

s/ye

ar$/

hous

ehol

d/ye

ar$/

met

ric

ton

(ton

)$/

m3

(yd3 )

*

All

sys

tem

s11

3,10

039

.36

31.4

57.

82(2

8.53

)(5

.98)

All

rea

r-lo

ad s

yste

ms

121,

100

40.8

332

.71

9.02

(29.

67)

(6.9

0)R

ear-

load

cur

bsid

e sy

stem

s13

8,00

040

.19

28.6

37.

72(2

5.97

)(5

.90)

Cre

w o

f 2,

1-s

ide

coll

ecti

on, t

wic

e/w

eek

141,

300

24.1

720

.78

6.02

(18.

85)

(4.6

0)C

rew

of

3, 1

-sid

e co

llec

tion

, onc

e/w

eek

26,5

0050

.41

34.1

18.

46(3

0.94

)(6

.47)

Rea

r lo

ad—

back

yard

ser

vice

78,9

0042

.43

40.3

811

.84

Cre

w o

f 2,

onc

e/w

eek

coll

ecti

on(3

6.63

)(9

.05)

All

sid

e-lo

ad s

yste

ms

(all

cur

bsid

e se

rvic

e)10

5,00

063

.39

29.2

66.

21(2

6.54

)(4

.75)

Cre

w o

f 1

100,

200

30.7

422

.95

4.92

(20.

82)

(3.7

6)C

rew

of

211

9,40

054

.79

48.1

79.

94(4

3.70

)(7

.60)

Onc

e/w

eek

coll

ecti

on86

,300

26.3

219

.07

4.17

(17.

30)

(3.1

9)Tw

ice/

wee

k co

llec

tion

123,

700

42.2

739

.43

8.12

(35.

77)

(6.2

1)

*Vol

ume

calc

ulat

ed o

n no

min

al b

ody

capa

city

tim

es tr

ips

to th

e di

spos

al s

ite,

ass

umin

g ve

hicl

e is

em

ptie

d at

end

of

each

wor

king

day

.N

ote:

Cos

ts, U

.S. d

olla

rs (

C.P

.I.,

$271

.7).

SOLID WASTE



Curbside collection can service more homes and collect more waste per day with fewer trucks than canbackyard collection. Backyard collection with a three-person crew will require about 1.6 times more fuel fora set number of customers than will curbside service.

Fuel consumption on packer vehicles is as much a function of engine operating hours as it is of distancedriven. A comparative breakdown showing percent of time for various activities of a collection vehicle ap-pears in Table 8.25.

Backyard service consumes less fuel because the vehicle idles more and has fewer compaction cycles peroperating hour than is required for curbside service. Curbside service consumes slightly more fuel per milebecause trucks spend a greater portion of their operating hour in driving and compacting. Fuel consumptionfor collection vehicles is outlined in Table 8.26.

8.48 CHAPTER EIGHT

TABLE 8.23 Advantages and Disadvantages of Different Frequencies of Collection (22)

Potential Potential Conditions thatAlternative advantages disadvantages favor alternative

Once per week or Less expensive Improperly stored Adequate storageless Requires less fuel waste can create provisions

odor and vector Cold to moderateproblems climate

Twice per week Reduces litter More expensive Quality of serviceReduces storage Requires more fuel provided more

requirements important criterion than economics

Warm climate More than twice Reduces litter More expensive Seriously restricted

per week Reduces storage Requires more fuel storage spacerequirements Dense population

TABLE 8.24 Advantages and Disadvantages of Curbside–Alley and Backyard Collection (22)

Potential Potential Conditions thatAlternative advantages disadvantages favor alternative

Curbside More efficient Cans at curb look High collectionLess expensive messy costsRequires less labor Special arrange- Unwillingness onFacilitates use of ments must be part of residents

paper or plastic made for handi- to pay higherbags capped and elderly taxes or user

Reduces collector Residents must re- chargeinjuries member day of

Requires less fuel collection Backyard No effort required More expensive Quality of service

by residents High labor turnover provided moreNo mess at curbs Increases number important criterion

of collector injuries than economicsRequires more fuel

SOLID WASTE



Time Study. The time needed to service a collection stop is a combination of the time required to set outthe containers and the time to empty the containers into the truck. Curbside collection crews apply a signifi-cant amount of work time to collection that would otherwise be used for set out. A time-and-motion studywas conducted in Ann Arbor, Michigan, on three-person setout crews in 1975. Backyard collection averaged1.10 mm per household, whereas curbside collection averaged 0.64 mm per household (7).

The time needed to collect the refuse after it has been set out is a function of the number and type of con-tainers at the stop, and not distinctly related to the weight of the containers. Figure 8.9 is a graph comparingthe time of collection with container characteristics (7).

Collection Schedules and Costs. Almost all residential collection systems operate on a four-, five-, or six-day work schedule. In communities using a four-day schedule, crews are often detailed for special pickup onthe fifth workday of the week. Twice-weekly collection systems must cover the collection area in 2 or in 2½days. If a two-day schedule is selected, crews are often used for special pickup on the remaining work day.

The type of service can vary in cost according to crew size and frequency of collection. The cost compar-ison in Table 8.27 is based on data from nine U.S. cities (2). The data compare the cost between once-week-ly and twice-weekly collection using either one-, two-, or three-person crews. Data must be used judiciouslybecause of the small sample size.

Commercial Collection. Commercial collection is primarily a specialty service with pickup frequency de-pendent on waste volumes and types. Putrescible wastes are collected more frequently than nonputrescible

SOLID WASTE 8.49

TABLE 8.25 Comparative Operating Activities for Residential CollectionVehicles in Percent

Type of service

Activity Curbside Backyard

Off-route travel and dumping 30 21On-route driving 21 13On-route idling 43 61Compaction 6 5

TABLE 8.26 Typical Fuel Consumption by Collection Type (22)

Fuel consumption

Type of service and engine gal/h (L/h)

CurbsideDiesel 2.4 9.0Gasoline 2.8 10.5

BackyardDiesel 1.4 5.5Gasoline 1.7 6.5

SOLID WASTE



8.50 CHAPTER EIGHT

FIGURE 8.9 Collection time for refuse cans, refuse bags, and other containers.

TABLE 8.27 Comparative Cost of CurbsideCollection by Frequency and Crew Size

FrequencyCrew size per week Cost ratio

1 1 1.001 2 1.622 1 1.132 2 2.763 1 1.863 2 1.84

SOLID WASTE



wastes. The generator and collector usually arrange service frequency at a specific point on the premises.Parks, hospitals, and other institutions may require daily if not more frequent collection.

Storage Containers

The collection agency should regulate storage container size and type, matching the container characteristicsto the collection system. A suitable storage container must be easy to handle and to keep clean. The contain-ers should contain odors, limit disease, and keep out animals and insects. Containers should be large enoughto limit the number of containers at a stop, but small enough to be lifted easily and safely by one person orbe appropriate to a mechanized system. In general, storage bins, 55-gal (208-L) drums, and cardboard boxesare unacceptable for storage.

Containers for Manual Collection. Two types of containers are acceptable for manual residential collec-tion: metal or plastic cans and bags. Cans should be sized from 20 to 32 gal (75 to 120 L); bags may be pa-per or plastic. Table 8.28 details the advantages and disadvantages of common storage containers.

Containers for Mechanized Collection. In efforts to increase productivity, several communities are usingmechanized containers for residential refuse collection. Residential mechanized collection is a new applica-tion of technology requiring more experience before it can be generally adopted. Currently, large containersfor use at single- or multifamily residences are the most common mechanized storage containers. Containersfor single-family residences are commonly 80-gal (300-L) heavy plastic mounted on wheels to facilitatemovement to the curb. Collection vehicles are equipped to pick up the container and empty it into the vehi-cle. Some of these systems use special refuse bodies. Because reliability and public acceptance have notbeen generally established for such systems, careful study of the community is needed before such a systemis selected.

Container systems to serve from two to four single-family households have been tried recently. The con-tainers are similar to commercial containers and are handled the same way. Their principal advantage is thatseveral homes are serviced by a single stop of the collection vehicle. Three disadvantages can be associatedwith such containers: (1) carelessness can lead to littering around the container; (2) reluctance to use con-tainers that allow neighbors to observe waste; and (3) objections to having containers on private property.

Unacceptable Containers. Although residents may want to use almost anything as a refuse container, sev-eral containers, such as bins, large drums, and cardboard boxes, are unacceptable.

Some older apartment and commercial buildings have stationary concrete or cement-block storage bins.The bins are unsanitary and inefficient, must be emptied by hand, attract insects and rodents, allow blowingpapers and odors, and if uncovered, can leach a foul liquid after rains.

Steel Drums and paper packing drums are also unacceptable. The drums usually do not have tight-fittinglids, allowing insects, birds, and animals access to the refuse. An empty metal drum weighs 35 to 40 lb (16to 18 kg), and over 100 lb (45 kg) when full. Full drums can be unmanageable for one collector and unsafedue to sharp edges.

Cardboard boxes are sometimes set out on the curb for collection. Boxes filled with garbage or liquids at-tract pests and are a hazard to the collector. Cardboard boxes nested and open for inspection or cardboardboxes filled with newspapers may be acceptable for collection.

Collection Equipment

The system manager should choose equipment suited to the characteristics of the collection area. Anticipat-ed service level, crew size, and route characteristics such as narrow alleys, restricted headroom, or turningradius will affect vehicle and body selection. Often more than one type or size of equipment will be needed

SOLID WASTE 8.51

SOLID WASTE



8.52

TAB

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Adv

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s

SOLID WASTE



in a service area. Table 8.29 outlines equipment and crew characteristics and the implications for equipmentoperations. The average life of a collection vehicle body is from 5 to 7 years.

Residential Collection Vehicles. Residential collection vehicles have a chassis and a body. The chassiscontains the drive train and operator’s cab. The body includes the receiving hopper, compactor, and storagecompartment. The chassis and body must be able to negotiate turns and pass under bridges along the route.

Collection equipment for residential services is classified by loading characteristics-rear-loaded or side-loaded. Consideration of loading hopper height and location is important in determining crew size and esti-mating productivity.

Compaction density, the measure of how much the body can compact the loose refuse picked up by thecrew, is a significant consideration in loader productivity. Compaction density is measured in pounds per cu-bic yard (kilograms per cubic meter). Figure 8.10 illustrates the relationship between packer capacity, com-pacted density, and households collected.

Rear Loaders. Rear loaders are suited to densely populated areas, where stops are frequent and waste vol-umes are high. Both sides of the street or alley are usually collected at the same time. Where three-personcrews are used, the driver helps with occasional heavy loads. On two-person crews, the driver usually dou-bles as a loader.

A number of manufacturers make rear-loading collection equipment (24). Units are available in 14 bodysizes, ranging from 9 to 32 yd3 (6.9 to 24.5 m3) capacity (exclusive of hopper).

Compaction densities among 29 models of packers sampled range from 500 to 1100 lb/yd3 (225 to 500

SOLID WASTE 8.53

TABLE 8.29 Factors to be Considered in Selecting Solid Waste Collection Equipment

Truck or crew factor Implication

Route considerations

Vehicle length Garage space, turns Vehicle height Underpass and garage door clearance Vehicle weight Bridge and road weight limits Vehicle turning radius Turns to streets, alleys, or cul-de-sacs Body volume Number of collection stops and trips to disposal area Compaction Number of collection stops, amount of waste per stop Number of axles Load limits

Crew considerations

Equipment access Crew dismounting, mounting Hopper height Crew loading effortHopper width Crew size and minimizing compaction cycles Safety features Crew safety

Cost considerations

Vehicle fuel economy Energy costs Capital costs Replacement and amortization costs Operating and maintenance costs Operating costs Labor wage rates Labor costs Equipment reliability DowntimeService DowntimeEase of repair Downtime

SOLID WASTE



kg/m3). Compaction density on three-quarters of the rear loaders is less than 750 lb/yd3 (445 kg/m3). The av-erage on-route efficiency that can be expected of rear-load equipment is between 80 and 85% of the manu-facturer’s rated maximum density.

Rear-load bodies have several advantages. The hopper height is lower than for most side-load bodies, sothe crew does not have to lift containers as high. Loading hoppers are wide, usually the full width of thebody, and the packer plate configuration allows large, bulky items to be accepted. The principal disadvan-

8.54 CHAPTER EIGHT

FIGURE 8.10 Number of households served by vehicle size and compacted density.

SOLID WASTE



tage of rear loaders is the complexity of the packer plate design and the accompanying higher maintenancerequirements.

Rear-load vehicles are emptied by gravity or by ejector. Gravity bodies are tilted so that the load slidesout in a manner similar to dump trucks. If a load hangs up and cannot be shaken out, the truck must be emp-tied by hand. The trend has been toward ejector bodies, because dump body packers are slower to empty andmore prone to overturning on the uneven surfaces of a landfill.

Side Loaders. Side loaders are best suited to densely populated areas where collection takes place on oneside of the street or for rural routes. Side loaders are most often employed for residential curbside pick up,for apartments, or for small commercial establishments. Side-load vehicles are particularly useful in narrowalleys with limited maneuvering room if large containers are used. A low step-in model cab with right-handdrive can be run by a one-person crew. If two-person crews are used, two sides of the street can be collectedat the same time, but the left-hand loader is exposed to traffic.

Thirteen side loader manufacturers offer 28 different body capacities ranging from 6 to 40 yd3 (4.6 to30.6 m3). Compaction density ranges from 300 to 800 lb/yd3 (178 to 475 kg/m3), generally lower than thecompaction capability of rear-load units (24). The compaction mechanism on side loaders is much simpler.The average on-route density will range from 85 to 95% of the manufacturer’s statement of maximum densi-ty.

Side loaders are emptied by ejection, using the same mechanism that loads the storage compartment andcompacts the load.

Specialized Collection Vehicles. Special collection vehicles are usually chosen to eliminate a cumber-some or costly collection situation or to improve productivity. Many of the vehicles are mechanized or auto-mated, requiring fewer crew members. European cities have used specialized equipment that lifts and emp-ties residential containers from the rear of the packer vehicle for many years. Other systems usingmechanical arms or mechanical lifting devices are being tried in the United States.

Commercial Collection Vehicles. Solid waste is collected from commercial establishments as well as resi-dences. Commercial establishments include businesses, industries, institutions, and often apartments withmore than four to six units. Commercial wastes differ in quantity and type from residential wastes and areusually stored in large containers rather than small bags and cans.

Vehicle Types. Four types of vehicles are normally used in commercial collection systems: rear, side, andfront loaders and specialized drop-off bodies. The least complex is a residential-type rear- or side-loadingcollection vehicle equipped to handle bulk containers. Bulk containers ranging in capacity from 1.0 to 8 yd3

(0.76 to 6.1 m3) are normally used with rear-loading equipment. Side-loading vehicles can accommodatebulk containers up to 6 yd3 (4.5 m3) capacity. The containers are equipped with casters to aid in maneuver-ing the container.

Front-loading equipment is operated by one person and is designed for use with commercial bulk con-tainers ranging in capacity from 1 to 10 yd (0.76 to 7.6 m3). The containers are not equipped with casters,since the truck maneuvers to the container. The container is picked up by hydraulic arms, lifted over thetruck cab, and discharged into a hopper in the top of the compaction body.

Front-loading vehicles are furnished by 11 manufacturers in 41 models ranging in capacity from 9 yd3

(6.88 m3) to 50 yd3 (38.2 m3). The density that can be obtained by the compaction body ranges from 400lb/yd3 (237 kg/m3) to 850 lb/yd3 (504 kg/m3) (24).

A number of specialized body designs with drop-off boxes are on the market for commercial collection.The box is picked up, transported to a disposal or processing location, emptied, and returned to the cus-tomer’s premises on a specially designed truck. Two forms are prevalent. The dumpster type provides anopen or closed bin that is lifted onto the truck by arms attached near the rear of the chassis. The other offersa tilting frame chassis that moves bins on and off the truck by chain or cable. The latter form is popularlycalled a roll-off system.

Specialized systems usually become feasible when at least four containers per day per chassis are ser-

SOLID WASTE 8.55

SOLID WASTE



viced. The large containers offer storage for bulky items, as well as large volumes of material. Bin contain-ers are available from 10 yd3 (7.65 m3) capacity to 50 yd3 (38.2 m3).

Roll-off bodies are available from 10 to 55 yd3 (7.65 to 42 m3) and may be enclosed or open. Special roll-off containers may be obtained for leaves, sludge, snow, liquid, and bulk debris. Enclosed containers can beequipped with push plates for ejecting loads.

Table 8.30 indicates the relative costs of commercial collection systems. Maneuvering Requirements. The container must be located so that the vehicle can be driven up to it and

have adequate room to lift it into the discharge position. Figure 8.11 illustrates the minimum maneuveringroom required by various collection vehicles.

Routing

Routing is the process of identifying the path a vehicle is to take in serving its daily route. After administra-tive policy on service level, operations, and labor are clearly established, routing objectives can be set andthe basic routing steps can be undertaken. Ideally, each collection vehicle should start the day empty and bepacked out when the truck makes its final run to the disposal site. The path can be set by trial-and-error,computer, or heuristic methods. The steps involved in establishing a collection route are

� Define the collection area� Assign disposal sites� Establish daily zones� Balance daily vehicle assignments� Route vehicles within daily district

Collection Area. The collection area is comprised of the territory to be serviced in a single collection cy-cle. The limits of the collection area are usually defined by political or geographical boundaries. The limitsof the collection area, together with disposal sites, transfer stations, or waste processing plants, should bemarked on a map. The collection area map should show major routes to each disposal option and any re-strictions, such as load or height limits.

Disposal Area. Disposal areas are those parts of the collection area assigned to a specific disposal or pro-cessing site. Where a system has only one or two disposal options, matching routes to disposal sites is rela-tively easy. Where several disposal sites are available, the problem becomes highly complex. Many factorsmust be evaluated in choosing where to route vehicles. Among these are tipping fees, site reliability and life,round-trip haul and disposal time, queuing costs, and vehicle wear and tear. In very complicated situations, a

8.56 CHAPTER EIGHT

TABLE 8.30 Relative Cost of Commercial Collection Systems

CostSystem type(crew of 1) Capital Annual Dollars per ton

Rearload 1.00 1.00 1.00Side load 0.70 0.040 0.98Front load 1.67 0.99 0.52Roll-off 1.33 0.90 0.24

Note:1 ton = 0.9 metric tons.

SOLID WASTE



computer program might be used to identify the most effective use of many different facilities. Suitable pro-grams are available through the EPA, major universities, and private consultants.

Collection Zones. Collection zones are set up by dividing the collection area into sections or zones for dai-ly service. All premises collected on Monday constitute one zone; those collected on Tuesday, another zone;and so on.

The average number of households assigned to each residential zone should be approximately equal. Dai-ly zones can be slightly unbalanced to accommodate distinct geographical boundaries. When collection as-

SOLID WASTE 8.57

FIGURE 8.11 Maneuvering room for various types of commercial collection vehicles with 6-yd3 (4.5-m3) container (a)Front-load vehicle; (b) rear-load vehicle; (c) side-load vehicle.

SOLID WASTE



signments are unbalanced, the heavier workload should be scheduled early in the collection cycle. Zonesshould be shown on a map kept at the central office.

Establishing collection zones for commercial and industrial districts is complicated by variations in thefrequency of collection. The zones should be established based on compacted volume to be collected. A ruleof thumb is that one compacted volume unit equals four loose volume units of residential or commercialwaste, or 2.5 volume units of loose industrial waste. The following formula is useful in estimating compact-ed cubic meters per zone.

Sz = (S1 + S2 + · · · + Sd)/Dc

where Sz = compacted volume per zone S1 = compacted volume at one-day collection frequency S2 = compacted volume at two-day collection frequency Sd = compacted volume at collection frequency equal to the days per cycle Dc = working days per collection cycle

Daily Vehicle Assignment. Each zone must be divided into an optimum daily workload for each collectionvehicle and crew. Dividing the zone into sections reflecting the number of households to be serviced by eachvehicle is called districting. Each district should be compact, consisting of streets clustered in the same geo-graphical area. Districts should cover contiguous blocks and not be fragmented throughout the zone. Dis-tricting permits the manager to estimate the number and size of the trucks needed to collect waste, evaluatecrew performance, and balance or equalize workloads.

Major considerations in balancing and districting are the productivity of the crew and the on-route time.Increases in either will lower costs.

On-route time is productive time and should be maximized to allow crews to collect as many stops aspossible in a working day. In general, the major variable is the time spent in travel to and from the disposalsite. The round-trip haul time can be kept low by selecting the proper number of appropriately sized vehiclesfor each zone. As much as 20% of the working day can be lost in travel to and from the route, delays, breaks,and personnel needs. The following formula will assist in making an estimate of on-route time.

Tr = 8 – [Tt + n(Td + Th) + Tb + Tl]

where Tr = time on routeTt = time and travel between garage and route and return at day’s end Td = time at disposal site including check-in and check-out, queuing, and emptying timeTh = Round-trip haul time from route to disposal site gate and return Tb = break time—coffee and lunchTl = time lost for delays, including personnel needsn = number of trips to disposal site per day (must be a whole number)

The formula assumes an 8-hr work day; n can be estimated as follows:

n = 1.25Hr × Wh /Cv × Cd

where Hr = households per route Wh = average weight of waste collected per household, lb (kg) Cv = capacity of vehicle, yd3 (m3)Cd = rated compacted density on vehicle, lb/yd3 (kg/m3)

1.25 = a constant reflecting efficiency and allowance for seasonal variation from average

n must be rounded up to the nearest whole number.

8.58 CHAPTER EIGHT

SOLID WASTE



Service Stops. The number of service stops each vehicle makes per day may be estimated based on dataprovided in Table 8.21. Alternatively the number of loads, including fractional loads taken to the disposalsite, may be multiplied by the number of services per load (N).

N = a(Cv × Cd)/Wh

where N = the number of services per load Cv = capacity of vehicle, yd3 (m3)Cd = compacted density, lb/yd3 (kg/m3)a = a constant accounting for efficiency, normally less than 1

Wh = weight per household or stop, lb (kg)

The number of trucks required to collect a zone may be calculated by dividing the number of householdsper zone by the number of services all working vehicles can collect per day. Each zone should have the samenumber of routes to maximize labor utilization. Differences in route lengths can be made to allow for varia-tions in zone sizes.

District Routes. The route is the path the vehicle takes to make the collections within its district. The ob-jective of routing is to direct the collection vehicle through the district so that wasted time is kept to a mini-mum. Routing does not include the path from the garage to the district or from the district to the disposalpoint.

Maps should be prepared showing the number and type (residential, apartment, commercial, institutional,industrial) of services per street segment. One-way, dead-end, and busy streets, and corner-lot residencesshould be marked. Each street segment should show truck direction by arrow and whether crews are to col-lect one or both sides of the street on a pass. A daily route map or route book should be kept on each vehicle.

A route can be set by trial and error, by computer, or by heuristic methods. Trial and error can take con-siderable time to arrive at an effective route, with inefficiencies persisting.

Computer routing programs can precisely optimize the route. Preparing for the computer program entailsmodeling stop locations, street pattern, one-way streets, and other details of the district. Collecting the datacan take considerable time and it must be continually kept up to date. Computer routing has the advantagesof speed and precision of analysis and ease of checking the effect of potential route changes.

Heuristic Routing. Heuristic routing is a compromise between the trial-and-error approach and the com-puter approach. Heuristic routing is more precise than trial and error and requires less preparation time thancomputer routing. The heuristic routing method was developed by the EPA in the mid-1970s (25). Themethod uses routing guidelines to set up the collection route. The heuristic rules are found in Table 8.31.

Routing requires a map showing street segments with number of services, garage and disposal site loca-tions, heavily traveled streets, and one-way streets. A starting point is picked, and a continuous route is se-lected by applying the heuristic rules and patterns. The routing should be terminated when the number ofservices selected for that district is approached. Routes should be terminated at readily identifiable geo-graphical features or artificial boundaries. Retracing route segments should be minimized, and fragmenting,or skipping uncollected areas within the route boundaries should be avoided.

Particular routings are efficient for certain block patterns. Other patterns should be considered wheneverthe grid has blocks arranged differently than shown in the basic routing patterns in Figure 8.12. Block pat-terns may be more simply identified by omitting unserviced intersecting streets before establishing blockpatterns.

Upon completing the routing for the first district, a starting point for the next route is chosen and the pro-cedure is repeated until all the districts within the zone have been routed. Once the initial routes have beenset, each should be checked for alternate routings or modifications. Particular attention should be paid to

SOLID WASTE 8.59

SOLID WASTE



isolated segments not serviced, deadhead distances, the appropriateness of particular routing patterns, andunique characteristics of the area such as low bridges or weight limits.

Source Separation

Materials can be recovered from the waste stream for recycling through source separation, the setting asideof waste materials at the point of generation. The glass, paper, metal, and other separated materials are mostoften collected, sold, and recycled. Two types of source separation programs are in common use. One typeuses centralized recycling centers to which materials are brought by generators. The other type providescurbside collection of materials to be recycled. This discussion focuses on curbside collection of source-sep-arated materials.

Collection Practices. To be effective, collection of source-separated material must be regular. Participantsmust be informed how to prepare materials for collection and when collection will take place. Generally col-lection intervals greater than two weeks significantly reduce participation in source separation programs.

Curbside collection is usually accomplished by using separate vehicles for each item, a single vehiclewith several bins for each source-separated item, or by racks attached to the regular collection vehicle. Sep-arate vehicles can cover more territory than the regular collection vehicle. Racks attached to regular collec-tion vehicles usually fill before the packer body does. Strategically placed bins usually allow the racks to beemptied in about 15 min of off-route time.

Separation Volumes. The approximate amount of material that can be collected through source separationmay be calculated as follows:

8.60 CHAPTER EIGHT

TABLE 8.31 Heuristic Routing Rules

1. Routes should not be fragmented or overlapping. Each route should be compact, consisting of street segmentsclustered in the same geographical area.

2. Collection plus haul time should be reasonably constant for each route in the community. 3. Collection routes should begin as near the garage as possible. 4. Right-hand turns are to be preferred to left-hand turns. 5. Heavily traveled streets should not be collected during rush hours. 6. One-way streets are best collected by starting near the upper end of the street, working down through a looping

process.7. Dead-end streets are to be considered as a segment of the street they intersect, since they can only be collected

by passing down that street segment. They must be collected by walking down, backing down, or making a U-turn. Left turns may be kept to a minimum by collecting dead-end streets when they are to the right of thetruck.

8. Steep hills should be collected on both sides of the street while the vehicle is moving downhill for safety, load-ing ease, collection speed, vehicle wear, and fuel conservation.

9. Higher elevations should be at the start of the route. 10. For collection from one side of the street at a time, it is generally best to route with clockwise (right) turns

around blocks. 11. For collection from both sides of the street at the same time, it is generally better to route with long, straight

paths across the grid before looping clockwise. 12. For certain block configurations within the route, specific routing patterns should be applied. 13. Corner-lot residents should be asked to place their waste on specific streets to eliminate the need to traverse an

intersecting street.

SOLID WASTE



8.61

FIGURE 8.12 Basic heuristic routine patterns (25).

SOLID WASTE



A = PHW × H × I × P × W

where A = the amount of items to be collected, lb (kg)PHW = weight of total waste per household per week, lb (kg)

H = total households on the route I = percent of total waste stream represented by the items separated

P = percent participation of households on the route W = weeks between source-separated collections

Table 8.32 indicates the average density of various items usually collected in source separation projects.

Antiscavenging Ordinance. Due to the value of secondary materials, many cities have experienced diffi-culties with unauthorized persons picking up source-separated materials. An antiscavenging ordinanceshould be part of a source separation program. Antiscavenging ordinances should not preclude volunteergroups from collecting newspapers or scrap metal as one of their traditional revenue producers.

Rural Collections

Rural waste collection is of special concern. Many publicly sponsored rural systems have used bulk contain-ers to substitute for the open dump and to facilitate collection. Bulk containers are located along major trav-el routes or near the site of previously operated open dumps. The resident takes waste to the container anddeposits it for periodic collection by commercial-type collection vehicles.

Container sites should have ample room to allow a passenger vehicle or light truck to completely clearthe highway to unload. In general, the container should be at least 10 ft (3 m) off of the road surface. Roommust also be allowed for maneuvering the collection vehicle to pick up the container without interfering withtraffic.

To estimate the amount of container capacity, the following data should be used:

Waste volume per tributary population 0.1 yd3 (0.08 m3) per week Density in the container 75 to 150 lb/yd3 (125 to 250 kg/m3)Maximum road distance container to 5 to 6 mi (8 to 10 km)

contributor

Bulk container sites must be properly maintained. Spills or vandalism can make these sites unpleasant.Animals can upset small containers or forage in containers where lids have been left open. Large wild ani-mals frequenting the site can discourage use of the bulk container system.

8.62 CHAPTER EIGHT

TABLE 8.32 Densities of Source-Separated Items

Typical loose density

Item kg/m3 lb/yd3

Newsprint (bundled) 385 64.9Aluminum (crushed cans) 55 92.7Ferrous (crushed cans) 150 252.8Plastics 20 33.7Glass (crushed bottles) 870 1466.3

SOLID WASTE



Transfer Operations

When the collection vehicle is filled, or at the end of the working day, the truck should be driven to a dispos-al site and emptied. In some areas, disposal sites are quite far from the collection area, resulting in too muchoff-route time for the collection vehicle and crew. To increase productivity, many collection agencies haveestablished terminals where the route vehicles can empty their loads and return quickly to collecting refusewhile a larger vehicle transports the loads of several collection vehicles to the disposal site. These terminalsare called transfer stations. The purpose of a transfer station is to keep off-route time of collection vehiclesto an acceptable minimum.

The most popular method of transfer is over-the-road haul using trucks. Barges are used by some coastalcities. Rail haul has been proposed as a method of removing solid waste from densely populated communi-ties to rural areas for disposal in strip mines. Costs and public opposition have been largely responsible forthe limited U.S. experience in rail haul.

In general, transfer stations have a main two-story building where collection vehicles unload into tractortrailers on the lower level. The refuse is usually compacted in the tractor to ensure economical loading.When a trailer is full, it is hauled to the disposal site and replaced by an empty trailer.

Transfer Station Economics. A transfer station is justified when the cost of transport from route to thetransfer station, transfer, and haul to disposal using larger transfer vehicles is less than the cost of transportfrom route to disposal by smaller vehicles.

Past efforts at establishing transfer economics have often used rules of thumb, such as that a station isjustified if one-way haul exceeds a certain distance. These rules are not adequate, because the off-route timeof the collection vehicle is the controlling cost factor. Other factors affecting the economics of transfer in-clude site considerations, such as traffic patterns, construction conditions, and capacity.

The costs of owning and operating a transfer station vary depending on station use. Data from severaltransfer stations in Ohio and Michigan yield the cost comparisons in Table 8.33.

Transfer Station Location. The principal reason for using a transfer station is to increase crew and truckproductivity. A transfer station should be near the center of the collection area, convenient to good haulroutes, and zoned industrially or, if necessary, commercially. The site should not be located in residential ar-eas. A site can be at some distance from the center of production without excessive economic disadvantage.

Figure 8.13 illustrates a means of determining whether to use a transfer station or to have collection vehi-cles transport waste to disposal sites. The line of great est slope illustrates the cost of operating a collectionvehicle, with a three-person crew, between the route and the disposal site. The intercept distance on the ver-tical axis is the cost of unloading time for this collection vehicle.

As the transfer station is seldom located at the end of a collection route, each collection vehicle will trav-el some distance to the transfer station. Therefore, the origin of the transfer costs line must be offset to ac-count for the time of travel to the transfer station by the filled collection vehicle. As long as the combined

SOLID WASTE 8.63

TABLE 8.33 Transfer Station Costs

Compactor Noncompactorstations stations

Average transfer cost per ton (metric ton) $ 7.13 ($ 7.86) —Average haul cost per ton (metric ton) $ 7.14 ($ 7.87) —Average disposal cost per ton (metric ton) $ 8.35 ($ 9.20) —Total cost of system per ton (metric ton) $22.26 ($24.93) $12.53 ($13.81)

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costs of travel by the collection vehicle and transfer to disposal remains below the cost of direct haul by thecollection vehicle, the transfer station is economical.

Transfer Station Design. Transfer stations should be attractive, convenient, and safe, with adequate stor-age. Station site, structure, equipment, staffing, routing, and backup can be designed following general de-sign criteria. Local and state regulations should be reviewed before beginning design.

Site. The site should have enough space for buildings, storage, vehicle maneuvers, and expansion. Topog-raphy should encourage drainage and have sufficient elevation change to accommodate a two-level building.Foundation conditions should be able to support heavy industrial buildings and imposed equipment loads.Approximately 3 acres (1.25 ha) is the preferred size, although smaller sites are operating.

Access to the site should be from good, all-weather roads. Adjacent streets should be wide enough to al-low transfer trailers to enter and leave the site without interfering with smooth traffic flow. Traffic should bevisible from the gate house. On-site roads should be designed for all-weather operation and gates shouldcontrol access.

The ramp outside the building should slope slightly away from the structure. Ramps should extend atleast 100 ft (30 m) in front of the doors and 150 ft (45 m) or more if semi-trucks will use the facility.

8.64 CHAPTER EIGHT

FIGURE 8.13 Economic analysis for transfer station siting.

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A listing of preferred utilities, roadway characteristics, fencing, signage, and miscellaneous site featuresis presented in Table 8.34.

Structures. Structures mask the operation from neighbors, help control noise and blowing paper, and pro-tect the operation from weather. Two structures are commonly found at transfer stations: the gate house andthe transfer structure, or main building. In some stations, the gate house is incorporated into the main build-ing design.

Drivers should have easy access to the gate house. Wicket-type windows will serve in dry weather, butdrivers appreciate getting out of the rain or snow to sign usage slips. Doors opening onto the drivewayshould open inward to prevent them from being damaged by traffic. Canopies and overhangs should be atleast 16 ft (5 m) above the roadway. The gate house should have sufficient room for the scale head, a desk,and file.

The main building should be enclosed. Partially enclosed or open transfer operations should be discour-aged because it is difficult to control dust, noise, and debris at such stations. Open sites also may be attrac-tive nuisances for children.

The building should be functional yet attractive in design. Steel modular buildings with pedestrian doorsat each level are often seen. Windows should be shatterproof. The building should be high enough to ac-commodate a 16-ft (5-m) high door. If gravity discharge vehicles are expected, at least 24-ft (7-m) highdoors are required to assure clearance if a driver leaves with the bed up. Full-width doors are preferable to

SOLID WASTE 8.65

TABLE 8.34 Design Considerations for Solid Waste Transfer Station Sites

Design consideration Specification

Preferred utilitiesPotable water Flushing water 100 gpm (6.4 L/s) @ 60–80 lb/in2 (42,200–56,250), also fire protection.

Sanitary sewers or septic tank and leach field

Storm drainage Telephone Electricity 220 V, 60 Hz, 3 phase Natural gas Roadways and ramps

All-weather design Asphaltic concrete or Portland cement concrete One-way roadways 12 ft (3.5 m) minimum width Two-way roadways 20 ft (6 m) minimum width Thickness According to base material and vehicle loading Turning radius 50 ft (15 m) inside wheel Grades Preferred, 6%; normal maximum, 8%; absolute maximum, 10% Ramps Preferred minimum width beyond building-75 ft

(23 m), if semi-trailers accepted, 150 ft (45 m) Parking Employee and visitor, 5–6 spaces. Trailers—adequate for number owned. Scale platform 70 × 10 ft (21 × 3 m) Fencing 8 ft (2.5-m) chain link with 3-strand barbed wire, 20-ft (6-m) entry gate Signage Speed limit

DirectionalCaution/safetySite identification-keep to minimum

Miscellaneous Fuel storage: vehicle and equipment, 3–4 months Hydraulic oil: 1 reservoir refilling stored in 55-gal (208-L) drums (minimum)

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doors and columns. Roofs should be high enough to accommodate the open rear gate of ejector vehicles orthe lifted body for gravity dump vehicles.

The building should be supplied with water to flush floors, which should be sloped toward floor drains. Asmall climate-controlled operator enclosure housing the controls and providing an unobstructed view of thedumping area is needed. Table 8.35 lists the principal design features of the transfer station main building.

Transfer Trailer Tunnel. Transfer stations usually have a tunnel for transfer trailers. The trailer is pulledinto the tunnel and spotted at a loading station or coupled to a stationary compactor. Tunnel widths shouldbe at least 16 ft (5 m) with vertical clearances not less than 16 ft (5 m).

Tipping Area. The tipping floor is the area where the collection vehicle unloads.

Transfer Techniques. The collection vehicle may discharge waste onto the tipping floor, into pits or hop-pers, or directly into transfer trailers. When vehicles are emptied onto the tipping floor, a front loader isneeded to place the refuse into the transfer trailer or into the hopper of an external compactor.

Direct discharge of loads into trailers requires the trailer top or hopper to be no more than 1 ft (30 cm)above the tipping floor level. Hoppers may be used to guide refuse into the opening of self-compacting trail-ers.

When stationary compactors are used to load vehicles, packer loads may be emptied into hoppers con-necting to the compactor or into pits. Hoppers should accommodate at least one packer load of refuse.Where many vehicles are expected, the hopper or pit should be large enough to store several vehicle loads.

The tipping area should be wide enough to accommodate the number of vehicles expected in the peakhour of the average day. In general, this will be 2 to 2.5 times the average hourly number of vehicles expect-ed on the average day. The average number of unloading spaces is obtained by dividing the daily number of

8.66 CHAPTER EIGHT

TABLE 8.35 Design Features of Transfer Station Main Building

Feature Specification

Vehicle doors 16 ft (5 m) high, full width of approach lane, motorized Door guards (minimum) 6-in (15-cm) pipe, filled with concrete, 4-ft (1.2-m) bury Tipping floor 40–50 ft (12–15 m) deep to shelter crew during unloading; width to accommodate

the number of vehicles Operator enclosure 50–60 ft2 (5–6 m2)Transfer trailer stall(s) 16 ft (5 m) wide × 16 ft (5 m) vertical clearance; deep enough to accommodate

trailer and compactor, plus 6 ft (2 m) with concrete floor; provide stairs to tipping floor.

Lighting Adequate for maintenance area Heating 60°F (16°C) maximum except operator shelters to 70°F (21°C); infrared heaters

will work well on the tipping floor Climate control Air conditioning in operator shelter; minimum of six air changes per hour in

tipping floor and pit area Fire protection Ionization-type smoke detectors; extinguishers at each unloading station (type A,

B, C); fire hoses and/or spray system Safety equipment Safety harness at each unloading area if pit or hopper equipped; emergency ram

stop button at each unloading station (compactor station only); first aid equipment

Drainage Slope tipping floor to drain; pit drains to sanitary sewer; storm drain at transfer trailer tunnel or stall entrance; sanitary drain for leakage from compactor

Communications Telephone, intercomMaintenance shop Fully equipped for machine and building maintenance

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anticipated vehicles by the time needed to maneuver and empty the average vehicle. The average packertruck can clear the tipping floor in 5 to 7 min from entry, while manually unloaded vehicles may take from15 to 30 min. Each unloading lane should be at least 12 ft (3.5 m) wide.

Tipping floors should be constructed of concrete designed to carry the heavy trucks. Floors should befree of columns or other obstructions and designed for the arriving vehicle to back straight to the unloadingpoint. Wheel stops should be provided where the collection truck backs to an open drop. The minimum dis-tance from the door of enclosed stations to the edge of a push pit or hopper should not be less than 40 ft (12m).

Where vehicles discharge their loads directly onto the tipping floor, ample space is needed to store theloads expected under peak conditions. Loads from compactor-type bodies may expand up to twice theircompacted volume when emptied. Loads cannot be stacked except by front loaders or cranes, and then onlyin a limited way.

Transfer Station Equipment. Transfer stations require fixed equipment, stationary compactors, transfertrailers and tractors, and backup equipment.

Scales. Scales should have a device to signal the drivers when weighing is complete without requiringthem to dismount. If billing is to be done on weighed loads, consideration should be given to automaticrecording, card-operated scales with a capacity of 100,000 lb (45,350 kg). Scale platforms should be 70 ft(21 m) long.

Fixed Equipment. A variety of fixed equipment is required in a transfer station. Some stations receiverefuse into hoppers, hydraulic push pits, or bridge-crane unloaded pits; others require vehicles to dump ontoa tipping floor.

The hopper is the simplest loading device. Collection vehicles back up to the hopper and unload into itsthroat. The cycling ram of the compactor empties the hopper. Some stations have collection vehicles unloadon the floor, using a front loader to push material into the hopper.

Push pits are designed for direct unloading. The push pit is equipped with a screw or hydraulically oper-ated push plate that pushes refuse from the pit into the ram of the stationary compactor. Hydraulic pits havea maximum length of about 50 ft (15 m); stationary screw pits may exceed 100 ft (30 m) in length. The rateof feed should be controlled by the operator.

Where push pits are installed, the end wall at the compactor should be sloped at 10 to 20° toward the pitto direct material into the compactor. Controls for operating the push plate and ram should be in the opera-tor’s station, which should afford a clear view of the entire pit, including the ram of the compactor. Emer-gency shutoff switches should be located at each discharge station.

Pits are sometimes emptied by overhead bridge cranes, which deposit material into one or more hoppersattached to compactors. Bridge cranes should be of heavy capacity and have at least 0.5-yd3 (1.25-m3) buck-ets. Pits using cranes should be armored with steel rails set vertically in concrete.

Pits must have safety harnesses, use of which should be mandatory for persons conducting manual un-loading operations. The harnesses may be suspended from the ceiling by a ¾-in (2-cm) nylon rope permit-ting a fall of no more than 3 to 5 ft (1 to 1½ m).

Stationary Compactors. Transfer station rams are large stationary compactors. They are the heart of thecompaction-type transfer station. Typical ram characteristics are outlined in Table 8.36.

Stationary compactors and auxiliary hydraulic equipment are usually placed on a concrete base on thelower level. The access opening to connect the trailer to the compactor should have a door and be at least 12ft (3.5 m) wide by 16 ft (5 m) high. A sump and drain should collect liquid that runs from the trailer duringloading and direct it into the sanitary system. Ionization-type smoke detectors should be located in the load-ing area with an audible alarm in the dumping area and operator’s station.

Mobile Equipment. A front loader is an essential part of operations where collection vehicles are un-loaded onto a tipping floor. The front loader should be equipped with a protective shield over the radiator,

SOLID WASTE 8.67

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rock-type treads on foam-filled rubber tires, and rear weights. Foam fill reduces downtime due to tire failure. Rear weights supply traction. A backhoe, used in place of weights, is also useful in distributing loadsand compacting open trailers. Standard safety equipment is required, including roll bars, seat belts, andbackup warning.

Transfer trailers receive the waste from the tipping floor; when full, they are taken to the disposal site andemptied. Trailers may be open or enclosed. The following formula is a method of estimating the number oftrailers required for a smoothly functioning transfer operation. At least one spare trailer should be providedat each transfer station. A summary of transfer trailer characteristics appears in Table 8.37.

Nt = T × Vt × (L + M + tr)/WD

where Nt = number of trailers (round to the nearest whole number) T = weight in tons (U.S. or metric) received on maximum day (2.2 × avg. day)Vt = average volume of trailers, yd3 (m3)L = loading time, mm

M = maneuvering time (hooking up, spotting, and unhooking), mmTr = round-trip travel time to disposal site (includes time on site), mm W = working time in a day, mm D = compacted density, lb/yd3 (kg/m3)

Open Trailers. Open trailers are used at noncompacting transfer stations. The transfer trailer is a semi-trailer with rearward expanding body shape and an open top. The top is covered during travel with light-weight metal mesh or canvas lids mounted on light frames hinged to the trailer sides. The lids can be put inplace from the ground by one person.

8.68 CHAPTER EIGHT

TABLE 8.36 Stationary Transfer Compactor Characteristics

Item Range

Rated displacement 5.8–11.2 yd3 (4.4–8.5 m3)Rated capacity 525–975 yd3/h (400–745 m3/h)Operating pressure 1300–1700 lb/in2 (914,000–1,195,000 kg/m2)Total thrust 75,000–127,000 lb (34,000–57,600 kg)Total weight 19,900–37,000 lb (9025–16,870 kg)Oil tank capacity 240–375 gal (905–1,420 L)

TABLE 8.37 Characteristics of Transfer Trailers

Approximate Maximum Capacity,Volume, weight, lb height and tons

Type yd3 (m3) (kg) length, ft (m) (metric tons) Remarks

Open trailer 70–130 20,000 (9100) 14 and 50 15–20 Dumping(53–99) (4.3 and 15) (13.6–18.1) may be by

gravity, livebottom, orejection

Closed trailer 45–75 (34–57) 20,000–25,000 13 and 40 15–20 Dumping by(9100–11,300) (4 and 12) (13.6–18.1) ejection

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A variety of methods are available to load open trailers by gravity from the tipping floor. These includehydraulic systems and mechanical unloaders. Hydraulic unloading systems are more reliable than mechani-cal unloaders.

Hydraulic unloading requires equipping trailers with engines, reservoirs, and hydraulic pumps mountedexternally at the front of the trailer. The hydraulic system operates a ram and push plate, unloading a trailerin less than 3 mm. Another approach uses hydraulically actuated floor planks moving in sequence. Unload-ing a 50-ft (15-m) trailer, thus equipped, takes about 6 mm.

Open-top transfer trailers may also be mechanically unloaded using a chain and flight conveyor. Chainconveyors are high-maintenance equipment. Chains must be properly adjusted to keep them from slippingon sprockets or breaking. A chain conveyor can unload a trailer in less than 4 mm.

Open-top transfer trailers are manufactured in lengths ranging from 38 ft (11.5 m) to 50 ft (15 m), andload capacity ranges from 45 to 130 yd3 (34 to 99 m3). For average municipal waste, densities of up to 400lb/yd3 (237 kg/m3) can be obtained using external compaction. Densities of 200 to 300 lb/yd3 (120 to 178kg/m3) can be expected without compaction.

Enclosed Trailers. Two types of enclosed transfer trailers are currently on the market. One is loaded by anexternal compactor; the other has a self-contained compaction system.

Externally compacted transfer trailers rely on a stationary compactor to achieve maximum load density.Units are locked to the stationary compactor during the loading while the compactor pushes the chargethrough the trailer’s rear door. Care must be taken not to overload the trailer or rebound will prevent thedoors from closing securely. An internal hydraulic ram and push plate are used to unload the trailers.

Self-contained compaction trailers use an internal push plate to compact the load. Solid waste is loadedinto an opening in the top front of the trailer. The plate then pushes the material to the rear of the trailer,compacting it against the rear doors. The cycle is repeated until the trailer is filled. Care must be taken toprevent charging the trailer while the ram is extended so that refuse is trapped between the plate and thefront of the trailer.

Enclosed compaction transfer trailers may not be able to achieve optimum compaction and remain withinlegal highway load limits.

Trailer capacities vary from 45 to 75 yd3 (34 to 57 m3). Maximum compacted densities for municipal sol-id waste range from 550 lb/yd3 (326 kg/m3) for the self-contained units to 800 lb/yd3 (475 kg/m3) for theunits loaded by external compactors. Trailer lengths range from 35 to 50 ft (11.5 to 15 m). Trailer sizes mustbe chosen to avoid exceeding highway load limits (including frost laws in northern states).

Transfer Tractors. The number of tractors needed to haul transfer trailers depends on the amount of refusebeing delivered, the round-trip travel time to the disposal site, and the capacity of the trailers. Enough unitsare needed to permit the maximum day’s receipts to be delivered in an 8-h period, allowing for a productivetime of approximately 5.6 h. Fifteen minutes should be allowed for maneuvering and emptying on site and10 min to drop the empty trailer and pick up a full one. The formula below indicates the number of neces-sary tractors, including one standby unit.

R =

where R = round-trip haul time Hw = working hours per day per person adjusted for unproductive time.

tr = Over-the-road travel time, round trip. M = Maneuvering time (hooking up, on-site travel, spotting, unloading and unhooking)

NT =(Nt – 1)�

R

Hw�tr + M

SOLID WASTE 8.69

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where NT = number of tractors Nt = number of trailers R = round-trip haul time

Backup Equipment. Contingency plans are necessary in the event of major equipment failure. Mobileequipment, such as loaders and semi-tractors, can be rented on short notice. Stationary compactors and pushpits will require diversion of collection vehicles to an alternate location if sufficient storage is not avail ableon the site. At least one spare transfer trailer should always be available.

Transfer Station Staffing. Although one person can operate a transfer station, safety requires two personsto be on-site at all times. Additional staff will be needed where heavy volume requires multiple pits or hop-pers. Larger stations are staffed by two or three persons, while smaller rural stations may be unstaffed.

PROCESSING

Solid waste has been subjected to a number of material processing operations, similar to many other materi-als. However, due to the unique composition and character of solid waste, great care is required in the appli-cation of the various unit processes. In some cases, unit operations that are very effective on powdery mate-rials are completely ineffective on solid waste. This section describes the application of shredders, trommelscreens, magnetic separation, air classification, and baling in solid waste processing.

Shredding

Solid waste is typically subjected to shredding prior to its introduction to any separation operation. Shred-ding not only breaks the material into smaller size, making it more uniform, but shredding also imparts ablending action to the material stream. Solid waste management programs that have resource recovery as anultimate goal often install shredding systems and separation systems afterward only when sufficient fundsare available to expand the system. Even though shredded solid waste still is disposed at the landfill, a num-ber of advantages accrue. These are (1) volume reduction, (2) reduced vector problems, (3) reduced fire haz-ard, (4) reduced odor potential, and (5) reduced litter problems.

Types of Shredders. The purpose of shredding is to make the material more uniform and, hence, more pre-dictable. This is done by chopping up the items and mixing the fragments together. Shredders are essentiallya rotating shaft with shaped weights or devices fastened on the periphery of the shaft. The devices (general-ly called hammers) are for the purpose of impacting or shearing the materials, or both. These machines aremanufactured in two basic configurations: horizontal and vertical shaft machines. As the name implies, themain shaft is either horizontal or vertical. Some machines are made with very heavy and massive rotors, andothers are made for higher-speed operation with lighter rotors.

Flail Mills. Flail mills are lightweight, relatively high-speed machines. Their application is primarily fortearing open bags of refuse and breaking up bundles of material in addition to providing some mixing ac-tion. This type of machine is not applicable to the single-stage milling of refuse, but should be followed by asecond shredder that performs a more thorough size reduction. These machines have either rods or chainsfastened to the rotor shaft and “flail” the refuse as it is fed to the machine (see Figure 8.14).

Impactors. On the opposite extreme to flail mills, there are impactors. These are massive machines withsolid rotors. As the name implies, they impact the refuse as it enters the machine. The machine consists oftwo basic components: the rotor and one or two impactor target plates. As the material enters the machine,the massive rotor (sometimes with teeth) impacts the refuse, breaking some of the material into smaller par-

8.70 CHAPTER EIGHT

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ticles. The material is also “shot” toward the impactor plates where the refuse undergoes a second potentialsize reduction. These machines are not designed and built with grates or other devices for sizing the materi-al. They are intended to break up the tough bulky items in the refuse stream for either subsequent milling orfor feed to a refuse incinerator where oversize bulky items cannot be fed into the next process. The impactorperforms excellently on friable materials, although those machines equipped with ripping teeth providesome size reductions by shearing the refuse.

Hammer Mills. Two types of hammer mills have been used for shredding refuse: the fixed hammer andthe swing hammer type. The swing-hammer hammer mill has been most widely applied on refuse, but, un-der special conditions, the fixed-hammer machine may be more appropriate.

The fixed-hammer hammer mill is designed as a shearing-type machine. It is capable of reducing items,such as glass bottles, but generally it is a “light-duty” machine and is not applicable to mixed refuse. In situ-ations where refuse has been presorted with bulky and other items removed, this type of machine will out-perform most others and will provide the smallest particle size for a given amount of power input. The ma-chine is essentially a series of knives or cutters bolted on the periphery of a disk (see Figure 8.15). A series

SOLID WASTE 8.71

FIGURE 8.14 Flail mill.

FIGURE 8.15 Fixed-hammer hammer mill.

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of disks are stacked on a rotor shaft, forming a rotor assembly. The machines are also equipped with a gratebar or screen that restricts the flow of material until it has been reduced to some acceptable size.

The most commonly applied machine for the shredding of both mixed and sorted municipal refuse is theswing-hammer hammer mill. These machines can be obtained with either a vertical or a horizontal shaft; thelatter being the more common design. The machines are similar to the fixed-hammer unit except that thehammer is allowed to pivot. There are a number of minor and some major variations in the design of eachmanufacturer’s mill.

Design Considerations. Shredding designs tend to emphasize either capacity or capability; the two are notsynonymous. In simplistic terms, capacity refers to the mass per unit of time a machine will shred while pro-ducing a given product, i.e., a certain particle size distribution. This tends to ignore the consideration of ca-pability, which is the ability of the machine to shred a given item. Generally, designers have placed greateremphasis upon capacity for the reason that subsequent processes perform more efficiently, while equipmentsuppliers have emphasized capability, since this represents the machine’s ability to perform at any given mo-ment and, therefore, is an indirect indication of potential downtime.

Particle Size Distribution. Studies of particle size distributions of refuse from shredders have concludedthat the particle size distribution can be mathematically described by a Rosin–Rammler relationship as fol-lows:

Yx = 1 – exp [– (x/x0)n ]

where Yx = the cumulative decimal fraction passing a given screen size x0 = size at 63.2% passing n = the index of distribution

Characteristic Particle Size and Power Requirements. The 63.2% passing-screen size is considered thecharacteristic particle size. For a given characteristic particle size x0, the power consumption per ton ofrefuse can be projected. In addition, the grate bar spacing versus characteristic particle size can be predicted,and from the data, the proper motor size and grate bar spacing can be determined for a desired x0. Suppliers’data should be consulted.

Shredder Selection. Consider, now, the selection of a shredder in the situation where the plant will acceptall types of refuse and minimum inspection and sorting will be performed. A shredder can be viewed as anenergy-storage device accepting energy from the motor, storing it, and at some other point in time transmit-ting it to the refuse. Newtonian physics says that there is energy associated with a moving body

Kinetic energy (KE) =

where M = mass V = velocity

Mass is defined as

M =

where w = weight g = acceleration due to gravity

Kinetic energy is also defined as

w�g

MV 2

�2

8.72 CHAPTER EIGHT

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KE =

but for rotational energy

V = �r

where � = revolutions or radians per unit time r = the radius.

The effective radius is the radius of gyration, which is the distance from the center of rotation about whichthe entire mass can be visualized as acting.

KE = (NR)2

Usual engineering terminology is to describe � as radians per second and N as revolutions per minute, and Ras the radius of gyration. In this case, K is a factor for adjustment of the units and is constant for each system(i.e., U.S. Customary or SI). Regrouping, we have

KE = (�R N2)

In examining the above equation, it is found that the kinetic energy released when the rotor slows down isnot a linear function but an exponential one. Each halving of the speed produces four units of energy release.For a given speed change it can be seen that doubling the radius of gyration will also produce four units ofavailable energy. While there is an optimum operating speed for each machine, the manufacturer can onlycontrol or truly influence the �R2 of the machine.

For any given speed of operation, the larger the �R2, the more energy it can store and release for shred-ding a given object. In general, machines of different �R2 (sometimes called rotating moment of inertia) willnot undergo the same speed change. Any given article contained in the refuse stream requires a givenamount of energy to reduce it in size. That object will remain in the shredder until it has absorbed, at least,that minimum amount of energy and can pass through the grate bars.

A machine with a small �R2 will release all of its energy to a given item and have none left for the otherrefuse that was fed with the “tough” object. Two possible things will occur. Either the object must be fed in byitself and no additional refuse fed until the machine has cleared itself, or the machine will come to a stop. Thefirst option requires careful operator control and reduces the actual production rate of the system in additionto a change in degree of homogenization. The alternate requires opening up the shredder and clearing therefuse out of the machine. Both situations are undesirable because of lost production time and because ma-chines with inadequate �R2 are prone to damage because they are not heavy duty enough for the application.

Some general guidelines can be drawn and used as guides in the selection of shredders. No municipalrefuse shredder should have a �R2 of less than 25,000 lb/ft2 (1200 kN/m2) unless the refuse stream has beenhighly refined by hand sorting and the refuse originated from households only. Machines of relatively small�R2 can handle tougher items when fed only that item; however, wear and tear on the machine is not a linearfunction (see Figure 8.16). As the limits of the shredder’s capability are approached, the maintenance willtend to increase very rapidly.

The following items should be excluded from all swing hammer mills regardless of �R2:

1. Items that are likely to explode, such as propane or butane bottles and gasoline cans 2. Large, thick-walled metallic objects, such as gas cylinders and pressure vessels

K�2g

K��2g

wV 2

�2g

SOLID WASTE 8.73

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3. Heavy wire rope and heavy industrial chain in lengths greater than circumference of rotor 4. Large blocks of plastic or rubber 5. Heavy truck and earth-moving-vehicle tires 6. Long, dense rolls of wovens, such as industrial carpeting, tarpaulins longer than rotor circumference, and

long pieces of conveyor belting

Motor Size. Generally, the motor is sized by the manufacturer to match a specific shredder. This is a func-tion of speed of rotation and �R2. In practice, as the grate bar spacing is changed, the capacity of the shred-der changes to match the available power. This is done by the system designer who uses the shredder motoramperage demand to control the feed conveyor speed (or vibrating feed frequency). The higher the shreddercurrent demand, the lower the conveyor speed.

When the refuse is household-derived, a conventional motor sized for particle size control will generallyprove satisfactory. When the application becomes more severe, the systems designer will want to select ahigh “pull out” torque motor. Simplistically, torque is a function of slip, or the difference between rated andactual rotational speed. In North America, common high-torque motors are in the range of 250%, while inmany European countries, motors will range as high as 450%. The actual selection of motor specification isproperly left to the electrical engineer, but the systems engineer should be cognizant of the severe duty re-quired of shredder motors. In almost all cases, reduced voltage starting will be required. To determine theproper level of starting voltage, the electrical engineer will need to know the range of �R2 the shredder mayhave. The pull out level and slip determine how much kinetic energy can be reclaimed from the shredder andstill recover without shutting the system down.

Particle Size. Because maximum particle size rather than average or mean particle size has been the ma-jor concern of the systems designer, most shredders are rated on the basis of nominal particle size. This is anarbitrary sizing, but is the screen size at which 90% of the material will pass a given opening. There seemsto be some minor difference between vertical- and horizontal-shaft machines. Figure 8.17 shows typical par-ticle size distributions for conventional grate spacings for horizontal-shaft machines.

In addition to changes in the capacity of a shredder resulting from changes in grate bar spacing, the sys-tems engineer should note that when refuse density changes, the shredder capacity, measured on a weightbasis, will change significantly (see Figure 8.18). Figure 8.18 was developed principally for a horizontalshaft shredder being fed raw medium-character refuse and shredding to 1.5-in (3.8-cm) nominal particle

8.74 CHAPTER EIGHT

FIGURE 8.16 Shredder capability relative to maintenance costs.

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size. It should be noted that volumetric capacity is relatively insensitive to changes in density. In effect,shredders have an intrinsic capacity that is indicative of its working internal volume.

Shredder Rating. Table 8.38 presents a typical manufacturer’s rating of its own shredders. (Note: Thenumbers have been rounded off slightly to eliminate specific manufacturer identification).

Special Design Aspects. Shredder design should also consider the following.Lubrication. When selecting a shredder, the system engineer should be aware of the type of lubrication

system being offered by the vendor. A circulating oil system with reservoir and oil filter is the most desir-able, and manually lubricated grease fittings the least desirable, although this is a function of bearing sizeand rated life.

Hammer and Grate Wear. Hammers are rapidly wearing items, and grate bars also tend to require fre-quent replacement. Tough items tend to rotate with the rotor for some number of revolutions before beingreduced sufficiently to exit from the machine. During this period, the grate bars perform some significantshearing action on the refuse as it passes by the grate bars. When the hammers wear down to the point thatthe clearance between the grates and hammers becomes excessive, two performance characteristics willchange. An increase in particle size will occur as the result of the hammer trying to extrude the refuse outthe grate, and shredder capacity will decrease as a result of partial plugging.

SOLID WASTE 8.75

FIGURE 8.17 Particle size distribution for conventional shredder grate spacing.

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Airflow. Because the rotor direction of travel (one side of the rotor) and the path of the refuse are thesame, any restrictions in refuse flow will also affect the air current inside the shredder. This air current helpsto carry the refuse through the machine. When refuse is first shredded, it fluffs up. After some vibration (go-ing over idlers on the discharge belt conveyor, etc.), it may compact to a higher density than the original rawrefuse. If sufficient space does not exist between the grates and the top surface of the belt conveyor, it willrestrict the airflow. A minimum of 2 ft (0.6 m) should be kept clear between the bottom of the shredder andthe top of the receiving conveyor.

A 4-ft (1.24-m) clearance is more desirable, and distances less than this should be used only when thereare severe space constraints. A quick way to determine if a shredder is air-bound is to release smoke in thearea where the rotor shaft goes through the shredder side. There generally is a gap between the shaft and theshell. If the machine is not air-bound, it will draw the smoke into the shredder; otherwise, the smoke willblow away.

8.76 CHAPTER EIGHT

FIGURE 8.18 Shredder weight and volume capacity relationships.

TABLE 8.38 Shredder Ratings

Feed opening, in Capacity, tons/h Horsepower �R2, lb·ft2

77 × 90 65 1000 248,00060 × 90 55 800 180,00054 × 90 45 800 100,00054 × 80 35 600 55,00054 × 50 25 400 45,00035 × 50 15 250 12,800

Note: 1 in = 2.54 cm; 1 ton/h = 0.9 t/h; l hp = 0.75 kW; 1 lb·ft2 = 0.042kg·m2.

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Entrance Velocity. When the refuse is being charged into the shredder, it must have sufficient velocity.With adequate velocity, it will be able to penetrate the space between the rows of hammers. Otherwise, thematerial will sit on the tops of the hammers and “bounce” back. A rule of thumb is that the theoretical ve-locity of the refuse should be at least 10% of the hammer tip speed.

Feed Chute Design. In the design of feed chutes, there are some practical considerations that the designermust take into account. Sealing around a feed chute is important. If a vibrating feeder is used to feed theshredder, then heavy rubber can be used to seal the gap between the end of the pan and the feed chute on thebottom and sides. When a mechanical conveyor is feeding the shredder, a rubber seal should be designed be-tween the bottom of the feed chute opening and the drip pan under the conveyor. Sealing the feed chute tothe sides of the conveyor becomes difficult depending upon the conveyor drive arrangement. Shredders willreject material up the feed chute, and any openings will be found and the refuse will escape. Minor addition-al expense in sealing more than offsets the cleanup labor costs for a system in which little attention was de-voted to this item. Whether a horizontal- or vertical-shaft shredder is selected, this type of action occurs. Thetop on the feed hood in some cases is sloped downward toward the opening. Any material striking the topwill tend to be deflected away from the opening and contain the material within the hood. In addition, rubbersheeting should cover the opening and, when it is planned that the shredder will accept bulky material,chains should back up the rubber sheeting. The designer should also provide for minimum exposure of thefeed equipment to the material that flies back. The head of the mechanical conveyor should not project intothe feed chute. The sketches in Figure 8.19 illustrate some of the points discussed.

Trommel Screens

A separation method based upon particle size uses the trommel screen, sometimes called a rotary screen orrotary tube. At this point, the designer should consult the various equipment suppliers, since many trommel

SOLID WASTE 8.77

FIGURE 8.19 Shredder feed chute characteristics.

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screens are custom manufactured and are of a specialized design. However, there are a number of manufac-turers of “standard” trommel screens for which the following discussion is applicable.

Design Considerations. The material transport capacity of a rotating tubular device is described by:

VF = 5.19N(R)3 � �where VF = volumetric flow rate of material, ft3/min (m3/min)

5.19 = dimensionless coefficient describing fundamental physics of unit N = rotational speed, r/min H = maximum bed depth of material, ft (m) R = radius, ft (m) S = slope angle of tube � = angle of repose (dynamic)

In addition, there is a maximum rotational speed called the critical speed, above which the material trav-els full circle in contact with the drum. One can calculate the critical speed from

N0 = ��(H/R) is called the bed depth fraction and can be calculated from the following:

= 1 – cos � �where � is the bed angle from Figure 8.20.

In practice, if the unit does not perform as well as anticipated, extra lifting blades can be added to the in-side to slow down or speed up the flow of material. The higher the moisture content, the longer the detentiontime. Trommel length should not be less than the equivalent of about two diameters. It is also generally goodpractice to remain below 50% of the critical speed for the design conditions. A bed angle of l20° is fairlyreasonable but should not be exceeded. This value gives slightly under a loaded area of 20%; that is, the tubeis running 20% full at the inlet side.

Trommel Selection. Table 8.39 shows standard sizes and rates for two different manufacturers. For esti-mating electrical needs, a value of 1 hp per ton per hour of capacity (0.95 kw per 0.9 metric ton per hour)can be used by selecting the next-larger-size motor available.

Magnetic Ferrous Separation

Magnetic separation of ferrous scrap is a relatively easy and inexpensive operation to accomplish. Scrap re-claimed from municipal refuse generally is classified as No. 3 Dealer Bundle. When scrap is subsequentlyagitated and air-classified, it can in some cases be cleaned of foreign material sufficiently to be reclassified.The ability to clean ferrous scrap reclaimed from municipal refuse is a function of the amount of “balling” itreceived in the shredding process. Shredders that impart higher shear and lower impact to the refuse willtend to produce a cleaner scrap. The cleanliness of the product seems a matter of hammer style and shreddersize, in terms of �R2. Ring hammers, as opposed to hourglass or boise-style hammers, will tend to roll or

��2

H�R

g0�R

1�2�

tan S�sin �

H�R

8.78 CHAPTER EIGHT

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grind the refuse as opposed to chopping it up. On the other hand, a shredder with a larger �R2 and heavyhammers, typically over 60 lb (26 kg) each, will tend to shear the material as a result of its ability to ripthrough the refuse with a minimum amount of hammer “lay-back,” i.e., hammer rotation about the hammerpin on impact. Hammer weight and �R2 are a function of shredder capability and size and not a function ofits style.

Types of Magnetic Separators. Magnetic separation is a process that was originally used for the enrich-ment of iron ore slurries. They are drum-style (see Figure 8.21) with permanent magnets mounted on the in-side in which the drum rotated while the magnet, covering only a segment, remained fixed.

SOLID WASTE 8.79

FIGURE 8.20 Trommel screen geometry.

TABLE 8.39 Standard Trommel Sizes

Diameter, ft Length, ftr/min

HorsepowerCapacity,

__________________ __________________ __________________

ton/h Mfg. A Mfg. B Mfg. A Mfg. B (Mfg. B) Mfg. A Mfg. B

up to 15 7 4 15 20 22 15 7.526 — 5.33 — 25 16 — 2037.5 — 6.66 — 30 13 — 3045 8 — 31 — — 50 —60 — 10.66 — 30 0 — 7580 10.5 — 47 — — 80 —

100 12 — 62 — — 100 —

Note: 1 ton/h = 0.9 t/h; 1 ft = 0.3 m; 1 in = 2.54 cm; 1 hp = 0.75 kW.

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In response to the need to produce a cleaner product, the various separator manufacturers developed aspecial line of refuse magnetic separators. These are suspended belt or “horseshoe” style and subject theferrous product to a spinning or flipping action that tends to produce a cleaner product. The automobileshredding industry, faced with the same problems, has generally adopted a two-stage drum magnet system(see Figure 8.22) with a crude air-cleaning system between the two separation stages. This configurationproduces a cleaner product but consumes more power. In situations where budget allows or the materialspecifications absolutely require the cleaner product, municipalities will adopt the system used by the scrapindustry.

The most widely applied separators to municipal refuse is the suspended belt or “horseshoe” style, which

8.80 CHAPTER EIGHT

FIGURE 8.21 Single-stage drum magnetic separator.

FIGURE 8.22 Two-stage drum magnetic separator.

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is a series of reverse-pole magnets with a belt traveling across them (see Figure 8.23). The first magnet is thepickup magnet, and the subsequent magnets are reverse-poled sequentially. As the tin can, etc., is magne-tized, the opposite pole attracts the end away from the belt, causing the item to flip or spin as it progresses. Insome cases, a gap is included between the pickup and the next magnet to allow the item to temporarily dropaway from the belt releasing any trapped material picked up with the metallic scrap. Steel pulleys and idlerscan become magnetized and cause the ferrous scrap to remain on the belt as well as building up high staticelectrical voltages. An electrical engineer should review the possible need for grounding in this area of theconveyor. Consultation should also take place with the various separator manufacturers.

Design Considerations. Magnetism, like light, varies in intensity with the square root of the distance. Thismeans that the performance will fall off drastically as the gap is increased small amounts. Generally, mag-netic separators are designed for a 6- to 8-in (15- to 20-cm) gap between the conveyor belt surface and thesurface of the separator pickup section. The surface speed of the separator is preferred to be about 50 ft/min(15 m/min) faster than the speed of the belt conveyor from which it is removing the ferrous material. Thepractice is to remove the ferrous scrap faster than it is being fed.

Recovery Efficiency. When considering recovery efficiency, it is recommended that the systems designeruse a value of 95%. Recovery efficiency will depend upon flux density and thus gap distance. Just about allseparators will operate at a far superior level (97 to 98%) most of the time; the recommended value takesinto consideration periods when the refuse bed on the conveyor is particularly thick or some other variable isaffecting performance. Table 8.40 can be used to select a magnetic separator.

Power Requirements. These units will vary in power consumption for the pickup magnet; the other mag-nets are almost always permanent magnets, and the belt drive will vary from 5 up to 10 hp (6.7 up to 13.4kW) depending upon the type of belt used.

Serving Conveyors. Because of the impact force, points, and edges contained by the scrap, some of theunits are equipped with aluminum armor plating in the center section of the belt. This reduces belt wear andincreases belt life.

The chuting and covers around the discharge from a magnetic separator require careful consideration.Because of the high speed, the separated metallics generally have a high velocity when released from thebelt or drum. A deflector plate with a replaceable liner should be designed for the reclaimed ferrous materi-

SOLID WASTE 8.81

FIGURE 8.23 Suspended belt/horseshoe magnetic separator.

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al to strike. The plate can be fabricated of steel when a distance of 2.5 ft (0.75 m) is maintained; otherwise,the metal will tend to interrupt the magnetic field pattern. The sides can be fabricated of rubber sheeting, butcare should be taken that the sheeting does not get caught in the moving parts of the machinery. The headpulley and idlers in the area of the magnet should also be heavily rubber-covered or made of rubber.

Air Classification

Air classification has been, and will probably continue to be, the major unit process for the beneficiation ofmunicipal solid waste second only to magnetic separation. In simplistic terms, a rising current of air effectsthe material separation because of the differential density of the refuse components. A key element in theseparation is the proper preparation of the feed material. Air classifiers operate because viscous drag onsome of the particles is able to overcome the force of gravity. If relative density is to be the criterion for sep-aration, then the viscous drag on each particle must be uniform regardless of the material. The implication isthat the more uniform the particle size, the more definable and predictable will be the separation. The fur-ther implication from fluid mechanics is that the larger the particle size, the higher the minimum upflow airvelocity for fluidization of the material.

Design Considerations. As illustrated in Figure 8.24, an air classification system consists of (I) an airtightfeeder, (2) a separation chamber with a top and bottom exit, (3) a receiving–settling chamber, (4) a primemover (vacuum fan), and (5) a heavies takeaway conveyor. Generally, the entire system is provided by a sin-gle manufacturer or supplier, except for the feed conveyor and two takeaway conveyors.

Air Classifier Performance. In considering the performance of an air classifier, the design engineer needsto be concerned with the recovery efficiency E1 and the rejection efficiency E2. The purpose of an air classi-fier is to separate the combustible from the noncombustible fraction. An excessive air velocity will minimize the rejection efficiency and recovery all of the burnable portion. However, the high degree of nonburn-able fraction in the recovered portion may not be tolerable in the burning process. At the same time, insuffi-cient air velocity will produce a clean recovered product, but the quantity may be so small as to make theseparation process uneconomic.

E1 =

and

combustibles recovered��

combustibles in feed

8.82 CHAPTER EIGHT

TABLE 8.40 Magnetic Separator Selection Data

Refuse feed Separator Conveyor Separatorrate, tons/h width, in width, in weight, lb

Up to 15 30 30 9,00030 36 36 9,50050 48 48 14,00070 60 60 17,500

100 66 72 22,000

Note: 1 ton/h = 0.9 t/h; 1 in = 2.54 cm; 1 lb = 0.45 kg.

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E2 =

Since the two efficiencies are not independent of one another, Figure 8.25 shows efficiencies of an airclassifier as a function of the throat velocity. The graph is typical of a common air classifier with a feed ofmunicipal refuse prior to magnetic separation at 30% moisture content (E1 and E2 on moisture-free basis).

Baling

Baling is a relatively simple process that does not alter the physical or chemical nature of the solid waste.Rather it is a compression process for the significant reduction of volume occupied by the solid waste. In ad-dition, baling produces a predictable product, the bale, that is both much easier to handle and produces muchsmaller voids at the disposal site. Baled solid waste is less prone to methane generation; generally will notsupport combustion; and produces a leachate of a less concentrated character.

Baler Operation. The solid waste is fed to a baler cavity; at this point the solid waste is then compressed intwo of the three major axes to a fixed dimension. A ram then compresses the solid waste along the third axisuntil the ram pressure reaches some predetermined level. The bale produced will have two of its three di-mensions fixed and the third dimension somewhat variable depending upon the amount and nature of thematerial originally charged to the baler. Depending upon the preset cutoff pressure for the third ram, the balemay require tying or binding up. Intermediate- and low-pressure baling will require banding, while high-pressure baling generally does not. The trade-off is between the cost of banding or strapping material and thecost of the additional energy to form a high-density bale. High-density balers are generally only available inthe higher-capacity units, i.e., 50 tons/h (45 t/h) or greater capacity.

noncombustibles rejected��noncombustibles in feed

SOLID WASTE 8.83

FIGURE 8.24 Air classification system.

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A number of factors affect the final density of the bale. These factors include not only the baler operationitself but also the amount and character of the refuse as well as time since the bale formed. Baler operationalcharacteristics include pressure of application and time of pressure application. Solid waste parameters af-fecting the final bale include moisture content, mechanical properties (elasticity, etc.), and size and numberof the individual items fed to the baler. Finally, all bales experience a “spring-back” effect, creating a vari-able time lag before bales reach their final density.

Baler Selection. The final density will be a function of the ram pressure. Below approximately 1000 lb/in2

(6900 kN/m2), unstable bales will be produced regardless of the other parameters. Intermediate pressures upto approximately 3000 lb/in2 (20,700 kN/m2) or slightly higher will produce stable bales if they are tied orbanded. Above about 3500 psi (24,150 kN/m2), a stable bale can be produced without the requirement fortying. Figure 8.26 shows the relationship between baler ram pressure and volume reduction.

Figure 8.27 indicates the typical range for final bale weight as a function of connected power. The shapeof the curve would be as expected from the pressure–volume change curve. The higher-power balers do notrequire wire tying even though the final bale weight is not significantly greater.

Bale Stability. Bale stability will increase as moisture content is increased. However, once a moisture lev-el beyond 30% (on a dry weight basis) is reached, bale stability again starts to decline.

Specific refuse character will also affect bale stability. For example, it is not desirable for any bale to con-tain more than one tire. Large amounts of lawn wastes, such as grass or leaves, will reduce bale stabilitywhen any bale contains more than 50% of these kinds of materials. On the other end of the spectrum, balers

8.84 CHAPTER EIGHT

FIGURE 8.25 Air classifier efficiencies.

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SOLID WASTE 8.85

FIGURE 8.26 Baler ram pressure and volume reduction.

FIGURE 8.27 Bale weight and connected power relationship.

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are very effective on “white” goods, such as stoves and refrigerators, and other ductile types of materials(sheet metal, etc.).

All bales will experience a spring-back effect or expansion of the bale upon being released from thebaler. Most of the expansion will occur rather immediately, but final volume may take a week or more toreach. Figure 8.28 shows typical volume change as a function of time after being released from the baler.The engineer should contact the various manufacturers for specific lines of solid waste balers.

Balefill. Disposal of solid waste bales (in a balefill) is an easier material-handling task to the extent thatmany operators of balefills claim that baling is more economical. Transportation of bales to the final dispos-al site as well as handling at the disposal site requires not only less sophisticated (and therefore less expen-sive) equipment but also requires less equipment in total. Balefills are generally operated by stacking balesend to end and three bales high. A balefill still requires daily soil cover, but due to the geometry of the bale,not only is volume saved but the soil cover volume is also less. Depending upon the original nature of thesolid waste, a balefill may occupy as little as 10% of what the same material would occupy if the refuse weredisposed in a conventional landfill.

RECOVERY AND REUSE

Recovery and reuse are essential elements of a integrated solid waste management plan. Energy recoveryand production offer a direct economic benefit and lessen demands on processing and disposal require-

8.86 CHAPTER EIGHT

FIGURE 8.28 Bale “spring-back” characteristics.

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ments. Similarly, the cost-effective recovery of solid waste components for reuse in industrial productionand source reduction techniques are improving overall control of solid wastes.

Pollution prevention or source reduction, including product reuse, is the first priority in an integrated sol-id waste management program. Source reduction includes the design, manufacture, purchase, or use of ma-terials, such as products and packaging, to reduce either the amount or toxicity of materials before they enterthe solid waste management system. Examples of source reduction activities include:

� Modifying residential, commercial, and industrial practices and functions to manage abusive waste ofmaterials

� Managing nonproduct organic wastes, such as food scraps and yard trimmings, through on-site compost-ing or other alternatives to disposal, such as leaving grass clippings on the lawn

� Designing products or packaging to reduce the quantity of materials or to make them easier to reuse� Improving packaging to reduce the amount of damage or spoilage to products� Reusing existing products or packaging� Extending the useful life of products to postpone disposal

Energy from Solid Waste

As early as the turn of the twentieth century, solid wastes were used to produce steam and electricity. Thetypes, quantities, and distribution of solid wastes with fuel potential, particularly urban solid wastes, havereceived increased attention over the past decade due to rapidly escalating energy costs. Companies or gov-ernments investigating converting solid waste into usable energy must evaluate the waste’s heating value andcompeting recovery technologies in estimating the energy recoverable from a waste stream.

Heating Values. The heating value (or energy content) of most solid wastes is roughly one-third to one-half the heating value of coal. However, there are many ways heating values are reported, so caution is nec-essary when comparing and evaluating such data.

The quantity of heat generated by complete combustion of a fuel is known as the heating value, heat ofcombustion, or caloric value. The heating value of a fuel may be determined directly by measurement of theheat evolved during combustion of a known quantity of the fuel in a calorimeter, or it may be estimated fromthe chemical or physical analysis of the fuel and the heating value of the several chemical elements or phys-ical components (28).

The higher heating value (HHV), or gross heating value, of a fuel is determined when the water vapor inthe products of combustion of a fuel is condensed, and the latent heat of vaporization of the water is includ-ed in the heating value of the fuel. Conversely, the lower heating value (LHV), or net heating value, is ob-tained when the latent heat of vaporization of the water vapor is not included in the heating value of the fuel(28).

European practice is to use the LHV, while in the United States the heating value of a fuel specified isgenerally the HHV. For most solid wastes, the HHV will range from 7 to 15% more than the LHV due to hy-drogen content variations, or (29).

Rough HHV = 1.11 LHV

Heating values are usually expressed in units of Btu per cubic foot (kilocalories per cubic meter) forgaseous fuels, Btu per gallon (kilocalories per liter) for liquid fuels, and Btu per pound (kilocalories perkilogram) for solid fuels. The heating value of solid fuels may be reported four ways: as-received, dry, ash-free, or dry and ash-free (28). Therefore, it is vital that both the presenter and user of data properly qualifyand understand the units applied to a given set of numbers.

SOLID WASTE 8.87

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Solid Waste Fuels. The heating value of nearly all solid waste fuels is a function of carbon content. Ashcontent is generally low, but the amount of moisture is highly variable and depends upon moisture genera-tion plus the effects of processing, handling, and storage. On a dry, ash-free basis the heating value can beestimated at 8000 Btu/lb (4400 kcal/kg); more resinous materials contain about 9000 Btu/lb (5000 kcal/kg).Table 8.41 lists the characteristics of some typical solid waste fuels.

Municipal solid waste and most other solid waste fuels can be burned without auxiliary fuel in their raw,as-received state, over a rather wide range of compositions. In fact, solid waste fuels containing as much as75% moisture and ash can be burned without auxiliary fuels (31). However, since water and noncom-bustible materials do not contribute to the heating value of the waste, handling and storing wastes to mini-mize their moisture content and processing wastes to reduce their ash content can greatly improve fuelquality.

Municipal Solid Wastes. The heating value of municipal solid waste (MSW) is dependent upon its com-position. Typical heating values of MSW as received at a resource recovery facility or disposal site rangefrom approximately 3000 to 6000 Btu/lb (1700 to 3300 kcal/kg). This variation can be attributed to seasonalfactors that influence the moisture and noncombustible content of the waste. However, the average value istypically 4500 to 5000 Btu/lb (2500 to 2800 kcal/kg) as received. Heating values are discussed further in thesection Refuse-Derived Fuels.

The heating value of a waste stream can be estimated if the waste stream composition is known. Variouswaste stream components have the following heating values (32):

Paper 7,750 Btu/lb 4,300 kcal/kgPlastic 18,000 Btu/lb 10,000 kcal/kgWood 8,000 Btu/lb 4,400 kcal/kgOther organics 2,000 Btu/lb 1,100 kcal/kg

Therefore, an example of how the heating value of a waste stream is approximated is indicated below (32):

8.88 CHAPTER EIGHT

TABLE 8.41 Characteristics of Solid Waste Fuels (30)

Higher heatingvalue, Btu/lb dry, Moisture, % as

Solid Waste Fuel ash-free received Ash, % dry

Black liquor (sulfate) 6,500 35 40–45 Cattle manure 7,400 50–75 17 Coffee grounds 10,000 65 1.5 Corncobs 9,300 10 1.5 Cottonseed cake 9,500 10 8 Municipal solid waste 9,000 20–50 20–40 Pine bark 9,500 40-50 5–10 Rice straw or hulls 6,000 7 15 Scrap tires 16,400 0.5 6 Wheat straw 8,500 10 4

Note: 1.0 kcal/kg = 1.8 Btu/lb.

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Heating value

Typical waste stream Btu/lb kcal/kg

Paper, 35% 7750 × 0.35 = 2712 4300 × 0.35 = 1505 Plastic, 10% 18,000 × 0.10 = 1800 10,000 × 0.10 = 1000 Other organics, 10% 2000 × 0.10 = 200 1100 × 0.10 = 110 ____ ____Total 4712 2615

This estimating procedure for energy potential is very useful in evaluating resource recovery alternatives.Also, it can be used to approximate the impact of source separation and other recycling activities, or otherchanges in waste composition on the energy content of the waste stream.

Energy Recovery Technologies. The major technologies used to recover energy from MSW include

� Waterwall incineration: Combustion of unprocessed MSW (mass burning) or processed MSW in a fur-nace with integral boiler tubes.

� Modular incineration: Combustion of MSW in relatively small two-stage, starved-air furnaces with heatrecovery boilers or heat exchangers.

� Refuse-derived fuels: A variety of technologies that produce solid fuel by processing MSW into com-bustible and noncombustible fractions. The resulting fuel can be cofired with fossil fuels or burned alonein a “dedicated boiler.”

� Pyrolysis: A variety of technologies that process MSW in an oxygen-deficient environment to producegaseous, liquid, and/or solid fuels.

� Anaerobic digestion: A developmental technology adapted from anaerobic digestion of wastewatersludges.

� Landfill gas recovery: Collection of gas generated during decomposition of landfilled MSW.

Energy Recovery Efficiencies. Currently there is no standard, accepted way to evaluate the energy recov-ery efficiency of resource recovery systems. There are several major contributing causes for differing effi-ciencies, including

1. Alternative ways of treating energy used by the process itself 2. The choice of system boundaries for which the calculation is made 3. The use of higher (HHV) or lower heating value (LHV) of the waste 4. Including or excluding the energy content of nonfuel materials

Under current conditions, it is possible to produce energy recovery efficiency figures to either enhance ordetract from the apparent attractiveness of a particular system (33).

Table 8.42 shows system energy efficiencies in terms of the energy content of the fuel produced, and interms of the output energy available as steam and electricity. The energy efficiencies in this table are “netsystem outputs,” and, are based on the HHV of the product minus the energy used to operate the recoverysystem divided by the HHV of the input waste. The energy available as steam and electricity was calculatedusing boiler and turbine efficiencies appropriate for each recovery system (33).

While comparison of energy recovery technologies on the basis of available steam and/or electricitymakes thermodynamic sense in terms of standard system boundaries, it ignores important characteristics ofthe various waste-derived fuels such as the quality of the fuel product and its transportability, which directlyaffect fuel economics (33).

Estimating Recoverable Energy. The quantity of energy that can be recovered from MSW depends onfour factors:

SOLID WASTE 8.89

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1. Waste input quantityThermal output

2. Waste energy content � 3. Boiler efficiency

Thermal utilization 4. System availability �

The thermal output of a recovery system is the product of the waste input quantity, waste energy content,and boiler efficiency. The quantity of waste generated or collected must be adjusted to reflect the recoveryfacility’s operating schedule (If x tons are collected in 5 days and the recovery facility operates 7 days, thedaily waste tonnages used for estimating energy recovery is 5x/7). Generally, the energy content of MSW isapproximated at 4500 Btu/lb (2500 kcal/kg) for unprocessed MSW and up to 8000 Btu/lb (4400 kcal/kg) forRDF.

Boiler efficiency is greatly influenced by the moisture content of the fuel. Typical boiler efficiencies are60 to 70% for mass burning and 70 to 75% for RDF combustion. A more detailed discussion of waste ener-gy content is provided in the next section, Refuse-Derived Fuels. The nomograph in Figure 8.29 provides amethod to estimate the thermal output of a combustion system. For example,

Unprocessed MSW: If waste input is 500 tons/day, energy content 4500 Btu/lb, and at 60% boiler effi-ciency, then the thermal output is 1.1 × 108 Btu/h for 24 h/day.

RDF: If waste input is 2000 tons/day, and the fuel yield is 1400 tons/day, energy content is 6000 Btu/lb,and at 75% boiler efficiency, then the thermal output is 5.2 × 108 Btu/h for 24 h/day.

The system availability is simply the percentage of the time the system is functional. System availabilitydepends on the recovery system chosen and the steam quality produced (see Figure 8.30). The availability ofRDF firing units is typically higher than mass burning facilities. However, for RDF systems with minimalstorage capacity, the system availability must take into account the reliability of the processing equipmentand the firing units.

Thermal utilization is the product of boiler efficiency and system availability and represents the fractionof solid waste heat content converted to steam on an annual average basis (36). Estimation of potential rev-enues from energy recovery projects should be based on thermal utilization, not thermal output.

Figure 8.31 relates the thermal output of the incinerator to quantities of steam and electric energy pro-duced. In general, steam of any pressure and temperature can be produced up to practical limits of about1200 lb/in2 and 900°F (8.3 MPa and 480°C). Above these conditions, it has been determined that corrosionmechanisms accelerate due to increased temperatures within the combustion chamber, leading to excessivedeterioration (40).

The selection of steam pressure and temperature is largely based on market demand. For example, satu-rated steam of 250 lb/in2 (1.7 MPa) might be produced to supply a chemical process plant, food processingplant, or district heating system; if electric generation were desired, steam might be produced at 850 lb/in2

and 750°F (5.9 MPa and 400°C), resulting in a higher energy content of each pound of steam, and therebyreducing the size of electric generating equipment required (40).

For example, to determine potential steam generation, enter Figure 8.31 at 5.2 × 108 Btu/h incineratoroutput of Figure 8.29. Move vertically upward to steam conditions of either 250 lb/in2 saturated, or 850lb/in2 at 750°F, for steam production of 443,000 lb/h and 498,000 lb/h, respectively (40).

To determine the potential electricity generated by the steam, move horizontally to the right from498,000 lb/h to the turbine exhaust conditions of 6 in Hg absolute and 2 in Hg absolute. These conditionscorrespond with reasonable average turbine exhaust back pressures for air-cooled condensers and water-cooled surface condensers, respectively. Now, read vertically to determine potential power generation of49,000 kW for the water-cooled condensers (40). Using air cooled condensers would reduce the power gen-eration approximately 10%.

The preceding discussion provides methods for estimating the heating value and the energy recoverablefrom municipal solid wastes. These topics are expanded upon in the next section.

8.90 CHAPTER EIGHT

SOLID WASTE



8.91

TAB

LE 8

.42

MS

W E

nerg

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ecov

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Sys

tem

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Com

pari

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and

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to f

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MS

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to e

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rici

ty__

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Ref

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Ref

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Ref

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Ref

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SOLID WASTE



8.92

FIG

UR

E 8

.29

Was

te in

put a

nd th

erm

al o

utpu

t (40

).

SOLID WASTE



The planning and implementation phases of a successful energy recovery project must address many top-ics beyond the scope of this discussion. System economics and community acceptance are typically the con-cerns that most frequently determine if a technically viable project is implemented.

Refuse-Derived Fuel

Utilizing MSW as fuels accomplishes two purposes-significant volume reduction and energy production.The basic technology for producing energy from MSW, incineration and steam turbine electrical generation,was developed during the early twentieth century. Over the past 15 years, new technologies have been devel-oped to recover materials from MSW while also producing an improved fuel product, generically describedas refuse-derived fuel (RDF). Some of these new technologies produce fuels compatible with existing powergeneration facilities or other energy users.

Municipal solid waste can be processed to concentrate the combustible components into an RDF by re-moving glass, ceramics, and metals. The resulting RDF will typically have an HHV of 4500 to 8000 Btu/lb(2500 to 4500 kcal/kg) (wet). RDF typically burns more efficiently than unprocessed MSW due to lower ashcontent, lower moisture content, and a higher degree of homogeneity. Because of these qualities, RDF re-quires less excess combustion air than mass burning processes. This in turn reduces the required size of allair pollution controls and air handling equipment (40, 41).

The removal of inert materials has the added advantage of producing more easily handled fuel, since theglass, stone, and metals can contribute to the deterioration of materials handling equipment. However, RDFproduction typically involves sophisticated integrations of many materials handling operations. Also, de-tailed market and waste stream evaluations are required in order to determine that technologies are appropri-ate in each specific situation (40).

SOLID WASTE 8.93

FIGURE 8.30 Thermal utilization and availability of waterwall incinerators (37).

SOLID WASTE



8.94

FIG

UR

E 8

.31

The

rmal

out

put,

stea

m p

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ctio

n, a

nd e

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rica

l pow

er g

ener

atio

n (4

0).

SOLID WASTE



The combustible components can be further processed to produce gaseous or liquid fuels via pyrolysis oranaerobic digestion.

Fuel Product Classification. The need for uniform definitions, specifications, and analytical proceduresfor producers and users of recovered commodities from refuse prompted the formation of ASTM CommitteeE-38 on Resource Recovery (44). This Committee has developed standard classifications for refuse-derivedfuels (Table 8.43). ASTM has also developed a standard test method for gross calorific value (E 711-81) anda standard method for RDF data conversion (E 791-81).

RDF production processes and final fuel products have also been described by the terms in Table 8.44.

Heating Value. Typical heating values, ash contents, and moisture contents of prepared fuels are summa-rized in Table 8.45. These typical values illustrate that processing MSW can significantly lower ash contentand moisture content as well as concentrating the waste’s combustible components. However, processingcan also significantly reduce the energy obtained from each ton of MSW due to controllable, but inevi-table, losses of combustible materials. Typically, RDF contains 85 to 99% of the paper and plastics inthe raw waste, while the majority of glass and metals are removed. These losses are reflected in processyields (weight percent) and energy yields, expressed as fuel Btus per pound (kilocalories per kilogram) ofMSW.

Typical process yields and energy yields of fuel products are also summarized in Table 8.45. The data inTable 8.43 represent general ranges expected for prepared fuels. For example, the heating value of RDF-1can range from 3000 to 6000 Btu/lb (1650 to 3300 kcal/kg) due to waste composition and moisture and ashvariations, but a good average range that can be expected currently is 4500 to 5000 Btu/lb (2500 to 2750kcal/kg) (46). The ranges presented are also affected by data availability, which varies with fuel product; thedata bases for RDF-1 and RDF-3 are large, while data for the other products are somewhat scarce.

The heating value of prepared solid fuels generally increases from RDF-l through RDF-4. The heatingvalue of RDF-5 is comparable to or slightly higher than that of RDF-3 because RDF-3 is typically used toproduce RDF-5.

The HHV of prepared fuels correlates with the fuel ash content and moisture content because ash andmoisture contribute little, if any, heating value to the fuels. The strongest correlation exists between heatingvalue and ash content plus moisture content. Figure 8.32 presents this relationship based on limited pub-

SOLID WASTE 8.95

TABLE 8.43 ASTM Classifications of Refuse-Derived Fuels (44)

Category Description

RDF-1 Municipal solid wastes (MSW) used as a fuel in as-discarded form [without oversize bulky waste]. RDF-2 MSW processed to coarse particle size with or without ferrous metal separation. [The particle size

of this material is such that 95 weight percent passes through a 6-in (15-cm) square mesh screen.]

RDF-3 Shredded fuel derived from MSW that has been processed to remove metal, glass, and other inorganics. This material has a particle size such that 95 weight percent passes through a 2-in (50-mm) square mesh screen.

RDF-4 Combustible waste processed into powdered form-95 weight percent passing 10-mesh screening (0.035 in or 0.89 mm).

RDF-5 Combustible waste densified (compressed) into the form of pellets, slugs, cubettes, or briquettes. RDF-6 Combustible waste processed into liquid fuel. RDF-7 Combustible waste processed into gaseous fuel.

Note: [ ] indicate wording in tentative classifications deleted from definitions.

SOLID WASTE



lished data. This correlation provides an easy way to estimate RDF heating value using the results from twoeasily performed tests.

Municipal solid wastes can be processed into a variety of fuel products to improve their heating value,combustion efficiency, storage life, and marketability. However, the disadvantages of solid waste processingmust be considered, particularly energy yield reduction and increased system complexity. The overall impactof energy yield reduction is demonstrated in the energy recovery efficiency comparison presented in the sec-tion Energy from Solid Wastes.

Ferrous Metals

There are several sources of scrap ferrous metals: from iron and steel production (home scrap), from conver-sion of steel to various metal parts and products (prompt industrial or processing scrap) and from discardedgoods from end users (obsolete or postconsumers scrap). Of these three types of ferrous scrap, home scrapmakes up 55 to 60% of the total. Prompt industrial and obsolete scrap account equally for the remaining por-tion. Virtually all home scrap and very large fractions of prompt industrial scrap are recycled directly withinthe production and conversion industries.

8.96 CHAPTER EIGHT

TABLE 8.44 Common Refuse-Derived Fuel Classifications

Common name Typical ASTM class

Coarse fluff RDF RDF-2Fine fluff RDF RDF-3Powdered RDF RDF-4Densified RDF or d-RDF RDF-5

TABLE 8.45 Characteristics of Refuse-Derived Fuels

EnergyMoisture Ash yield,

Fuel content, content, Process Btu/1b Referenceproduct HHV, Btu/lb* % % yield, % MSW sources

RDF-l 4500–5000 15–30 24 90–100 4000–5000 44–46RDF-2 4600–4680 26–29 20–21 80–95 3680–4400 44, 45,

47RDF-3 4800–6500 18–25 7–21 50–85 2970–4510 36, 44,

45–48RDF-4 7500–8000 3–5 12–15 40–50 3120–3940 36, 48 RDF-5 5760–6640 15–23 9–23 †

† 42, 43 RDF-6 8490 (94,000 Btu/gal) 14.1 2.1 23 1950‡ 45RDF-7 270–320 Btu/ft3 6.0 † † 3380 45

*Unless otherwise noted.†Not available.‡Energy yield does not include energy in charcoal by-product. Notes: 1.0 kcal/kg = 1.8 Btu/lb; 3.8 L = 1.0 gal; 28.3 L = 1.0 ft3.

SOLID WASTE



The categories of obsolete or postconsumer scrap most obvious to the public are automobiles and dis-carded ferrous metals in municipal solid waste. Spurred by economic incentives of increasing scrap prices,shredding and magnetic processing of junk automobiles has increased over the last decade. In 1980, a pro-jected 35% of the recycled obsolete scrap came from junk automobiles. An estimated 10 million tons of fer-rous scrap made up largely of steel cans was discarded in the municipal solid waste stream in the UnitedStates in 1981. In this same period, there were 48 source separation programs collecting ferrous metals and27 facilities that had mechanized systems for separation of ferrous metals from municipal solid waste. Acombined total of some 140,000 tons or 1.5% of the available ferrous cans were recycled.

Magnetic Separation. Magnetic separation of ferrous metals from solid waste is one of the most devel-oped and simplest material separation processes in resource recovery. However, for magnetic separation tobe effective, unprocessed waste must first undergo processing—generally size reduction or screening—tofree the metal from bags and containers that would inhibit or prevent separation. In addition, it appears thatshredding of the ferrous metals is necessary—either as part of the mixed waste or the recovered product—towork the metal to free attached or entrapped nonmetallic contaminants.

SOLID WASTE 8.97

FIGURE 8.32 Higher heating values of prepared fuels versus moisture and ash content.

SOLID WASTE



There are several types of magnetic separators that have been applied to solid waste: suspended drummagnets, suspended self-cleaning belt magnets, and magnetic pulleys. The suspended belt separator hasmultiple magnet assemblies over a single belt for the purpose of tumbling, dropping, and reattracting themetal to free loose contaminants.

Magnetic Pulley Separators. Pulley-type magnets have diameters between 1 to 4.5 ft (0.3 to 1.5 m) andwidths of 1 to 6 ft (0.3 to 1.8 m) to match the conveyor belt on which they are mounted. Pulley-type magnetsare generally not applied to primary separation of ferrous metal from solid waste because of the tendency toentrap and carry over significant amounts of organics with the ferrous product. They have, however, foundapplication in secondary scalping of small amounts of ferrous metals in waste streams following primarymagnetic separation. Pulley-type magnet drive rotational speed and power requirements will be determinedby the design and capacity of the belt conveyor on which it is mounted. Pulley-type magnets normally con-tain permanent magnets that have no external power requirements.

Suspended Drum Separators. Suspended drum magnets range in diameters of 3 to 6 ft (1 to 1.8 m) andwidths of 4 to 8 ft (1.2 to 2.4 m). The size is selected to match the width of the stream of material fed to theseparator, the material height and particle size, and the field strength requirements (and thus the magnet as-sembly size) to achieve the required performance. Rotational speeds match the feed stream velocity and aregenerally in the range of 5 to 30 r/min. Magnet power requirements would be 3.8 to 11.3 kW for the drivemotor and 7 to 18 kW for the magnet power supply.

Typical application and performance have been reported on two suspended drum magnets in a solid wasteprocessing plant in New Orleans, Louisiana (4). One drum is designed for 95% ferrous recovery from a 36.5metric ton per hour stream of minus 4 in (100 mm) trommeled undersize solid waste product. It is 3.5 ft(1.07 m) in diameter and 4.5 ft (1.37 m) in width, rotates at 28 r/min (3.8-kW drive) and contains electro-magnetics with a 7-kW power supply. The drum is cleated and rotates against material flow. Testing at thedesign throughput of 36.3 metric tons per hour and a larger-than-design gap between the belt and drum sur-faces—15 versus 12 in (0.38 versus 0.29 m)—the recovery efficiency of the magnet tested at 90%.

The second suspended drum separator was designed to recover 95% of the ferrous metals from a 75 ton(68 metric ton) per hour shredded solid waste stream. The drum is 4 ft (1.22 m) in diameter, 6 ft (1.83 m)wide, rotates at 25 r/min (7.5-kW drive) and powered by a 1 1-kW supply. Because of significantly higherand more variable feed material burden height than predicted, the gap between the drum and belt was raised20 in (0.53 m) compared to the 14-in (0.36 m) design gap. Ferrous metal recovery efficiency was thus sub-stantially below design testing at only 28%.

The location of the magnetic separator varies with the type of waste processing system. The primarymagnet is normally located immediately after the initial shredding or screening equipment. Secondary mag-nets are often utilized to retrieve the 10 to 20% of the ferrous metals missed in the primary stage.

Experience suggests that it is necessary to magnetically separate the metal product a second time or pos-sibly reshred or air-classify after separation to free and separate nonmetal contaminants. The contaminationlevel for the suspended drum product in the New Orleans facility discussed above averaged 15%, well abovethe 4% specification and necessitating the addition of a ferrous product cleanup system. This system con-sisted of an air knife, light ferrous metal shredder, and a secondary suspended-belt-type magnetic separator.Performance tests indicated the ability to meet the 4% contaminant specification but indicated a reduction of37% in the amount of the recovered ferrous product due to losses in the secondary processing.

Reshredding as part of secondary processing will also increase bulk density. A high bulk density, 1700 to2500 lb/yd3 (800 to 1200 kg/m3) is desirable for transportation of the ferrous metal product and is requiredfor some scrap markets. The secondary cleanup system in New Orleans increased the density of the recov-ered ferrous product from an as-separated density of 450 lb/yd3 (210 kg/m3) for partially crushed cans to820 lb/yd3 (380 kg/m3) for the coarsely shredded cans.

Separation of ferrous metals from incinerator residue is generally more efficient than from processedwaste because of the absence of organic materials that inhibit separation or contaminate the product. How-ever, because of oxidation and alloying of metals and nonmetals during the combustion process, incinerated

8.98 CHAPTER EIGHT

SOLID WASTE



ferrous scrap may not meet some industry specifications. In 1981, nine incinerator facilities were reportedlyseparating ferrous metals, although not all were able to find markets for the recovered product.

Markets. The potential markets from ferrous metals recovered from municipal solid waste would bereached through brokers, detinners, or scrap dealers and include iron and steel mills and foundries, thedetinning industry, the copper industry (for use in precipitation processes) and production of ferroalloys.Each market has different requirements for chemical and physical properties of scrap. Standard specifica-tions for municipal ferrous scrap covering each of these end uses are shown in Tables 8.46 and 8.47.

Contracts for purchase of municipal ferrous scrap typically establish the price as a percentage of the ironand steel industry composite prices, such as No. 1 heavy melting steel scrap or No. 2 bundles. Thepercentage factor will vary for the particular scrap quantity composition, location (transportation), and the

SOLID WASTE 8.99

TABLE 8.46 Municipal Ferrous Scrap Chemical Specifications (48)

Composition, %a

Copperindustry Iron and Iron and

precipitation steel steel Detinning FerroalloyElement process) foundries production industryb production

Phosphorus, max — 0.03 0.03 8.8— 0.03 Sulfur, max — 0.04 0.04 8.8— —Nickel, max — 0.12 0.08 8.8— —Chromium, max — 0.15 0.10 8.8— 0.15Molybdenum, max — 0.04 0.025 8.8— —Copper, max — 0.20 0.10 8.8— 0.20Aluminum, max — 0.50 0.50 4.00e 0.15Tin — 0.30 maxd 0.30 max 0.15 min f 0.30Lead, max — 0.03 0.15 8.8— —Zinc, max — 0.06 0.06 8.8— —Iron (metallic), min 96.0 — — 8.8— —Silicon, max — — 0.10 8.8— —Manganese, max — — — 8.8— 0.35Carbon, max — — — 8.8— 0.6Titanium, max — — — 8.8— 0.025Total combustibles, max 0.2c 4.0 4.0 8.8— 0.5g

Metallic yield, min — 90.0 90.0 8.8— 90.0

aExperience has shown that material that has been incinerated probably will not meet these requirements.bA minimum of 95 weight percent of the material delivered shall be magnetic. Nonmagnetic material attached to the original

magnetic article may be included in the minimum requirement.cThe scrap shall be appropriately processed (for example, by burning or chemical detinning) to be virtually free of com-

bustibles.dFor steel castings, the requirement for tin content is 0.10 max percent.eNot based on melt analyses due to aluminum losses during melting; to be determined by a method mutually agreed upon be-

tween the purchaser and supplier.f Refer to sections on magnetic fraction and chemical analysis of tin in Methods E 701. Normal separation of white goods and

heavy iron yields tin contents equal to or greater than 0.15 weight percent. Lesser tin contents would impact severely the value ofthe scrap to detinners.

gThe scrap shall be appropriately processed (for example, by burning or chemical detinning) to be virtually free of com-bustibles.

SOLID WASTE



respective market. Historically, the market prices for iron and steel scrap are quite erratic. This has madeprojections of revenues for recovered metal very tenuous, and during some periods of depressed markets,made ferrous scrap virtually impossible to sell at any price. Nonferrous Metals Nonferrous scrap isclassified into three types: home scrap, prompt industrial scrap (new scrap), and old scrap (obsolete orpostuser scrap). Most home and prompt industrial scrap is recycled internally by primary or secondaryproducers. The sources of old nonferrous scrap is mainly junk automobiles and municipal solid waste. Themajor nonferrous metals recovered from auto shredders include zinc, aluminum, copper, and stainless steel.MSW is a potentially large source of erratic. This has made projections of revenues for recovered metal verytenuous, and during some periods of depressed markets, made ferrous scrap virtually impossible to sell atany price.

Nonferrous Metals

Nonferrous scrap is classified into three types: home scrap, prompt industrial scrap (new scrap), and oldscrap (obsolete or postuser scrap). Most home and prompt industrial scrap is recycled internally by primaryor secondary producers. The sources of old nonferrous scrap is mainly junk automobiles and municipal sol-id waste. The major nonferrous metals recovered from auto shredders include zinc, aluminum, copper, andstainless steel. MSW is a potentially large source of nonferrous scrap, particularly aluminum. Approximate-ly 1% of the MSW is nonferrous metals of which two-thirds is aluminum and the remainder primarily brass,copper, zinc, and stainless steel. During the 1970s, efforts at recycling of nonferrous metals from municipalsolid waste were directed at recovery of aluminum.

In 1979, an estimated 1.15 million tons (1 million metric tons) of aluminum containers and packagingwere available in MSW as old scrap for recycling in the United States. Of this, about 65% were all-

8.100 CHAPTER EIGHT

TABLE 8.47 Municipal Ferrous Scrap Physical Specifications (48)

Property

Bulk density,End use lb/ft3 (kg/m3) Form

Copper industry (precipitation 30 (480) max Loose, shredded as agreed upon between process) purchaser and supplier, shall not be balled or

baleda

Iron and steel foundries 50 (800) min Loose, balled, or baledb as agreed upon between purchaser and supplier

Iron and steel production 75 (1200) min Loosec or baledb as agreed upon between purchaser and supplier

Detinning industry 25 (400) max Shredded, 95 weight percent shall be –6, +½ in (–152, +12.5 mm); shall not be balled, baled, burned, incinerated, or pyrolyzed

Ferroalloy production 50 (800) min Loose, as agreed upon between purchaser and supplier

aVarious consumers may establish gage limitations on the material they purchase.bIndustry practice is to specify a maximum bale size that may vary among users.cExperience has shown that if the size range is 95 weight percent, –2, +¼ in. (–50, +6.3 mm), the bulk density requirement can

be met and the material will be loose and free flowing.

SOLID WASTE



aluminum cans. In 1981, some 30% of all-aluminum cans were recycled, virtually all through source separa-tion programs.

Source separation refers to separation of selected components from solid waste at the point of discard andtransfer of the separated fractions (by the homeowner or by separate collection) to collection centers, sec-ondary dealers, or direct consumers of secondary materials. The separation is done by hand and involveslimited use of mechanical equipment. In 1980, there were over 2500 locations in the United States for recy-cling of source-separated aluminum cans.

Mechanical Separation. Mechanical methods for nonferrous metal recovery include flotation (density),electromagnetic separation, electrostatic separation, and preconcentration and handpicking. A description ofeach method follows.

Flotation. Flotation systems utilize stages of controlled water elutriation (for effective specific gravitiesbetween 1.1 and 2.0), heavy liquids (for specific gravities of 1.5 to 3.0), and dense media (water slurries ofmagnetite, galena, or ferrosilicon for effective specific gravities of 2.5, 3.3, and 3.5, respectively). In sys-tems applied to MSW, several flotation stages with specific gravities between 1.2 and 3.0 are used to sepa-rate glass, aluminum, and other nonferrous metals by differences in material densities.

Typically size reduction, air classification, magnetic separation, and screening precede a combination ofwater flotation and heavy media separation. Proper feedstock preparation is necessary to control the size andshape of the metals and to minimize the levels of organic and inorganic fines. Shredded aluminum withfolds and pockets may entrap air, which bouys the particles, or entrap dense solids that settle and increaseparticle weight and media losses and product contamination. Organic materials can adsorb heavy liquids andmedia increasing operating costs. Organics and fine inorganics can change media specific gravity and vis-cosity thereby effecting separation efficiency.

Water elutriation and heavy media separators, in conjunction with magnets and screens, have also beenused for recovery of nonferrous metals from incinerator residues in pilot-scale and experimental programs(51). Because nonferrous metals such as aluminum may oxidize, melt, and be lost in the undergrate siftings,or alloy with other materials during incineration, sufficient marketable nonferrous metals are usually not re-coverable to justify commercial applications.

Electromagnetic. Electromagnetic or eddy current separators employ the principle of electromagnetic in-duction to separate conductive nonferrous metals. Utilizing modulating electromagnetic fields or the motionof the metal moving through the magnetic field of an array of permanent magnets, eddy currents are gener-ated in conductive metal particles that in turn interact with the magnetic field and cause the particles to bedeflected out of the separator.

The flow scheme for the electromagnetic nonferrous separation system in the New Orleans, Louisiana,facility is shown in Figure 8.33. This particular system utilizes two parallel electromagnetic separators oper-ating on a feedstock from a series of screens, classifiers, and magnets designed to preconcentrate aluminumcan stock. The importance of efficient preconcentration is indicated in results from this demonstration facil-ity. Although the aluminum recovery efficiency for the electromagnetic separators was as high as 85 to 90%,because of losses of aluminum in preceding screening and classification processes, the corresponding over-all aluminum recovery rate for the facility averaged 35 to 40%.

Each electromagnetic separator has four sets of magnets, each set having a magnet above and below a 22-in (0.56-m) wide conveyor belt. The gap between the top and bottom magnet was nominally 3.5 in (90 mm).The belt carried 1 to 3 tons/h (0.9 to 2.7 Mg/h) of waste at a belt speed of between 250 to 500 ft/min (75 and150 m/min). Conductive nonferrous metals concentrated in the feedstock to about a 5% level are ejected lat-erally off both sides of the belt by the electromagnets. The magnets operate at 480 V and 60 cycles with eachset consuming about 7.5 kW.

The effect of aluminum shape and belt speed on electromagnet separation efficiency is shown in Table8.48. For nominal operation, each separator recovered between 85 and 90% of the aluminum from a feedstream of 1 ton/h (0.9 Mg/h) and belt speed of 350 ft/min (105 m/min).

SOLID WASTE 8.101

SOLID WASTE



Electrostatic. Electrostatic separators utilize a 30 to 50 kV field generated from electrodes located abovea stream of particles as they flow onto a grounded metallic drum. The drum is usually 10 to 30 in (0.25 to0.75 m) wide and 1.5 to 3 ft (0.5 to 1 m) in diameter. The nonconductors (typically glass and organics) retaina static charge long enough to be attracted and held to the drum, while conductors (metals) dissipate theircharge quickly and are repelled from the drum and thereby separated. Processing of the electrostatic separa-tor feedstock to even a greater degree than electromagnetic feedstock is necessary. The feed must be quite

8.102 CHAPTER EIGHT

FIGURE 8.33 Ferrous, aluminum, and glass recovery processes at New Orleans.

SOLID WASTE



small [< 1 in (25 mm)] and very dry (< 1% moisture), and the level of organics (paper) kept at a minimum toavoid interferences and minimize separator volumetric loadings. These requirements have limited wide-spread application of electrostatic separators to nonferrous metal recovery. The application of an electrostat-ic separator for metal separation in a pilot glass recovery facility is discussed in Ref. 52.

Depending mainly on market specifications for the metals, the product from an electromagnetic or elec-trostatic separator may require additional processing to remove glass and loose organic contaminants carriedwith the metal product and to separate other nonferrous metals from aluminum. In the New Orleans facility,such nonmetal contaminants averaged 14% of the separator product. This cleanup may be accomplishedwith an air knife, screen, and/or additional stages of electromagnetic (or heavy media) separators. The alu-minum product may also require shredding or baling to increase density for economic transportation. Shred-ding in New Orleans raised the density from 130 to 520 lb/yd3 (60 to 240 kg/m3).

Friction Slide. The first commercial preconcentration and handpicking system for separation of alu-minum from mixed MSW is located in Houston, Texas. The process flow is shown in Figure 8.34. The plantis designed to process 60 tons/h (56 × 103 kg/h) of unprocessed MSW.

The friction slide used in the Houston plant exploits the differences in bounce and frictional resistancewith an inclined flat-belt conveyor fed near the lower (tail) end. More rigid or round particles (metals, denseorganics) will bounce, slide, or roll off the bottom of the slide and are conveyed to an air knife and hand-picking while more flexible, damp material (paper, textiles, organics) is carried up and discharged as aresidue off the top. Human pickers remove aluminum from a slow-moving belt and the product is then flat-tened and shipped to market.

Mechanical recovery of nonferrous metals from MSW has been discontinued at several plants and is notbeing applied in newer installations for several reasons: relatively low and fluctuating concentrations of met-als in the waste, current absence of efficient and reliable technology for separation and sorting of mixedmetals, and lack of widespread markets.

Product Quality. A specification for municipal aluminum scrap has been developed. It covers two classes(based on fines content) and six grades (based on chemical composition). Table 8.49 presents the chemicaland physical requirements for municipal aluminum scrap. The market value and reuse of the recovered scrap(for example, as aluminum can stock or in wrought alloys) will depend on grade quantity and location ofmarkets (transportation). Use of recycled aluminum does provide significant energy savings. Less than 5%of the energy required to produce an aluminum ingot from ore is needed to produce aluminum ingot from re-cycled metal.

Glass Products

With the exception of glass scrap (cullet) generated in glass manufacturer conversion plants, municipal solidwaste offers the largest source of waste glass for recycling. On the average, 10 to 11% of the MSW stream is

SOLID WASTE 8.103

TABLE 8.48 Effect of Can Shape and Belt Speed on Electromagnetic Separator Performance (49)

Percent recovery by can shapeBelt speed

m/min Flattened Deformed Whole Total

91 97.7 98.4 100 98.4122 92.9 98.4 100 98.0152 90.7 95.0 86.6 94.5

Note: 1 m/min × 3.3 = ft/min.

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glass, of which an estimated 90% is container glass. The raw materials for glass production are comparative-ly inexpensive and readily available in most areas. However, there are incentives for recycling glass in the re-duction in energy and water consumption and improvements in the melt reaction for glass manufacturingand also in reduction in solid waste disposal requirements.

In the 1980s nearly all of the recovered and recycled glass from MSW was recovered through source sep-aration programs, where the householder segregates the glass for separate collection or for delivery to a re-cycling center. However, during the 1970s, several approaches to mechanical separation of glass from mixedmunicipal waste were developed and applied in commercial-scale facilities.

The technology for mechanical recovery of glass cullet from mixed municipal waste involves either frothflotation or optical sorting. Due to relatively low product values coupled with the technical and operationalcomplexities, none of the four first-generation froth flotation or optical sorting systems have operated regu-larly, and further commercialization of glass recovery has not extended beyond these earlier plants. Equip-ment and markets for recovery and use of glass for secondary applications (i.e., fiberglass, aggregate, glass-ware, architectural treatments) has also been explored in one full-scale facility.

8.104 CHAPTER EIGHT

FIGURE 8.34 Ferrous and aluminum recovery processes at Houston.

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Froth Flotation Separation. Froth flotation is a process developed in the minerals industry when fine-sized particles [0.002 to 0.35 in (0.05 to 9 mm)] are selectively floated to the surface of a slurry by means ofattached air bubbles. A surface conditioning agent (for example a coca amine) that preferentially coats glassparticles and makes the surfaces hydrophobic (water repellent) is mixed with water and added to a precon-centrated and finely sized (less than 20 mesh) fraction containing the glass and inorganic contaminants (ce-ramics, stones, and metal). The treated slurry is then aerated in a separation chamber where the glass israised by attachment of air bubbles effected by the reagent and removed from the nonglass contaminants,which are separated in the tailings (sink) product. Additional secondary cells may be used to reclean theproduct or attempt to raise more glass from the tailings. A complete discussion of the froth flotation processis provided in Refs. 53 and 54.

Figure 8.35 shows the process flow for the prototype froth flotation glass recovery system located in a re-source recovery plant in New Orleans. Also shown is the mass balance data for the glass fraction based on amunicipal waste input of 100 tons/h (90 metric tons/h) or about 160% of the facility design capacity. Thespecifications for the principal equipment items are provided in Table 8.50.

The performance characteristics of the glass recovery system revealed during the shakedown and testingin New Orleans were as follows:

SOLID WASTE 8.105

TABLE 8.49 Municipal Aluminum Scrap Specification (50)

Chemical requirements

Composition, maximum % allowable

Element* Grade 1 Grade 2 Grade 3 Grade 4 Grade 5 Grade 6

Silicon 0.30 0.30 0.50 1.00 9.00 9.00 Iron 0.60 0.70 1.00 1.00 0.80 1.00 Copper 0.25 0.40 1.00 2.00 3.00 4.00 Manganese 1.25 1.50 1.50 1.50 0.60 0.80 Magnesium 2.00 2.00 2.00 2.00 2.00 2.00 Chromium 0.05 0.10 0.30 0.30 0.30 0.30 Nickel 0.04 0.04 0.30 0.30 0.30 0.30 Zinc 0.25 0.25 1.00 2.00 1.00 3.00 Lead 0.02 0.04 0.30 0.50 0.10 0.25 Tin 0.02 0.04 0.30 0.30 0.10 0.25 Bismuth 0.02 0.04 0.30 0.30 0.10 0.25 Titanium 0.05 0.05 0.05 0.05 0.10 0.25 Others (each) 0.04 0.05 0.05 0.08 0.10 0.10 Others (total) 0.12 0.15 0.15 0.20 0.30 0.30 Aluminum Balance Balance Balance Balance Balance Balance

Physical requirements

Density—To be agreed upon between purchaser and seller Fineness—Class A-Not less than 1 weight percent fires minus 12 mesh

Class B—Not less than 3 weight percent fires minus 12 mesh Loose combustibles—Not more than 2 weight percent loose combustible material Moisture—Not more than 0.5 weight percent moisture Metal recovery—Minimum metal recovery of 85% (per ASTM procedure) Magnetics—To be agreed upon between purchaser and seller

*By agreement between the purchaser and the seller, analysis may be required, and limits established for elements or com-pounds not specified in this table.

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8.106 CHAPTER EIGHT

FIGURE 8.35 Nominal glass mass balance at New Orleans glass recovery system (49).

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� Glass losses in the dry portion of the processing system (prior to the mineral jig) were sensitive to the lev-el and fluctuations on infeed rate, and ranged from 40 to 50%.

� The capacity of the “wet” portion of the glass recovery system (the mineral jig through flotation) was 4tons/h (3.6 metric tons/h); however, it was operated at 1.7 tons/h (1.5 metric tons/h). Increased or fluctu-ating feed rates were detrimental to system performance and glass product quality.

� Glass recovery in the wet portion of the glass recovery system tested at 80% of the input glass.� Prefloat cells containing water in reagent were found necessary to float and remove grease and other or-

ganic materials carried over from the jig.� The vacuum filter lowered moisture content of the glass product to 5% and the dryer to less than the 1%

required in the glass product specification.

Table 8.51 lists the glass product specifications as established in ASTM E708-79, Standard Specificationfor Waste Glass as a Raw Material for Manufacturer of Glass Containers, as well as the mean results ofanalysis of the glass product from New Orleans. A comparison shows that the glass product met the specifi-cations in all categories except in moisture and minus 140 mesh fines.

Pilot tests on a froth flotation of glass incinerator residue has been reported (51) but no commercial ap-plication has developed.

Optical Separation. In an optical sorter, the intensity of light transmitted through a particle passingthrough the device is measured by a photocell. By means of sensing or comparing the intensity to a back-ground slide, the particle is identified as opaque (nonglass) or flint, green, or amber and accepted or divert-ed by a rapidly acting ejector located below the sensor. Because most sorters are binary devices, severalstages may be required to make these multiple separations.

Optical sorting also requires considerable processing to prepare the appropriate feedstock. Clean, dryparticles sized in the range of 0.25 to 2 in (6 to 50 mm) and as free of metal and other contaminants as pos-sible are required. Particles smaller than 0.25 in (6 mm) cannot be recovered with this equipment, whicheliminates shredders or flails in the feed preparation system. Alternative size reduction schemes might em-

SOLID WASTE 8.107

TABLE 8.50 New Orleans Glass Recovery Equipment Descriptions (49)

Item Description

Two-deck screen CE Tyler Model F-800; 3 m long by 1.5 m wide; 5° declined; 1.5-kW electromechanical drive; 114- and 57-mm-diameter perforated screen decks

One-deck screen Vibraretics, 3 m long by 0.6 m wide; 0.38-kW electromechanical drive; 26 mm × 75 mm slotted screen deck

Mineral jig WEMCO Remer jig; 4.9 m long by 1.5 m wide; 6-mm mesh screen panels; 9.5 m and 13 mm ragging; 5° declination; 1.5 and 5.6-kW drives

Rod mill Marcy Mfg., 1.2 m diameter by 2.4 m long; center overflow discharge type; 37 grinding rods (typical); 25 r/min; 37.5-kW drive

Sizing screen CE Tyler Model 1F-800; 3 m long by l.5 m wide; 1.5-kW electromechanical drive; 6-mm and #20 wire mesh screen decks

Hydroclones (2) Krebs Models Dl0B and D6B (modified); 254 and 152 mm diameters, adjustable apex Flotation cells Denver Equipment Model 18-Special, Type A; 0.67-m3 cells arranged as two prefloat,

three rougher, three cleaner Flotation reagent Sherex MG 83A at 5% concentration with 1% (wt) concentration pine oil frothing aid Vacuum filter Dorr Oliver; 1.2 m diameter; 430 mmHg vacuum Dryer Joy Manufacturing Holoflite Model l-D-2410; 1.25 MBtu/h oil fired heater for 650°F

operation; twin revolving flights

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ploy primary screening, shear shredders, or hydropulping. Further concentration of the glass can be accom-plished by air classification and screens, separation of organics, and washing of the glass by flotation; metalseparation by flotation; or electrostatic separation and drying. The complete process flow for the first full-scale commercial application of an optical sorting system in Hempstead, N.Y., is shown in Figure 8.36.

Paper and Plastics

On the average, paper comprises about 35% and plastics about 6% of the United States MSW stream. Theproportion of individual paper categories varies considerably with location, but typically newspapers com-prise 25%, corrugated 35%, and other categories and mixed papers, the remaining 40% of the paper frac-tions. Of the plastics, polyolefins (including polyethylenes and polypropylenes) account for 75% of the plas-tics with styrene polymers contributing 15% and polyvinylchlorides (PVCs) about 10%.

In spite of their relatively high concentrations, mechanical separation of either paper or plastics frommixed MSW in the United States has not developed beyond limited research or pilot-scale programs. Thesignificant quantity of paper that is being recycled [18 million tons (16.2 million metric tons) or 26% of thetotal U.S. consumption of paper and paperboard in 1981] is virtually all from commercial and residentialsource separation prior to discard into the waste stream. In 1981, there were 228 residential source separa-tion programs recycling paper, of which three-quarters collected newspaper and one-quarter collected mixedwaste paper. Source-separated waste paper is primarily used to make new paper and paperboard products(86%), with the remainder used for such products as cellulose insulation and building products (3%) or ex-ported (11%).

Mechanical recycling of paper and plastics from mixed wastes has been limited by both technical andeconomic factors. For paper, testing has shown that the yield of recycled fiber is very low (less than 30% of

8.108 CHAPTER EIGHT

TABLE 8.51 Glass Specifications and Product Analysis Result from New Orleans (55)

ASTM MeanSpecification recovery I

Parameter Value result

Percent moisture, wet-weight basis <0.5% 1.0Percent plus 6-mm, dry-weight basis 0% 0Percent minus U.S. std. #140, dry-weight basis �15% 15.8Percent organic materials, dry-weight basis � 0.2% or � 0.4 ± 0.16

0.05%Percent Fe2O3 mixed-color glass >0.1 and/or 0.16 Percent Cr2O3 mixed-color glass >0.0015 % Trace Percent SiO2 65–75 % 73.4 Percent A12O3 soda-lime 1–7 2.0 Percent CaO + MgO glass 9–13% 10.3Percent Na2O 12–16% 13.5 %Total magnetic materials mixed-color glass �0. 14% –—Percent total inorganic nonmagnetic materials �0.6% 0.59Refractories: number of particles per pound

Minus #20, plus #40 mesh �2 0Minus #40, plus #60 mesh �20 12

Nonmagnetic metals: number of particles per pound Plus #20 mesh �1 0

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the available paper fiber) due to losses in the dry-screening and air-classification stages required to concen-trate the fiber fraction and further fiber losses in the pulping, centrifugal cleaning, screening, and dispersionsystems required to remove contaminants. No compositional data on production material are yet available toestablish the markets and thus the economic viability of paper recovery.

Recycling of plastics from mixed municipal waste is complicated by lower concentrations, a variety insizes, density, and other properties (melting points), and frequent lamination with other plastic and nonplas-tic material. The technologies that have been explored for concentration of a mixed-plastic product fromwaste include multiple stages of air classification, thermal contraction of film plastics followed by air sepa-ration, and use of electrostatic separators.

Even were a clean mixed-plastic product recovered, economic reuse requires further separation of themixed product by plastic (or resin) type. Surface wetting and density separation techniques have been ex-

SOLID WASTE 8.109

FIGURE 8.36 Ferrous, aluminum, mixed nonferrous, and glass recovery process at Hempstead, N.Y.

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plored but not demonstrated on a large scale. The potential for reuse of mixed plastics is primarily with thethermoplastics fraction that can be remelted. Thermoplastics comprise some 80% of the plastics in thewaste, while the remaining 20% are thermosetting plastics “set” after forming. Markets for reformed ther-moplastics need to be sought.

TREATMENT AND DISPOSAL

Following separation, including by-product recovery, and volume reduction processes, solid wastes are pre-pared for treatment (stabilization) and final disposal. The most common method is by use of a landfill. Oth-er methods of interest are land reclamation, composting, incineration, pyrolysis, and wet oxidation.

Whether reclaiming a strip mine or developing a recreation area, the engineer must consider costs and de-sign limitations and subsequent maintenance by good soil management practices. In general terms, the prin-cipal considerations are land requirements, site soil and topographic conditions, site access, and land andoperating costs. In some cases, ground and surface water monitoring may be required during and after thedisposal process.

Sanitary Landfill

Sanitary landfilling is defined as “an engineered method of disposing of solid waste on land in a mannerthat protects the environment, by spreading the waste in thin layers, compacting it to the smallest practicalvolume, and covering it with compacted soil by the end of each working day or at more frequent intervals ifnecessary” (56). Sanitary landfills are and will continue to be the principal method of solid waste disposal.

In the United States, the design and operation of sanitary landfills are regulated by 40 CFR 258, whichestablishes minimum national criteria for all solid waste landfills that are not regulated under Subtitle C ofRCRA and that receive municipal solid waste, or codispose sewage sludge with municipal solid waste, oraccept nonhazardous municipal waste combustion ash. The U.S. Environmental Protection Agency providesextensive technical guidance to meet the regulation requirements (94, 95). Landfills that receive construc-tion and demolition debris only, tires only, and nonhazardous industrial waste only are regulated under 40CFR 257.

This section outlines the salient points that must be considered in the design of a sanitary landfill by thecivil engineer. Because of the volatile nature of construction and equipment costs, the cost figures given inthis section should be used as a guide only. Also, heavy equipment used in sanitary landfill operations is be-ing improved and revised continually. Engineers who need to determine sanitary landfill equipment require-ments should contact equipment suppliers and observe equipment used by existing facilities in the area.

Preliminary Determination of Landfill Requirements. Every proposed sanitary landfill has some basicrequirements that must be determined prior to commencing any design activity. Typical determinations fol-low.

Estimate of Solid Waste Quantity. If accurate records are not available on the amount of solid waste gen-erated in an area, then the amount may be estimated by one of the following methods.

1. The population to be served by the sanitary landfill must be determined. This population is multiplied by5 lb (2.26 kg) per capita to arrive at an estimated daily weight of solid waste generated. The generationrate per capita estimate can be broken down as half residential and half commercial and industrial.

2. In communities with very large commercial or industrial establishments, it is prudent to estimate thelarge establishment’s per capita solid waste generation. If this figure is significant, an adjustment in theper capita figure can be made.

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3. A fairly accurate estimate of the amount of solid waste can be determined by counting the types of vehi-cles entering the existing landfill (or landfills) that will be replaced by the new facility. Table 8.52 liststypical unit weights for various collection vehicles.

Estimate of Landfill Space Required. The annual landfill space requirements can be determined by thefollowing formula.

VA = W/1100

where W is the annual weight in pounds (kilograms) of solid waste generated in the area, and 1100 is therefuse compaction in pounds per cubic yard (650 kg/m3).

Landfills should be designed for a minimum depth of solid waste of 20 ft (6 m) and a minimum life of 10years.

Type of Waste Delivered to Site. A survey should be made to determine if any nontypical solid waste is tobe delivered to the proposed site. Particular attention should be paid to wastes that will require special han-dling by the operating personnel. It is also important to identify any potential hazardous wastes that present-ly are accepted at the existing sanitary landfill. Hazardous wastes should be excluded from sanitary land-fills.

Traffic on Site. It is necessary to know how solid waste will be delivered to the site. The most importantdetermination is the percentage of vehicles that will be off-loaded by hand. It can take 30 mm to unload apickup truck with 300 lb (90 kg) of solid waste and 5 mm for a rear-load packer with 10,000 lb (3000 kg).Un loading times for all vehicles must be kept to a minimum.

Requirements of Landfill Operator. The operator of the sanitary landfill might have requirements thatwill affect the design. The anticipated hours of operation and number of days per week the site will be openmust be determined. The operator might also have special equipment that he or she wants to use in the oper-ation.

Requirements of State Regulatory Agency. Every state has its own set of landfill standards. These stan-dards must be fully understood prior to undertaking any investigation. Many states have laws dealing withdevelopments in wetlands, floodplains, or other types of environmentally sensitive areas. The regulatingagency for these requirements might be different than the landfill regulating agency. Also, certain federalregulatory agencies may have jurisdiction.

Site Selection. Selection of a sanitary landfill site is often more a social–political process than an engi-neering process. The selection process should involve the evaluation of at least two potential sites within thestudy area. A typical site-selection scenario would be as follows.

Accumulate Available Data. Land-use maps, topographic maps, water well logs, soil conservation servicesoil maps, highway maps, and bridge loading information should all be used in the study.

Location Restrictions. In consideration of both the potential effects that a sanitary landfill may have on

SOLID WASTE 8.111

TABLE 8.52 Typical Unit Weights for Various Collection Vehicles

Type of vehicle Unit weight

Car or pickup 200 lb (91 kg each) Rear-loading packer 550–820 lb/yd3 (320–487 kg/m3)Side-loading packer 450-700 lb/yd3 (267–415 kg/m3)Top loading packer 400–500 lb/yd3 (237–297 kg/m3)Compacted roll-off container 450–550 lb/yd3 (267–326 kg/m3)Open top roll-off container 250–400 lb/yd3 (148–237 kg/m3)

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the surrounding environment and the effects that natural and human-made conditions may have on the per-formance of the landfill, location restrictions apply (94). Floodplains, fault areas, seismic impact zones, andunstable area restrictions address conditions that may have adverse effects on landfill performance thatcould lead to releases to the environment or disruptions of natural functions, such as floodplain flow restric-tions. Airport safety, floodplains, and wetlands criteria are needed to the location of landfills in areas wheresensitive natural environments and/or the public may be adversely affected.

Establish Minimum Site Size. The geometry of a site is very important. Required setbacks from roads andother natural features make portions of a site unusable. A square site maximizes the amount of land availablefor actual solid waste disposal activity.

The fixed cost of engineering, land, roads, buildings, and environmental controls for a site with a 20-yearlife are not much different than for a site with a 10-year life.

Obtain Soil Borings. Soil borings are required on the most desirable of the sites. Permission to take theseborings from private landowners is sometimes difficult to obtain. For some sites, this dilemma can be over-come by requesting per mission from the local road agency to take borings in the public right-of-wayfronting the site.

Prepare Budgetary Cost Estimate. Budgetary cost estimates must be prepared for each of the selectedsites. The cost estimates must be of sufficient detail to allow a comparison among the various sites. The cap-ital cost items must be on an annual basis. Items included in the cost estimates should be

� Land cost� On-site development costs (roads, fences, leachate control, liners, etc.)� Off-site costs (bringing access roads up to anticipated load-carrying capacity)� Cost of closing the site when it is filled� Cost of perpetual care for the site, including the transportation and treatment of any leachate� Anticipated annual operating cost

Select the Most Desirable Site. Barring any political or social constraints, the site to be recommendedshould be the one with the lowest cost (disposal plus trucking) per ton.

The landfill evaluation, design, and approval process can be a long and drawn-out ordeal with often neg-ative results. For these reasons, long-term real estate options instead of an outright land purchase is desir-able. A typical option would require the payment of a nominal sum during the option period (normally oneyear) with the balance to be paid only upon receipt of a sanitary landfill operating license.

Hydrogeologic and Soils Investigation of Selected Site. Prior to the detailed design of a sanitary landfill,a hydrogeologic and soils investigation of the site is performed. The state regulatory agency normally hassome specific criteria for performing this task. The investigation should include as a minimum the follow-ing.

Topographic Map. A topographic map at a sufficient scale and contour interval so that the character ofthe study area is clearly defined should be prepared. Convenient scales are 1:1200 (1 inch = 100 ft) horizon-tal and 2 or 4 ft vertical contour interval.

Soil Borings. Soil borings should be taken at intervals and depths such that the nature of the soil stratacan be determined. Soil samples should be collected at 5-ft (1.5-rn) intervals. These samples should besaved for testing. When groundwater is encountered, it is important that several soil borings completely pen-etrate the aquifer and its aquiclude. The driller must accurately record the depth at which groundwater isfirst encountered. If possible, the hole should be left open several hours and a second measurement ofgroundwater depth taken. The elevation of the ground at each boring location should be determined to thenearest 0.15 ft (5 cm).

Existing Water Well Logs. Water well logs for wells drilled in the surrounding area should be collectedfrom the appropriate agencies (normally a public health department). The well logs will provide some indi-cation of the location, depth, capacity, and water quality of the aquifer.

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Set Groundwater Observation Wells. A series of groundwater observation wells [2 in (5 cm) diameter]should be established around the perimeter of the site. These wells should be set in porous soils and pene-trate the groundwater at least 10 ft (3 m). A survey crew should establish the top elevation of the well to thenearest 0.01 ft (0.5 cm). The cap of the well should be vented and provided with a locking device to preventtampering. A sufficient number of wells (three minimum) must be set to accurately determine the slope anddirection of groundwater flow. The wells can also be used as groundwater sampling points.

Groundwater Contour Map. The groundwater elevations recorded from the observation wells should beplotted on the site topographic map. A groundwater contour map should be superimposed on the site topo-graphic map.

Soil Tests. It is important that tests be run on

� Clay soil that will serve as an in situ impermeable barrier or be used to construct a clay liner. Hydrometer,Atterberg limits, and permeability tests should all be run on selected clay samples.

� Saturated porous soils that require a grain size analysis.

Groundwater Recharge Area Impact. The impact the proposed landfill will have on the recharge capabil-ity of the groundwater aquifer must be determined. A large lined landfill might remove sufficient rechargearea from the groundwater system to alter the character of the aquifer.

The landfill site investigation process is a progressive one. It must be designed to minimize the cost andallow the owner and/or regulatory agency an opportunity at each step to review the accumulated data. Thehydrogeologic and soils study, which is the first actual on-site operation, must be sequenced to allow the en-gineer the opportunity to abort the project if conditions other than those originally anticipated are encoun-tered.

Water Quality Control Requirements at Selected Site. Since leachate is by far the primary source ofwastewater of concern at a landfill, the spatial and hydrogeological characteristics of the selected site are im-portant. Also, the selected sites proximity to wastewater collection and/or treatment facilities needs to be de-termined.

In addition to leachate, other sources of landfill wastewaters are: gas collection condensate, truck/equip-ment washwater, drained free liquids, laboratory wastewaters, and contaminated stormwater. Additionalsources of wastewaters generated by landfills may include contaminated groundwater, noncontaminatedstormwater, and sanitary wastewaters. These wastewaters are described below.

� Leachate is liquid that has passed through or emerged from solid waste and contains soluble, suspended,or miscible materials removed from such waste. Over time, the potential for certain pollutants to moveinto the wider environment increases. As water passes through the landfill, it leaches pollutants from thedisposed waste, moving them deeper into the soil. One measure used to prevent the movement of waterpollutants from the landfill site is a liner integrated with a leachate collection system. Leachate also maybe collected through the use of slurry walls, trenches or other containment systems.

� Gas collection condensate is liquid that has condensed in a gas collection system during the extraction ofgas from the landfill. Gases, such as methane and carbon dioxide, are generated due to microbial activitywithin the landfill and must be removed to avoid hazardous conditions. The gases tend to contain highconcentrations of water vapor that is condensed in traps staged throughout the gas collection network.The gas condensate contains volatile compounds and accounts for a relatively small percentage of flowfrom a landfill.

� Drained free liquids are aqueous wastes drained from waste containers, such as drums and trucks, orwastewater resulting from waste stabilization prior to landfilling. Landfills that accept containerizedwaste may generate this type of wastewater. Wastewaters generated from these waste processing activitiesare collected and usually combined with other landfill generated wastewaters for treatment at the waste-water treatment plant.

SOLID WASTE 8.113

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� Truck/equipment washwater is generated during either truck or equipment washes at landfills. Duringroutine maintenance or repair operations, trucks and/or equipment such as loaders, compactors, or dumptrucks used within the landfill are washed and the resultant wastewaters are collected for treatment. In ad-dition, many facilities wash the wheels, body, and undercarriage of trucks used to deliver the waste to theopen landfill face upon leaving the landfill.

� Laboratory-derived wastewater is generated from on-site laboratories that characterize incoming wastestreams and monitor on-site treatment performance.

� Contaminated stormwater is runoff that comes in direct contact with the waste or waste handling andtreatment areas.

� Noncontaminated stormwater includes stormwater that flows off the cap or cover of the landfill and doesnot come in direct contact with solid waste. Noncontaminated stormwaters discharged through municipalstorm sewer systems or that discharge directly to waters of the United States are subject to National Pol-lutant Discharge Elimination System (NPDES) stormwater permit requirements.

� Contaminated groundwater is water below the land surface in the zone of saturation that has been conta-minated by landfill leachate.

Preliminary Design. The next major step toward completion of the project is the preliminary design stage.Discussion follows of the major areas that must be investigated during this stage.

Review Accumulated Data. Information gathered during the earlier phase of the project is now accumu-lated and used to establish some criteria for the site. Examples of data use follow.

� The soils report will indicate the nature of on-site soils and the need for any liners or leachate collectionsystem. Generally, porous soils [permeability greater than 0.00028 ft/day (1 × l0–7 cm/s)] will require aliner system and some type of leachate collection.

� The hydrogeologic report will provide the designer with information as to how deep the site may go be-low ground surface. The location of permanent groundwater monitoring wells can be determined.

� The site topographic map prepared for the hydrogeologic investigation will be used as the master sheetfor the design. It may be necessary to have the map enlarged in order to properly layout all the details ofthe site. It is always a good idea to make several reproducible copies of this map.

� The original estimate of the site’s daily traffic and quantity must be updated.� The landfill operator’s equipment limitations must be fully understood. This item requires special consid-

eration, since a landfill is primarily a materials-handling operation.

Establish Operating and Design Parameters. In general, the standard operating requirements for a sani-tary landfill should be developed to ensure the safe daily operation and management at the facility. Dailycover, liners, and leachate controls are common to all landfills. Other operating requirements include (94):

� Hazardous waste exclusion. A program must be developed and implemented to detect and prevent dis-posal of regulated hazardous wastes or PCB wastes at the sanitary landfill. Hazardous wastes may begases, liquids, solids, or sludges that are listed or exhibit the characteristics described in 40 CFR 261.Household hazardous wastes are excluded from the regulation, and wastes generated by conditionallyexempt small-quantity generators (CESQGs) are not considered regulated hazardous wastes for thesepurposes.

� Disease vector control. Disease vectors such as rodents, birds, flies, and mosquitoes typically are attract-ed by putrescent waste and standing water, which act as a food source and breeding ground. Normally,application of a daily cover is sufficient to control disease vectors; however, other vector control alterna-tives may be required. These alternatives could include: reducing the size of the working face; other oper-ational modifications, such as increasing daily cover thickness, changing cover type, density, placementfrequency, and grading; repellents, insecticides or rodenticides; composting or processing of organicwastes prior to disposal; and predatory or reproductive control of insect, bird, and animal populations.

� Explosive gas control. Methane production rates will vary spatially within a landfill unit as a result of

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pockets of elevated microbial activity but, due to partial pressure gradients, differences in gas composi-tion are reduced as the gases commingle within and outside the landfill unit. Monitoring is necessary toestablish if the concentration of methane gas generated by the facility exceeds 25% of the lower explosivelimit (LEL) for methane in facility structures, excluding gas control or recovery system components, andif the concentration of methane gas exceeds the LEL for methane at the facility property boundary. Ifthese concentrations are exceeded, abatement measures are necessary.

� Air monitoring. Sanitary landfill operations must not violate any applicable requirements developed un-der a State Implementation Plan (SIP) developed to comply with the Clean Air Act. Open burning of sol-id waste, except for the infrequent burning of agricultural wastes, silvicultural wastes, land-clearing de-bris, diseased trees, or debris from emergency clean-up operations, should be prohibited.

� Facility access. The development of landfill facilities should include means to control public access toprevent illegal dumping, public exposures to hazards at the site, and unauthorized vehicular traffic. Gen-erally, unauthorized persons are unfamiliar with the hazards associated with landfill facilities. Access tofacilities should be controlled through gates that can be locked when the site is unsupervised.

� Run-on/run-off control systems. The stormwater control system must include a mn-on control system toprevent flow onto the active portion of the landfill during the peak discharge from a 25-year storm and arun-off control system from the active portion of the landfill to collect and control at least the water vol-ume resulting from a 24-hour, 25-year storm.

� Surface water requirements. The operation of the sanitary landfill should not create a point or nonpointdischarge of pollutants to surface waters. Otherwise, abatement measures must be included in the facilitydesign.

� Liquid restrictions. Only household liquid wastes and leachate or gas condensate derived from the land-fill operation are permissible. All other bulk or noncontainerized liquid wastes should be excluded fromthe sanitary landfill.

� Record keeping requirements. Records should be maintained to document the day to day activities at thesanitary landfill and to provide regulatory agency reporting data.

In addition, operating and design parameters should be establish for the following:

� The maximum distance a heavy refuse truck can travel from an all-weather service road must be deter-mined. A good design will minimize the distance a truck must travel over refuse. In wet northern climatesthe maximum desirable distance a refuse truck should travel over a refuse-filled area is 800 ft (245 m).

� The desired width of the landfill’s working face must be determined. A narrow working face minimizesthe daily cover required but could also result in excessively long waiting times for the refuse vehicles. Ifthe mix of unloading vehicles contains more than 30% car and pickups, then serious consideration shouldbe given to constructing an on-site transfer station for these hand-unloaded vehicles. Generally, the nar-rowest possible working face is the least costly to operate and control.

� Landfilled solid waste must be covered with a minimum of 6 in (15 cm) of earthen material at the end ofeach operating day, or at more frequent intervals if necessary, to control disease vectors, fires, odors,blowing litter, and scavenging. Alternative materials of an alternative thickness may be acceptable ifdemonstrated that the alternative material and thickness satisfies the daily cover objectives.

� The amount of daily cover must be calculated. The hauling of this cover is an important cost element inthe landfill operation. The daily surface area of cover required is calculated by multiplying the width ofthe working face times the length of the compacted refuse. Compaction equipment requires a length atleast equal to 10 times its wheelbase for proper refuse spreading and compacting.

� The cost of moving this daily cover with the operator’s available equipment must be calculated. This costis computed by multiplying the hourly rate for the cover equipment (operator wages included) times thenumber of hours required to place the cover. Hourly equipment rental rates are available from most heavyconstruction equipment dealers.

� An estimate of the in-place refuse density must be made. In well-run landfills using modern compactionequipment, an average in-place density of 1100 lb of refuse per cubic yard (650 kg/m3) of landfill volume

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is a reasonable estimate. Typically, the in-place density of refuse will range from 950 to 1500 lb of refuseper cubic yard (560 to 890 kg/m3).

� Storm drainage facilities should, as a minimum, be designed to accommodate a 25-year storm. An opti-mum design would not allow any rainwaters to flow across the landfill work area. Storm drainage for theactual landfill work areas must be designed so that rainwater in contact with the working face and thus re-quiring possible containment and treatment is not mixed with other stormwaters. Stormwaters falling ondisturbed but unfilled areas of the landfill should be kept out of the leachate collection system but shouldpass through sedimentation controls prior to exiting the site.

� Landfill costs are optimized when the site is designed to accommodate as great a depth of solid waste aspossible. This is especially true when expensive liners and leachate collection systems are required.Therefore, the maximum height that the landfill may go above and below natural ground must be estab-lished. The below-ground distance is usually determined by the proximity of groundwater or the excava-tion limitations of available equipment. The above-ground height can often be a volatile social–politicalissue. It is important that the environmental and cost advantages of placing greater depths of refuse ver-sus using more surface area be clearly documented. When going above ground with a landfill, the com-pleted side slopes should be no steeper than 1 unit vertical to 4 units horizontal. This slope will allowgrass mower operation and it will not erode as much as steeper slopes.

� All landfills must allocate some capital for on-site roads, fences, maintenance buildings, utilities, and agate house. The following discusses some minimum requirements for each of these capital items.

Landfill roads must be of sufficient width and strength to handle the anticipated traffic in all kinds ofweather. The biggest item that must be determined is the number of refuse compaction vehicles that will beusing the site. These heavy vehicles, when loaded, meet or exceed allowable axle loads. Most state roadagencies have established design standards for roads that would be similar in heavy truck traffic to the land-fills. These standards should be used for determining the road’s structural section. If design standards arenot available locally, The Asphalt Institute has an excellent design manual entitled Thickness Design-As-phalt Pavement Structures for Highways and Streets (57). It is always a good idea to pave the entrance road,at least to the gatehouse. This paved road, in addition to its strength and smooth riding surface, will also pro-vide a cleaning area for mud-caked tires on the vehicles exiting the site.

The landfill’s entrance and those portions of the site fronting public roads should be fenced. Fencing theremainder of the site will depend on terrain, vegetation, population density in the area, and state and localrequirements. In heavily populated areas or along high-traffic roads, it is a prudent practice to fence the en-tire work area.

Landfill equipment costs are the largest single operating cost. Proper maintenance of this equipment re-quires frequent oil changes, cleaning of the radiators, and cleaning of areas on the machines where oil andflammable debris can accumulate. During winter months and inclement weather, routine maintenance is bestperformed inside a heated building. A maintenance building helps reduce equipment costs.

An investigation of the availability of utilities is an important first step in any design. Typical questionsthat must be answered are

� Is natural gas available for building heat?� Does the available electrical system meet the electrical needs of the pumps, compressors, and other

equipment needed to operate the landfill? If the proper voltage is not available, what is the cost to deliverit to the site?

� Is a public water system available or must on-site wells be installed?� Is a public wastewater system available for domestic sewage or must a septic system be installed?� Can the public wastewater system accept the high-strength leachate that might be discharged, and if so

where is the outlet point?

The type of gatehouse will be dictated by local conditions. The gatehouse should be located at least 200

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ft (60 m) from the entrance. This will allow the backup of vehicles during rush hours without spilling outonto the public road.

The need for scales is a much discussed topic. For large sites with over seven years life, the cost of a scalebecomes insignificant. The following are several advantages of scales:

1. Scales provide an equitable method for pricing disposal fees. 2. Tons is the accepted unit of measure for solid waste engineering. By weighing the refuse received, the op-

erator is establishing a valuable data base. This data base can be used to compute in-place density andcollection truck performance.

Daily Cover Requirements. Daily or more frequent placement of 6 in (15 cm) of cover is necessary tocontrol disease vectors, fires, odors, blowing litter, and scavenging. This temporary cover controls diseasevectors (birds, insects, or rodents that represent the principal transmission pathway of a human disease) bypreventing egress from the waste and by preventing access to breeding environments or food sources. Cover-ing also reduces exposure of combustible materials to ignition sources and may reduce the spread of fire ifthe disposed waste bums. Odors and blowing litter are reduced by eliminating the direct contact of wind anddisposed waste. Similarly, scavenging is reduced by removing the waste from observation. The depth of cov-er and/or frequency should be increase to insure these objectives are satisfied (94).

Alternative materials of an alternative thickness may be acceptable if demonstrated that the alternativematerial and thickness satisfies the daily cover objectives. Demonstrations can be conducted in a variety ofways. For example, demonstrating alternative covers may be accomplished by: side by side (six inches ofearthen materials and alternative cover) test pads, full-scale demonstration, and short-term full-scale tests.

Alternative daily cover materials may include indigenous materials or commercially available materials.Indigenous materials are those materials that would be disposed as waste; therefore, using these materials isan efficient use of landfill space. Examples of indigenous materials include (94): ash from municipal wastecombustors and utility companies; compost-based material; sludge-based material, such as sludge treatedwith lime and mixed with ash or soil; construction and demolition debris (which has been processed to forma slurry); and shredded automobile tires.

Commercially developed alternative daily covers are available. Some of these alternative materials re-quire specially designed application equipment, while others use equipment generally available at mostlandfills. Examples of the types of commercially available daily cover materials are (94):

� Foam that usually is sprayed on the working face at the end of the day� Geosynthetic products such as a tarp or fabric panel that is applied at the end of the working day and re-

moved at the beginning of the following working day� Slurry products (e.g., fibers from recycled newspaper, wood chip slurry, clay slurry).

Liner Requirements. The prevailing theory of sanitary landfill design is to prevent water from enteringthe refuse mass and also to prevent leachate (water in contact with refuse) from entering ground or surfacewaters. Downward percolation of the leachate can be retarded if the in situ soils for the landfill are clayswith a thickness and permeability sufficient to satisfy regulatory requirements. If the in situ soils do not sat-isfy the requirements, a liner must be designed. Liners may be constructed of either clay, synthetic fabrics(geomembranes), or soil-additive mixtures. Specific comments on each type of liner follow.

Clay liners are constructed by spreading the clay in thin layers and compacting each layer to a predeter-mined density. The minimum total thickness of these liners is 2 ft (0.6 m). The minimum thickness shouldbe adequate to obtain adequate compaction to meet the hydraulic conductivity requirement, to minimize thenumber of breaks or imperfections through the entire liner thickness that could allow leachate migration,and to inhibit hydraulic short-circuiting of the entire clay liner layer.

The suitability of a clay for use as a liner is determined by laboratory tests on the clay. These tests are hy-drometer, Atterberg limits, permeability, and density. The hydrometer and Atterberg limits tests will serve toidentify and classify the soil. Permeability tests are run on disturbed samples of the clay. For each test, the

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density of the clay must be determined. The minimum clay liner density will be the density occurring at thedesired permeability.

Clay liners are relatively inexpensive to install if the clay is within a reasonable haul distance. The linerprovides excellent protection against leachate migration. However, clay liners cannot be installed during wetor freezing weather. In northern climates, the days available for clay placement are limited.

Because of their inherent impermeability, the use of geomembranes in landfill liner construction has in-creased. Geomembranes are relatively thin sheets of flexible thermoplastic or thermoset polymeric materi-als. The design of the side slope, specifically the friction between natural soils and geosynthetics, is criticaland requires careful review.

Geomembranes are made of one or more polymers along with a variety of other ingredients. The poly-mers include a wide range of plastics and rubbers differing in properties such as chemical resistance and ba-sic composition (58, 59). The polymeric materials may be categorized as follows:

� Thermoplastics, such as polyvinyl chloride (PVC)� Crystalline thermoplastics, such as high-density polyethylene (HDPE), very-low-density polyethylene

(VLDPE), and linear low-density polyethylene (LLDPE)� Thermoplastic elastomers, such as chlorinated polyethylene (CPE) and chlorosulfonated polyethylene

(CSPE)

The polymeric materials used most frequently as geomembranes are HIDPE, PVC, CSPE, and CPE. Thethicknesses of geomembranes range from 20 to 120 mil (58, 59). The recommended minimum thickness forall geomembranes is 30 mil, with the exception of HIPE, which must be at least 60 mil to allow for properseam welding (58).

Depending on the type of membrane, several bonding systems are available for the construction of bothfactory and field seams. Bonding methods include solvents, heat seals, heat guns, dielectric seaming, extru-sion welding, and hot wedge techniques. To ensure integrity of the seams, a geomembrane should be seamedusing the bonding system recommended by the manufacturer (60).

If locally available soils do not possess properties to achieve the specified hydraulic conductivity, soil ad-ditives can be used. Soil additives, such as bentonite or other clay materials, can decrease the hydraulic con-ductivity of the native soil (96).

Bentonite is a clay mineral (sodium-montmorillonite) and may be obtained in a dry, powdered form thatis relatively easy to blend with on-site soils. Bentonite expands when mixed with water (hydration). Thisproperty allows relatively small amounts of bentonite (5 to 10%) to be added to a noncohesive soil (sand) tomake it more cohesive (96).

The most common additive used to amend soils is sodium bentonite. The disadvantage of using sodiumbentonite includes its vulnerability to degradation as a result of contact with chemicals and waste leachates.Calcium bentonite and other materials, including lime, cement, and other clay minerals such as atapulgite,may be used as soil additives (60, 97).

The percent of additive that is added to the soil is determined by preparing several different mixtures andrunning permeability tests on these mixtures.

Like geomembrane liners, soil-additive liners must be placed on a clean, smooth surface. The desiredamount of additive is added to the soil surface and thoroughly mixed and compacted. The liner is placed inlifts, with each lift being no thicker than 6 in (15 cm). The total thickness of the soil-additive liner is usuallythe same as a clay liner. At least 1 ft (30 cm) of clean sand cover should be placed over the liner to protect itduring refuse filling.

When determining the liner best for the project, it is important that the following questions be answered:

1. Can the liner be placed on the desirable side slopes? 2. Is liner material delivery schedule reliable enough or must an inventory be kept on site?

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3. Can the liner be placed throughout the year or must sufficient liner be placed before the onset of winteror a rainy season?

4. Can the liner be placed with landfill personnel or must private contractors be retained?

All landfill liners must have leachate collection systems included in the design. The collection systemmust be designed to minimize the leachate hydraulic head on the liner. The liner design must also incorpo-rate provisions to isolate portions of the liner system in the unlikely event of a liner failure. This isolation isaccomplished by dividing the total liner system into a series of minisystems. Each minisystem has its ownliner and collection system with lined berms or cell walls preventing leachate flow between minisystems.

Leachate Collection and Treatment. Leachate is the liquid that has passed through or emerged from solidwaste and contains dissolved, suspended, or immiscible materials removed from the solid waste. The charac-teristics of leachate vary with the age of the landfill and the material placed in the landfill. Table 8.53 illus-trates some typical data on leachate composition. Since all sanitary landfills generate leachate and leachatehas very strong wastewater characteristics, collection and treatment of leachate is a necessary part of theoverall facility.

Rainwater falling directly on the landfill and off-site stormwater flowing across the site are the majorsource of water for leachate generation. Good design dictates that as much off-site stormwater as possible bediverted around the landfill. The critical periods for leachate generation occur during operation. The landfilldesign must incorporate features that will allow all water not in direct contact with uncovered refuse to flowoff the site and bypass the leachate collection system. It is important that the refuse mass be kept as dry aspossible. The placement of refuse in trapped stormwater must be avoided.

The function of the leachate collection system is to collect and convey leachate out of the landfill and tocontrol the depth of the leachate above the liner. The leachate collection system should be designed to meetthe regulatory performance standard of maintaining less than 12 in (30 cm) depth of leachate, or “head,”

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TABLE 8.53 Composition of Leachate from Landfills (61)

Constituent* Range Typical

BOD5 (5-day biochemical oxygen demand) 2000–30,000 10,000 TOC (total organic carbon) 1500–20,000 6,000 COD (chemical oxygen demand) 3000–45,000 18,000 Total suspended solids 200–1000 500 Organic nitrogen 10–600 200 Ammonia nitrogen 10–800 200 Nitrate 5–40 25 Total phosphorus 1–70 30 Ortho phosphorus 1–50 20 Alkalinity as CaCO3 1,000–10,000 3,000 pH 5.3–8.5 6 Total hardness as CaCO3 300–10,000 3,500 Calcium 200–3,000 1,000 Magnesium 50–1,500 250 Potassium 200–2,000 300 Sodium 200–2,000 500 Chloride 100–3,000 500 Sulfate 100–1,500 300 Total iron 50–600 60

*All units in milligrams per liter except pH.

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above the liner. This head allowance is a design standard and may be exceeded for relatively short periods oftime during the active life of the landfill.

Leachate is generally collected from the landfill through sand drainage layers, synthetic drainage nets, orgranular drainage layers with perforated plastic collection pipes, and is then removed through sumps orgravity drain carrier pipes. The principal components of the leachate collection system are (96):

� A low-permeability base. The bottom liner should have a minimum slope of 2% for effective gravitydrainage through the entire operating and postclosure period. Settlement estimates of the foundation soilsshould set this 2% grade as a postsettlement design objective (99).

� A high-permeability drainage layer. The drainage layer is constructed of either natural granular materials(sand and gravel) or synthetic drainage material (geonet) placed directly on the bottom liner, or on a pro-tective bedding layer (e.g., geofabric) directly overlying the liner.

� Perforated leachate collection pipes. The collection pipes are located within the high-permeabilitydrainage layer to collect leachate and carry it rapidly to a sump or collection header pipe.

� Protective filter layer. If necessary, a filter layer is placed over the high-permeability drainage material toprevent physical clogging of the material by fine-grained material.

� Leachate removal system. Collection sumps or header pipe system are installed at low points so leachatecan be removed for holding/treatment.

The relative performance of design options for the leachate collection system and liner layers may becompared and evaluated by the HELP (Hydrologic Evaluation of Landfill Performance) model (100, 101).The HELP model was developed by the U.S. Army Corps of Engineers for the U.S. Environmental Protec-tion Agency and is widely used for evaluating expected hydraulic performance of landfill cover/liner sys-tems (96).

The HELP program calculates daily, average, and peak estimates of water movement across, into,through, and out of landfills. The input parameters include soil properties, precipitation and other climato-logical data, vegetation type, and landfill design information. Default climatologic and soil data are avail-able but should be verified as reasonable for the site modeled. Outputs from the model include precipitation,run-off percolation through the base of each cover layer subprofile, evapotranspiration, and lateral drainagefrom each profile. A summary of outputs is produced, including average monthly totals, average annual to-tals, and peak daily values for several simulation variables (96).

The leachate collection systems must be designed to support heavy equipment loads during operationsand the superimposed refuse load when the landfill has been completed. Figure 8.37 shows a typical planview for a small lined site. Figures 8.38 and 8.39 are sections through the landfill and illustrate the layout forthe leachate collection system.

Each lined cell has its own collection system, hydraulically independent from the other cells. In the eventof a cell liner failing, the other liners will be unaffected.

Leachate discharging from the collection system should be stored in a lagoon. The lagoon serves as anequalization and pumping basin and sampling and monitoring point. The lagoon must be designed to holdthe stormwaters from a 1-in (2.5-cm) 25-year storm plus the amount of leachate that would normally be gen-erated over the anticipated holding period of the lagoon. (In areas where the leachate must be trucked to adisposal point or held for spraying on top of the landfill, the lagoon should have at least a 90-day holding ca-pacity.) Leachate generation will be greatest during the operational period and will decrease once the site iscapped with an impermeable final cover. Some jurisdictions do not require the impermeable final cover if aleachate collection system is provided. Because the final amount and strength of the leachate cannot be de-termined, it is best to install an impermeable final cover.

The high strength of the leachate makes it difficult to treat by itself. Fortunately, if the wastewater andlandfill service areas are identical, the quantity of leachate is very small compared to the wastewater flow.[Leachate from a 15-year, 25-ft (7.5 m) deep landfill will generate less than 0.1% of the daily wastewaterflow.]

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8.121

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8.122

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8.123

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.39

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.

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If possible, it is best to discharge to an existing wastewater plant. However, it is important to understandfully the pretreatment requirements of the receiving plant. Several leachates investigated have exceeded thereceiving plants acceptable heavy metal concentrations and required pretreatment.

Recycling leachate through the existing landfill has been practiced. This has usually been accomplishedby spraying leachate over the top of the operating landfill. If done in hot weather, some evaporation is alsoachieved. Tittlebaum (60) has reported that raising the pH of the leachate prior to spraying has proved bene-ficial. A major problem to spraying is odor control. Recycling might be difficult to accomplish after the siteis closed, especially if the site is used for recreation.

Various methods of on-site leachate treatment are available and have been tested in the landfill industry.Among the physical/chemical treatment technologies in use are:

� Equalization. Equalization dampens variation in hydraulic and pollutant loadings, thereby reducingshock loads and increasing treatment facility performance.

� Neutralization. Neutralization dampens pH variations prior to treatment or discharge.� Coagulation/flocculation. Coagulation/flocculation provides pollutant removal through aggregation of

colloidal solids.� Gravity separation. Gravity-assisted separation allows suspended matter to become quiescent and settle

and free oils (lighter than water) to become quiescent and float.� Emulsion breaking. The addition of a deemulsifier, such as heat or acid, breaks down emulsions to pro-

duce a mixture of water and free oil and/or an oily floc.� Chemical precipitation. The addition of chemicals to wastewater converts soluble metal salts to insoluble

metal oxides, which are then removed by filtration.� Chemical oxidation/reduction. By chemical addition, the structure of pollutants are changed so as to dis-

infect, increase biodegradation and adsorption, or convert pollutants to end products.� Air/steam stripping. Air/steam stripping involves the removal of pollutants from wastewater by the trans-

fer of volatile compounds from the liquid phase to a gas stream.� Flotation. Injection of fine air bubbles causes suspended solids to float to the surface where they are re-

moved by skimming.� Sand filtration. Monomedia or multimedia sand filtration involves a fixed (gravity or pressure) or moving

bed of porous media that traps and removes suspended solids from water passing though the media.� Ultrafiltration. Extremely fine grade filters are used to remove organic pollutants from wastewater ac-

cording to the organic molecule size.� Reverse osmosis. Reverse osmosis relies on differences in dissolved solids concentrations and selective

semipermeable membranes to allow for the concentration of dissolved inorganic pollutants.� Fabric filters. Fabric filters screen suspended matter by means of a cloth or paper barrier.� Carbon adsorption. In this process, wastewater is passed over a medium of activated carbon, which ad-

sorbs certain pollutants, primarily organics.� Ion exchange. Selected resins placed in contact with wastewater remove contaminants of similar charge.

Biological wastewater treatment technologies used in the landfill industry for treatment or pretreatmentof leachate include:

� Aerobic systems. Aerobic systems utilize an acclimated community of aerobic microorganisms to de-grade, coagulate, and remove organic and other contaminants.

� Activated sludge. Activated sludge is a continuous flow, aerobic biological treatment process that em-ploys suspended-growth aerobic microorganisms to biodegrade organic contaminants.

� Anaerobic systems. Anaerobic systems involve the conversion of organic matter in wastewater intomethane and carbon dioxide by anaerobic microorganisms.

� Facultative systems. Facultative systems stabilize wastes by incorporating a combination of aerobic,anaerobic, and facultative (thriving in either aerobic or anaerobic conditions) microorganisms.

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� Rotating biological contactors. Rotating biological contactors (RBCs) employ a fixed-film aerobic bio-logical system adhering to a rigid medium mounted on a horizontal, rotating shaft.

� Trickling filters. In this process, wastewater passes over a structure packed with an inert medium (e.g.,rock, wood, plastic) coated with a biological film of attached microorganisms capable of absorbing anddegrading organic pollutants.

� Sequential batch reactors. A sequence of batch operations in a single reactor containing acclimated mi-croorganisms is used to degrade organic material. The batch process allows for equalization, aeration,and clarification in a single tank.

� Powdered activated carbon biological treatment. The addition of granular activated carbon to biologicaltreatment systems enhances the removal of certain organic pollutants.

� Nitrification systems. These systems use nitrifying bacteria to convert ammonia–nitrogen compounds toless toxic nitrate–nitrite compounds.

� Denitrification systems. These systems convert nitrate–nitrite to nitrogen gas under anoxic–anaerobicconditions.

� Land application. Spray irrigation or other techniques are used to apply the wastewater to the land fortreatment by a combination of biological, chemical, and physical processes.

� Wetlands treatment. These systems employ natural or man-made wetlands systems that treat wastewaterutilizing natural processes of sedimentation, adsorption, and organic degradation.

The treatment sequence employed at any particular facility may vary with the pollutant characteristics ofthe leachate generated at the landfill. The optimal treatment system at a facility depends upon many factors,including permit requirements, design considerations, landfill acceptance criteria, and management prac-tices. The EPAS new source performance standards (40CFR445) for treatment of leachate and other nonhaz-ardous waste landfill wastewaters before discharge to surface waters are presented in Table 8.54 (102–105)

Various forms of equalization and aerobic biological systems, including aerated lagoons, activated sludgesystems, and sequential batch reactors, are the most widely used treatment technology in the landfill indus-try. These biological systems generally utilize high retention times to enhance performance by reducingvariations in raw wastewater flow and pollutant loads.

Vent Landfill Gases. The two principal gases generated in a landfill are carbon dioxide (CO2) andmethane (CH4). CO2 is generated during the early (aerobic) stages of landfill life. CH4 is generated duringthe later stages of landfill life (anaerobic). Figure 8.40 illustrates gas production and composition from anexperimental landfill.

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Table 8.54 Nonhazardous Leachate Treatment Limitations for Discharges to Surface Waters

Maximum for anyPollutant or pollutant property one day Monthly average

BOD5, mg/L 160 40 TSS, mg/L 89 27 Ammonia, mg/L 5.9 2.5 Zinc, mg/L 0.20 0.11 Alpha Terpineol, mg/L 0.059 0.029 Benzoic Acid, mg/L 0.23 0.13 p-Cresol, mg/L 0.046 0.026 Phenol, mg/L 0.045 0.026 Toluene, mg/L 0.080 0.026 pH, units (range) 6.0–9.0 6.0–9.0

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The explosive CH4 is the most worrisome for landfill designers. The lighter-than-air CH4 will rise until itmeets an impermeable barrier. It is then deflected laterally until an escape is found. There are many ways tovent a new landfill. The best vents are probably the ones that are incorporated with the leachate collectionmanholes as shown in Figure 8.38. These manholes are on the perimeter of the site and have the best chanceof being preserved during operations. Placing vents near the high point of the landfill after completion butprior to final cover placement is another method with a good chance of survival.

Prepare Preliminary Site Layouts. The last steps in the preliminary design stage are the determination ofthe operating methods, preparation of alternative site layouts, computation of earthwork quantities, and thepreparation of the preliminary cost estimates for each alternative. The following is a summary of the re-quired tasks.

Preliminary layouts for the site are most easily studied after plotting on the site topographic map. A mapscale of 1:1200 (1 in = 100 ft) and a 2-ft contour interval is standard. The map used in the hydrogeologicstudy should be satisfactory.

Since earthwork computations will be required, cross sections through the site must be plotted. A gridsystem should be established and cross sections plotted every 100 ft (30 m) along the principal axis of thesite and at 200-ft (60-m) intervals along the minor axis of the site. A good cross section scale is 1:600 (1 in= 50 ft) horizontal and 1:60 (1 in = 5 ft) vertical.

The work area for the site must be defined. The desired setbacks from roads and other physical barriersmust be plotted. The designer must also determine the desirable methods of operation. The slope of the ex-isting ground, direction of prevailing wind, amount of topsoil and dirt to be stockpiled and the equipment tobe used will all influence operating methods.

8.126 CHAPTER EIGHT

FIGURE 40 Gas production from an experimental landfill (62).

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Special attention must be given to the movement of all equipment on the site. Landfills are subjected toheavy traffic peaking, i.e., all refuse trucks will arrive at the site at the same time. Paved roads must be laidout so that the destructive tracked landfill equipment does not have to cross them.

The movement of cover material is one of the landfill’s principal activities. The site layout must providefor efficient movement. Double handling of excavated material must be minimized. In northern climates,consideration must be given to preventing the cover material from freezing. During freezing weather, themovement of heavy equipment over cover material will drive frost deeper into it. The unfortunate result isthe impossible task of excavating the frozen cover material.

Figure 8.39 shows a combination of the so-called trench and area fill methods. The trench fill methodconsists of excavating a trench (cell) below the ground surface. The trench is filled by depositing refuse atthe top of the trench and spreading it down a ramp to the bottom. The trench can be excavated usingdraglines or scrapers. The excavated earth can be stockpiled on the adjoining trench for later use as dailycover. The trench method leaves a wedge of undisturbed earth between trenches that serves to isolate eachtrench. The amount of cover material needed for trench fills is less than other operating methods becauseonly the top of the refuse must be covered. A disadvantage of the trench fill is the space taken up by thewedge between trenches.

The top portion of the landfill section illustrated in Figure 8.39 is an area fill operation. Area fill opera-tions take place on large open areas. The refuse is placed on top of the area and spread and compacted. Dai-ly cover is transported to the area, usually by scraper. Landfill operations above ground are normally areafills.

The locations of the active fill areas, roads, fences, buildings, and other physical features must be plottedon the site topographic map. The proposed top and bottom elevations of the fill are plotted on the cross-section sheets. These elevations should be plotted on both the cross sections drawn along the principal axisat 100-ft (30-m) intervals and the minor axis at 200-ft (60-m) intervals. The cross sections are used to com-pute earthwork quantities and to layout leachate facilities.

The volumes of earth to be excavated can be computed by the average end area method. The formula forvolumes by this method is

V = L

where V = volume (ft3 or m3) between sections A1 and A2

L = length (ft or m) between sections A1 and A2

A = area, ft2 or m2

The amount of earth excavated must be balanced with the needs of the landfill for daily and final cover.In addition, the dirt excavation operation must be evaluated to determine if it can keep ahead of refuse fill-ing. Typical earth-moving operations for which quantities must be computed are

1. Stripping and stockpiling topsoil for future use 2. Excavating trenches or portions of the area fill to the desired grade 3. The placement and compaction of a clay or soil-additive liner 4. The placement of the earth cushion over any liner 5. The hauling and placement of daily cover 6. The excavation and placement of the final cover 7. The placement of topsoil over the final cover

Estimates of the development cost must be prepared. Typical line items in a cost estimate are

1. Site clearing 2. Strip and stockpile topsoil

A1 + A2�

2

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3. Site fencing 4. Utilities 5. Gate house 6. Scale 7. Maintenance building 8. Leachate lagoon 9. Leachate collection system

10. Trench excavation (six months’ needs) 11. Liner installation (six months’ needs) 12. Earth cushion over liner 13. On-site roads, culverts, and berms 14. Off-site road improvements 15. Land purchase 16. Engineering 17. Equipment

An important part of any estimate is the determination of the cash-flow requirements for site develop-ment. This is especially true in areas where liner placement might be delayed for four months because of in-clement weather. The establishment of a critical-path diagram for site development will be of great assis-tance in any cash-flow analysis.

Final Design. The final design documents are used to obtain a permit from the regulatory agency and de-velop the site. These documents must be sufficiently detailed to satisfy both requirements. As-built changesthat occur during the operating life of the site are also recorded on these documents.

Prepare Plans for State Regulatory Agency. The plans submitted for a permit or license must show in suf-ficient detail the design for the landfill. Most states have specific submittal requirements. As a minimum thesubmittal should include

� The soils and hydrogeologic study.� The landfill layout, clearly showing the location of fill areas, storm drainage, roads, fences, buildings,

leachate facilities and borrow areas. This layout is plotted on the site topographic map.� Sufficient cross sections to show the bottom and top elevations of the fill, liner systems, the finish side

slopes, and the location of any leachate facilities.� Plan sheets detailing the groundwater monitoring systems, sedimentation control structures, leachate sys-

tems, roads, and other facilities to be constructed on the site.� An operating plan for the site should be prepared. This plan would outline the workers and equipment

needed to operate the site. The planned method for collecting and disposing of the leachate must be ad-dressed. The plan must also include the estimated daily tonnage, the estimated life of the site, and a state-ment on how the site will be monitored after closing.

Preparation of Construction Documents. Once a permit/license is obtained for a site, it is necessary tobuild it. Since most landfill operators do not have the capabilities to construct all portions of the site, itmight be necessary to prepare plans and specifications for the various individual projects. Separate contractdocuments might be prepared for

� Fences� Roads� Buildings� Liner systems� Trench excavation� Equipment

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� Periodic testing of groundwater wells� Scales

Much time and effort will be saved if the designer is able to use sheets of the plans submitted to the regu-latory agency for the contract project plans.

Like all civil engineering projects, a permanent system of control points must be established in the field.These control points will be used during the construction and operation of the landfill for horizontal layout.A series of benchmarks must also be established around the site.

Prepare Final Cost Estimate. Table 8.55 is a typical final cost estimate for a sanitary landfill. Item 1 ofthe final estimate lists the development costs for the landfill. These costs will be incurred prior to the open-ing of the site. Normally, the money for this work is borrowed at current interest rates. This money will berepaid over the operating life of the site. Included in the estimate is an item for purchase of a truck and tankto haul leachate from the holding lagoon to a nearby treatment plant.

Item II of the final estimate is an estimate of the annual operating costs for the landfill. Included in thisestimate is an item for the cost of final cover and perpetual care of the facility. These costs must be collect-ed prior to the closing of the site.

Item III of the final estimate states the unit cost of operating the site.

Landfill Closure. Key objectives for landfill closure are establishment of a low-maintenance cover sys-tems and minimization of the infiltration of precipitation into the waste. Landfill closure technology, design,and maintenance procedures continue to evolve as new geosynthetic materials are developed and as perfor-mance history is evaluated for the relatively small number of landfills that have been closed using modernprocedures and materials (94).

At a minimum, the closure system should include an erosion (vegetative) layer and an infiltration layer.For specific site conditions, closure system options are a biotic, drainage, and/or gas vent layer. The place-ment or vertical position of these layers is presented in Table 8-56.

The closure cover system should be designed to minimize infiltration and erosion and be designed andconstructed to:

� Minimize infiltration by the use of an infiltration layer that contains a minimum of 18 in (45 cm) of anearthen material

� Minimize erosion of the final cover (top layer) by the use of an erosion layer that contains a minimum 6in (15 cm) of earthen material that is capable of sustaining native plant growth

� Control permeability to less than or equal to the permeability of any bottom liner system or natural sub-soils present, or a permeability no greater than 1 × 10–5 cm/sec, whichever is less.

Other important technical issues in design development include the:

� Degree and rate of postclosure settlement and stresses imposed on soil and/or membrane liner compo-nents

� Long-term durability and survivability of cover system� Long-term waste decomposition and management of landfill leachate and gases� Environmental performance of the combined final cover system and bottom liner

Erosion Layer. The thickness of the erosion layer is influenced by depth of frost penetration and erosionpotential but in no case be less than. This layer also is referred to as the vegetation layer. Vegetative coversare advantageous as they improve the appearance of the site, control erosion of the final cover system, andshould require only minimal maintenance. The vegetation component of the erosion layer should have thefollowing specifications and characteristics (106):

� Locally adapted perennial plants that are resistant to drought and temperature extremes� Roots that will not disrupt the low-permeability layer

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8.130

TABLE 8.55 Sanitary Landfill Final Cost Estimate

Estimated quantity Unit used Unit price, $___________________ ___________________ ___________________Customary Customary Customary

Work item units SI units units SI units units SI units Amount, $

Item I: Development costs

Install monitor wells 200 60.96 Foot Meter 14. 50 47.57 2,900.00Install sediment traps 3 3 Each Each 300.00 300.00 900.00Clear site 36.6 14.8 Acre Hectare 1,450.00 3,585 81 53,070.00Excavate overburden 120,000 91,752 Cubic yard Cubic .95 1.24 114,000.00

meterMuck excavation 5,000 3,823 Cubic yard Cubic 2.50 3 27 12,500.00

meterConstruct earth berms 10,000 7,646 Cubic yard Cubic 0.85 1.11 8,500.00

meterInstall leachate manhole 7 7 Each Each 400.00 400.00 2,800.00Install sanitary manhole 5 5 Each Each 600.00 600.00 3,000.00Install leachate sewer 1,610 490.728 Foot Meter 8.75 28.71 14,087.50Install sanitary sewer 500 152.4 Foot Meter 20.00 65.62 10,000.00Excavate leachate 4,300 3,287.78 Cubic yard Cubic 1.10 1.44 4,730.00

lagoon meterInstall lagoon PVC liner 27,216 2,528.3664 Square foot Square 0.41 4.41 11,158.56

meterFence site 4,770 1,453.896 Foot Meter 2.75 9.02 13,117.50Grade entrance road 700 213.36 Foot Meter 8.50 27 89 5,950.00Place asphalt base 1,000 907.18 Ton Metric ton 19.00 2094 19,000.00Place asphalt surface 500 453.59 Ton Metric ton 25.00 27 56 12,500.00On-site transfer station 110 33.528 Foot Meter 150.00 492 13 16,500.00Gate house I I Lump sum Lump sum 15,000.00 15,000.00 15,000.00Purchase and install I I Lump sum Lump sum 35,000.00 35,000.00 35,000.00

scaleMaintenance building I I Lump sum Lump sum 120,000.00 120,000.00 120,000.00On-site service road 500 152.4 Foot Meter 6.00 19.69 3,000.00PVC liner anchor trench 3,200 975.36 Foot Meter 0.50 1.64 1,600.00Place PVC liner 478,125 44,417.8125 Square foot Square 0.36 3.88 172,125.00

meterPlace liner sand cushion 478,125 44,417.8125 Square foot Square 0.05 0.54 23,906.25

meterPlace topsoil and seed 2 0.81 Acre Hectare 780.00 1,925.93 1,560.00Install waterwell 1 1 Lump sum Lump sum 3,000.00 3,00000 3,000.00Install septic tank I I Lump sum Lump sum 3,200.00 3,20000 3,200.00Purchase 40CY 3 3 Each Each 3,000.00 3,000.00 9,000.00

containersPurchase leachate truck I I Each Each 53,000.00 53,000.00 53,000.00Purchase leachate tank 1 1 Each Each 4,500.00 4,500.00 4,500.00Purchase land 82.89 33.55 Acre Hectare 1,495.96 3,695.98 124,000.00Pave public approach 5,820 1,773.936 Foot Meter 9.50 31.17 55,290.00

road_________

Total development cost 928,894.81_________

Estimated life of site is ten (10) yearsCurrent prevailing interest rate is 9%Estimated annual development cost is $144,900 144,900.00_________

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� The ability to thrive in low-nutrient soil with minimum nutrient addition� Sufficient plant density to minimize cover soil erosion� The ability to survive and function with little or no maintenance� A variety of plant species sufficient to continue to achieve these characteristics and specifications over

time

Deep-rooted shrubs and trees are generally inappropriate because the root systems may penetrate the in-filtration layer, creating preferential pathways of percolation. Plant species with fibrous or branching root

SOLID WASTE 8.131

TABLE 8.55 Sanitary Landfill Final Cost Estimate (continued)

Estimated quantity Unit used Unit price, $___________________ ___________________ ___________________Customary Customary Customary

Work item units SI units units SI units units SI units Amount, $

Item II: Annual operating costs

A. Personnel Equipment operators (3) 85,000.00 Gatekeeper 20,000.00Laborer (1) 22,000.00Engineering time 28,000.00_________

Total annual personnel cost 155,000.00

B. EquipmentLandfill compactor 75,000.00Crawler loader 35,000.00Earthmover 85,000.00Leachate truck 15,000.00_________

Total annual equipment cost 210,000.00

C. Miscellaneous Costs Utilities, tools, etc. 22,00000Liners: installed 75,00000Leachate treatment 3,500 00Other Contracts 15,000.00Final cover and perpetual care 52,000.00_________

Total annual miscellaneous cost 167,500.00_________

D. Total annual operating cost 532,500.00

Item III: Unit cost

Annual development cost 144,900.00 Annual operating Cost 532,500.00

Total annual Cost 677,400.00Estimated tons per 300 272.154

operating day Operating days per year 306 306Annual tons of solid 91,800 83,279.124

waste Cost per ton (customary) 7.38

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systems are best suited for this application and may include a large variety of grasses and shallow-rootedplants. The timing of seeding (spring or fall in most climates) is critical to successful germination and estab-lishment of the vegetative cover (106). Temporary vegetative covers may be grown from fast-growing seedstock, such as ryegrass.

Selection of the soil for the vegetative cover (erosion layer) should include consideration of soil type, nu-trient and pH levels, climate, species of the vegetation selected, mulching, and seeding time. Loamy soilswith a sufficient organic content generally are preferred. The balance of clay, silt, and sand in loamy soilsprovides an environment conducive to seed germination and root growth (107).

In some cases, the erosion layer may be asphalt or concrete. These materials promote run-off with negli-gible erosion; however, they will deteriorate due to thermal expansion and deformation caused by subsi-dence. In other cases, crushed rock may be spread over the landfill cover in areas where weather conditions,such as wind, heavy rain, or temperature extremes, would be expected to cause deterioration of vegetativecovers (106).

Biotic Layer. Deep plant roots or burrowing animals (collectively called biointruders) may disrupt the

8.132 CHAPTER EIGHT

TABLE 8.56 Solid Waste Landfill Closure System Layers

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drainage and the low hydraulic conductivity layers, thereby interfering with the drainage capability of thelayers. A 30 cm (12 in) biotic barrier of cobbles directly beneath the erosion layer may stop the penetrationof some deep-rooted plants and the invasion of burrowing animals. Geosynthetic products that incorporate atime-released herbicide into the matrix or on the surface of the polymer also may be used to retard plantroots. The longevity of these products requires evaluation if the cover system is to serve for longer than 30 to50 years (108).

Drainage Layer. The drainage layer in a final cover system redirects percolating water that has infiltratedthrough the erosion layer after surface mn-off and evapotranspiration losses. By removing water in contactwith the low-permeability layer, the potential for leachate generation is diminished. Caution should be takenwhen using a drainage layer because this layer may prematurely draw moisture from the erosion layer that isneeded to sustain vegetation.

A permeable drainage layer, constructed of soil or geosynthetic drainage material, may be constructedbetween the erosion layer and the underlying infiltration layer. If a drainage layer is used, a filter layer, com-posed of either a low-nutrient soil or geosynthetic material, may be placed between the drainage layer andthe cover soil to help minimize clogging of the drainage layer by root systems or soil particles. With soil orgeosynthetic material, the filter layer should be at least 12 in (30 cm) thick with a hydraulic conductivity inthe range of 1 × 10.2–2cm/sec to 1 × 10–3 cm/sec.

When granular drainage layer material is used, the filter layer should be sloped at least 3% at the bottomof the layer. Greater thickness and/or slope may be necessary to provide sufficient drainage flow as deter-mined by site-specific modeling (106). Granular drainage material will vary from site to site depending onthe type of material that is locally available and economical to use. Typically, the material should be nocoarser than 0.3 75 inch (0.95 cm), classified according to the Universal Soil Classification System (USCS)as type SP, smooth and rounded, and free of debris that could damage an underlying membrane (106).

When geosynthetic materials are used, the filter layer (preferably a nonwoven needle-punch fabric)should be placed above the geosynthetic material to minimize intrusion and clogging by roots or by soil ma-terial from the top layer.

Gas Vent Layer. Landfill gas collection systems serve to inhibit gas migration and typically are installeddirectly beneath the infiltration layer to collect combustible gases (methane) and other potentially harmfulgases (hydrogen sulfide) generated by microorganisms during biological decay of organic wastes. The col-lection system diverts these gases via a pipe system through the infiltration layer to the gas vent layer.

The gas vent layer is usually 12 in (30 cm) thick and should be located between the infiltration layer andthe waste layer. Materials used in construction of the gas vent layer should be medium- to coarse-grainedporous materials or geosynthetic materials with demonstrated equivalent performance. Venting to an exteri-or collection point may be provided by pipes configured laterally throughout the gas vent layer to channelthe gases to vertical risers or lateral headers. If vertical risers are used, their number should be minimized, asthey are frequently vandalized, and located at high points in the cross section (106). Since condensates willform within the gas collection pipes, the design should address drainage condensate drainage from lowpoints.

Infiltration Layer. The infiltration layer must be at least 18 in (45 cm) thick and consist of earthen mater-ial that has a hydraulic conductivity (coefficient of permeability) less than or equal to the hydraulic conduc-tivity of any bottom liner system or natural subsoils. If a membrane is in the bottom liner, there must be amembrane liner in the final cover to achieve a permeability that is less than or equal to the permeability ofthe bottom liner. For units that have a composite liner with a membrane liner, or naturally occurring soilswith very low permeability, such as 1 × 10–8 cm/sec, the infiltration layer also should include a syntheticmembrane as part of the final cover. Landfill units with poor or nonexistent bottom liners possessing hy-draulic conductivities greater than 1 × 10–5 cm/sec must have an infiltration layer that meets the 1 × 10–5

cm/sec minimum requirement. The infiltration layer is designed and constructed in a manner similar to that used for soil liners (107),

with the following differences:

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� Because the cover is generally not subject to large overburden loads, the issue of compressive stresses isless critical unless postclosure land use will entail construction of objects that exert large amounts of stress.

� The soil cover is subject to loadings from settlement of underlying materials. The extent of settlement an-ticipated should be evaluated and a closure and postclosure maintenance plan should be designed to com-pensate for the effects of settlement.

� Direct shear tests performed on construction materials should be conducted at lower shear stresses thanthose used for liner system designs.

Earthen material used in the construction of the infiltration layer should be free of rocks, debris, and othersuch material that may increase the hydraulic conductivity by promoting preferential flow paths. To facili-tate run-off while minimizing erosion, the surface of the compacted soil should have a minimum slope of3% and a maximum slope of 5% after allowance for settlement. Final side slopes, which often are greaterthan 5%, should be evaluated for erosion potential.

Membrane and clay layers should be placed below the maximum depth of frost penetration to avoidfreeze–thaw effects (106). Infiltration layers may be subject to desiccation, depending on climate and soilwater retention in the erosion layer. Fracturing and volumetric shrinking of the clay due to water loss may in-crease the hydraulic conductivity of the infiltration layer.

When a membrane is used as an infiltration layer, the membrane should be at least 20 mils in thickness.Some membrane materials may need to be a greater thickness; for example, a minimum thickness of 60 milsis recommended for HDPE because of the difficulties in making consistent field seams in thinner material.Increased thickness and tensile strengths may be necessary to prevent failure under stresses caused by con-struction and by waste settlement during the postclosure care period. The hydraulic performance, strength,resistance to sliding, and actual thickness of membranes should be considered.

Hydraulic Performance. The design of a final cover is site-specific and the relative performance of coverdesign options may be compared and evaluated by the HELP (Hydrologic Evaluation of Landfill Perfor-mance) model (100, 101). The HELP model may be used to estimate the hydraulic performance of the finalcover system design. Information provided by the HELP model includes surface run-off, duration and quan-tity of water storage within the erosion layer, and net infiltration through the cover system to evaluatewhether leachate will accumulate within the landfill (107).

Settlement and Subsidence. Waste decomposition and consolidation can cause excessive settlement andsubsidence of the final cover system. This can impair the integrity of the system and can result in:

� Ponding of surface water on the erosion or infiltration layer� Interference with operation of the gas collection pipe system� Fracturing of low-permeability infiltration layers� Structural failure of membrane liners

The extent and rate of waste settlement is an estimate at best. Records of the type, quantity, and locationof waste materials disposed may be useful. Compacting the waste daily or landfilling baled waste will re-duce the settlement from consolidation.

Sliding Instability. The slope angle, slope length, and overlying soil load limit the stability of componentinterfaces (membrane with soil, geotextile, and geotextile/soil). Soil water pore pressures developed alonginterfaces also can dramatically reduce stability. Unstable slopes may require remedial measures to improvestability as a means of offsetting potential long-term maintenance costs.

LAND RECLAMATION

Using the principles and practices of sanitary landfill design, otherwise useless land may be reclaimed withsolid wastes without concern for development of health problems. In practice, land reclamation is most

8.134 CHAPTER EIGHT

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commonly associated with selection of a landfill site that will provide for treatment and disposal of solidwastes.

Land reclamation may also be achieved when the end products of solid waste treatment are applied topoor-quality or disturbed lands. Compost and the residue from incineration and other thermal processes maybe used. Soil conditioning is best achieved with the application of compost or codisposal with stabilizedwastewater sludges.

Composting

Composting is the biodegradation of the organic constituents in wastes (solid wastes and wastewatersludges). Through the microbial activity taking place during composting, organic matter is decomposed intoa stable, humus-like substance. At the same time the heat produced can result in pathogen destruction. Com-posting is an ancient practice whereby farmers have converted organic wastes into soil amendments. Theseamendments were used to stabilize soils from erosion, provide nutrients, and replenish depleted organic mat-ter that was lost through intensive farming.

Composting of organic wastes and residues remained more of an art than a science until about 50 yearsago. Until this period, there were several developments of mechanical or intensive systems in Europe, suchas the Itano process in 1928, Beccari in 1931, and VAM in 1932. The Europeans continued to develop andinstall composting systems in Europe, South America, and Asia.

In 1974, the U.S. Department of Agriculture at Beltsville, Maryland, developed the “static pile” method.This method is currently being used by over 200 municipalities throughout the United States. Principal ex-amples of municipalities using this process are Durham, N.H., Bangor, Me., Portland, Me., Greenwich, Ct.,Camden, N.J., Philadelphia, Pa., Washington, D.C., Columbus, Ohio, and Windsor, Ontario, Canada.

Public Health Issues. The major public health issues associated with composting using solid wastesmixed with sewage sludge are pathogens, heavy metals, and odors.

Pathogens. Sewage sludge contains numerous pathogenic organisms. The four principal groups are bac-teria, viruses, protozoa and helminths. Many of these pathogens survive the wastewater treatment and duringthe process are deposited in the sludge.

The heat generated during composting, as a result of the activities of thermophilic organisms, is capableof killing all four groups of pathogens present in sewage sludge. The efficiency of pathogen destruction de-pends on the ability of the process to subject the sludge to uniformly high temperatures. Figure 8.41 showstypical temperatures achieved by the static aerated pile method. Similar temperatures can be achieved usingthe “within-vessel” method. Windrow composting generally results in lower temperatures.

In September 1979, the U.S. Environmental Protection Agency (EPA) issued regulations title “Criteriafor Classification of Solid Waste Disposal Facilities and Practices” (40 CFR Part 257). Two major processcategories were described: (1) processes to significantly reduce pathogens (PSRP), and (2) processes to fur-ther reduce pathogens (PFRP). PSRP were designed to result in 90% reduction of bacteria and viruses, andPFRP were aimed at reducing pathogens to negligible numbers.

Heavy Metals. Heavy metals are present in sewage sludge as a result of domestic and industrial dis-charges. Normally, domestic sewage contains very low levels of heavy metals. However, industrial dischargecan result in large quantities of various elements. The EPA is primarily concerned with those elements thatcan accumulate in food crops and are toxic to humans. The elements in sludge of greatest concern to humanhealth are cadmium, lead, arsenic, selenium, and mercury. Only cadmium is normally found in sewagesludge at levels that, when applied to soils, can be absorbed by plants and accumulate in edible parts, there-by entering the food chain.

Odors and Vectors. Sewage sludge contains volatile malodorous compounds, such as mercaptans, ska-toles, phenols, and hydrogen sulfide. During composting these compounds can produce unpleasant odors.

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Selection of the proper process, designing the facility, and managing it properly will result in an operationfree of malodors.

Vectors, such as rats and flies, are not found in sewage sludge composting operations. However, they maybe present in solid waste or garbage composting operations if the facility is not kept clean or managed prop-erly.

Biological Processes. Rapid biodegradation of the sludge and stabilization of the organic fraction as com-post depends mainly on the optimal interaction of temperature, oxygen, moisture, and the carbon/nitrogenratio.

Temperature. The microbial population changes continually during composting. As the temperaturechanges from ambient to mesophilic [104 to 113°F (40 to 45°C)] bacteria multiply and will reach levels ofseveral million per gram. If the heat produced during the biological activity is contained within the system,the temperature will progress from the mesophilic stage to the thermophilic stage. Mesophilic bacteria arethen replaced by thermophilic bacteria and other organisms, such as fungi and actinomycetes. Spore-form-ing bacteria as well as thermophilic actinomycetes are found at temperatures exceeding 158°F (70°C). Mostof the decomposition occurs in the thermophilic stage. As indicated earlier, the high temperatures above131°F (55°C) will effectively destroy most pathogens. After several weeks of composting, temperatures be-gin to decrease and eventually the thermophilic organisms give way to mesophilic ones.

Oxygen. Composting can occur under anaerobic (lack of oxygen) or aerobic conditions. Aerobic com-posting is not only faster but also does not produce malodors. Consequently, sewage sludge of solid waste

8.136 CHAPTER EIGHT

FIGURE 8.41 Maximum, minimum, and mean temperatures recorded during the composting of raw sludge by theBeltsville aerated pile method.

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composting is generally conducted under aerobic conditions. Aerobic conditions can be obtained by period-ically turning the mass of organic materials or by forcing air through the mass.

Oxygen levels between 5 and 15% are optimum. Oxygen levels below 5% may result in anaerobic condi-tions, which may slow the decomposition process and produce undesirable odors. Oxygen levels above 15%are indicative of excessive aeration and may result in loss of heat with slower decomposition and poorpathogen destruction.

Moisture. The optimum moisture levels for composting are between 50 and 60% by weight. Below 40%,decomposition is reduced, and above 60% the pore space necessary for aerobic composting is blocked bywater and anaerobic conditions can occur. Moisture also affects the processing and materials handling incomposting operations.

Carbon/Nitrogen Ratio. The carbon/nitrogen (C/N) ratio is one of the most important parameters affect-ing the role of decomposition of organic materials. Carbon is an energy source for the microorganisms,whereas nitrogen is necessary for protein synthesis. The ideal C/N ratio is between 25 and 30. Sewagesludge has a low C/N ratio (10 to 15), whereas solid waste has a high ratio (35 to 80). The use of bulking ma-terials with sewage sludge enhances the C/N ratio and the use of sewage sludge in combination with solidwaste improves the latter’s ratio.

Process Flow. There are two principal methods for aerobic composting. One method utilizes agitation orturning to induce aerobic conditions, whereas the other method employs mechanical means of forcing airinto the system. Figure 8.42 illustrates the process flow for the forced-air system. The agitated system, oftencharacterized by the windrow method, may not require a bulking material, or the bulking material may bethe dried compost.

In most cases sewage sludge is mixed with a bulking material, solid waste, or dried compost. Good, thor-ough mixing is essential. In the case of solid waste, grinding is essential in order to increase the surface areaand accelerate composting. The mixture is then placed in windrows, aerated piles, tanks, drums, or silos.The mixture is composted for 14 to 21 days. Depending on the facilities available, climate, and materialshandling, drying may or may not be needed. Drying facilitates screening, bulking material recovery, and sol-id waste separation. Screening also produces a uniform product for distribution and marketing.

Design Considerations. The selection of the composting system and the design of facilities depend onsuch aspects as the site of operations, climate, sludge and solid waste characteristics, and types of bulkingmaterial available.

Site. The two most important site considerations are location and land availability. Ideally, the compost-ing site should be located near a landfill or, in the case of sludge, adjacent to a wastewater treatment plant.This will reduce materials handling and facilitate operations. Proximity to residences or industry impact the

SOLID WASTE 8.137

FIGURE 8.42 Typical flow scheme of the composting operation.

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facilities required and costs. Nonintensive composting operations will generally require 1 acre (0.4 ha) ofland for 6 dry tons (5.4 metric tons) of sludge solids produced per day.

Climate. Cold ambient temperatures rarely affect the composting process unless the solid waste or sludgeis frozen. Precipitation primarily affects the mixing and screening phases of the process. In areas of highprecipitation or when it occurs in periods of high intensity, certain phases of the process should be undercover. Cover may be necessary to facilitate materials handling and ease of operation.

Sludge Characteristics. The percentage of solids impacts materials handling, bulking material selection,and the quantities needed. A sludge with 17% solids could require 3 parts of bulking agent to 1 part sludge,whereas at 25% solids, between 1.5 and 2 volumes of bulking material are needed.

The chemical characteristics of the sludge may also impact the bulking material selection and quantityused. Since sewage sludge often contains undesirable heavy metals, the bulking material dilutes the heavymetal content and reduces their availability to plants. Furthermore, the bulking material enhances the C/Nratio and often improves the product.

Solid Waste Characteristics. Solid waste consisting of paper, metals, garbage, glass, and plastics needs tobe ground. The finer the particle size, the faster and more efficient is the composting process. Separation ofmaterials reduces materials handling and results in a better product. Some processes first separate while oth-ers compost and then screen. The former is preferable.

Bulking Materials. Bulking materials serve three functions. They adjust the moisture content of the mass,adjust the C/N ratio, and provide structure or porosity to the mass. The most common bulking materials arewoodchips, sawdust, solid waste, shredded rubber tires, straw, leaves, brush chips, bark, and compost. Im-portant characteristics are particle size, moisture content, and adsorbancy. Bulking materials also affect theprocessing time, materials handling facilities, and product characteristics. High carbonaceous or celluliticmaterials generally require long curing periods and large particles need to be ground. In the static pile sys-tem, the preferred particle size is 1.5 to 3.0 in (3.8 to 7.6 cm).

Materials Handling. Municipal or industrial waste composting is essentially materials handling that mustbe cognizant of the biological requirements of the system. Mixing is best done by auger feed mixers or pug-mills. Conveyance to the specific composting site is accomplished with front-end loaders or conveyers. Re-moval of the material after composting depends on the method used and is done by front-end loaders,augers, and conveyers or digging equipment with elevators and conveyers. Numerous screens are available,from trommel to circular and horizontal shakers. Efficiency of the screen is primarily dependent on moisturecontent. Materials should not be screened if the moisture content exceeds 50%. Below 35% dust could be amajor problem. Screen size depends on the use of the product and the desirability of recovering the bulkingmaterial. Most composting operations use screens with 0.25 to 0.50 in (0.6 to 1.2 cm) mesh size. The prod-uct can be marketed in bulk or bagged.

Methods of Composting. There are three principal methods for composting solid waste and sewagesludge. These are windrow, static pile, and in-vessel.

Windrow Methods. The windrow system consists of mixing the sludge with a hulking material or previ-ously dried sludge and periodically turning the mass. Open windrow systems are often adequate for digestedsludge but are not suitable for undigested (raw) sludge. Odor problems can be very severe when raw sludgeis composted in the windrow.

Static Pile. This method was developed at the Department of Agriculture research station at Beltsville,Maryland, in 1975. Currently, this is the most widely used system in the United States. It consists of mixingthe sludge with a bulking material and placing the mixture over perforated pipe, i.e., an aerating system(Figure 8.43). The mixture is then insulated with screened or unscreened compost. Air is introduced into themixture through a blower system. Negative (suction) or positive (blowing) pressure is used and the rate ofairflow is controlled to maintain proper oxygen and temperature. Air is also used to increase moisture re-moval. After composting for 14 to 28 days, the material is usually screened to recover the bulking materialfor reuse. If drying is necessary, the material can be moved for a short period into a drying shed or left out-

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doors to cure and dry. It is preferable to dry and screen before curing because it reduces the material to behandled and provides for more efficient use of the bulking material.

Vertical Systems. Vertical enclosed systems are usually free flowing and consist of either round or squarebins, silos, or towers. Materials enter the top of the unit and are extracted at the bottom after composting fora given period of time. Hulking or carbonaceous material is added prior to entering the units. Aerobic condi-tions are maintained by forcing air through the sludge-bulking material mix from the bottom.

Off gases and odors can be controlled through removal and scrubbing. Temperature or other parametersare monitored through ports along the vertical wall. Moisture control is minimal because the air warms up asit progresses through the composting mixture. The heated air condenses at the top when it reaches the coolmass of new material.

Material is usually removed from the bins or silos after 14 days. At this time it undergoes curing. Curingcan take place in a second unit essentially designed in the same manner, or the compost can be cured in ashed or outdoors. The length of the curing will depend on the facilities, i.e., covered or not, and whether airis induced into the compost. Curing usually takes place for several weeks.

Agitated Bed. This enclosed horizontal system consists of an aerated bed contained in a horizontal bin. Apremixed bulking agent and sludge is introduced into the bin. The material in the bin can be periodicallyturned by mechanical means. Material is removed from the bin mechanically. Composting in bins occursover a period of 14 to 21 days. Curing takes place outside the bins either in an open or covered area and canbe accelerated by induced air.

Pugflow System. This horizontal type of system consists of a totally enclosed bin with a hydraulic ramthat moves the materials through the unit. Only pilot models are available at present. One unit in Europe isbeing used in an industrial application. There is no history of operations or data on process efficiency.

Rotating Drum. The system consists of a large-diameter rotating drum whereby sludge and bulking mate-rials are introduced and retained for short time periods (24 to 48 h). These systems have been primarily usedfor cocomposting of solid waste and sludge. Material ejected from the drum must be further composted andthen cured. These additional steps can take place in the windrow or static pile. A cocomposting system iscurrently being operated in the United States.

Economics. Composting cost at various communities indicates a wide range from $42 to $144 per dry ton(0.91 t) of sludge. The primary reason for this wide discrepancy is the result of improper site and facilities

SOLID WASTE 8.139

FIGURE 8.43 Composting with forced aeration.

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design, which result in excessive operating costs and rising costs of bulking material, The major factors af-fecting operating costs are bulking material cost, recovery product value and remuneration, and labor. Prop-er materials, handling, and equipment selection can result in efficient bulking material recovery and savingsof 15 to 25% of the other costs.

Economy of Scale. The effect of facility’s size on economics of composting is shown in Table 8.55. Thehypothetical numbers were developed for facilities from 1 to 10 dry tons/day (0.91 to 9.1 t/day) and at twodifferent sludge solids contents. Relatively little additional economy of scale will result above 10 drytons/day because equipment and labor are already fully utilized. Solids content affects sludge volume, bulk-ing agent mix ratio, land area, and materials handling costs. Equipment and labor are the two major items af-fected by the capacity of the facility.

The amount of land required for composting depends on the volume of sludge to be processed, solidscontent, and the system used. In its simplest form the static pile method requires approximately 1 acre for 5to 6 dry tons (0.4 ha for 4.5 to 5.5 t) of sludge produced. The windrow system requires considerably moreland since current windrow machines produce low [4-ft (1.2-m) high] windrows. The in-vessel system re-quires less space than the static pile method. Land costs do not greatly affect the overall cost. Compostingoperations should preferably be sited at the treatment plant.

Table 8.57 shows the sensitivity of capital and O&M costs for the simplified static pile system. This sys-tem does not include any buildings and assumes that the facility is part of the wastewater treatment plant op-eration. In cold, humid areas a drying building may be desirable.

The bulking agent and labor have the greatest impact on total costs for 5- and 10-dry-tons/day (4.5- and9.l-t/day) facilities. Equipment cost is much more sensitive to facility size. Sludge solids content affectslarger facilities to a much greater extent. The basic equipment requirements for composting facilities aremixing, materials handling, and screening equipment.

Product Market Value. Composting of sewage sludge results in a marketable end product. Depending onthe quality and uniformity of the material, the product has a value as an organic soil conditioner.

Table 8.58 shows the effect of price change or remuneration for the sale of compost on the cost of com-posting. In many cases the value of the product is equivalent to potting media or topsoil. The value of $115represents a hypothetical figure for a facility and its O&M cost assuming the compost is distributed at novalue. At $5 per cubic yard ($6.60/m3) the total costs (i.e., capital and O&M) would be reduced to $60 perdry ton ($66/t) of sludge. Thus, market development can result in substantial savings to a compost facility.

8.140 CHAPTER EIGHT

TABLE 8.57 Baseline Composting Costs, $/Dry Ton

Solids content

15% 25%

Solids content capacity, dry tons/day

Item 1 5 10 1 5 10

Site 10 10 10 5 5 5 Equipment 46 15 12 46 15 8 O&M labor 55 33 22 55 23 16 Bulking agent 40 40 40 18 18 18 Other 17 17 17 17 17 17 Total costs 168 116 101 141 78 64 Sale of compost 38 38 38 22 22 22 Net costs 130 78 63 119 56 42

Note: 1 dry ton (2000 lb) = 0.9 metric tons.

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Utilization and Marketing. Sludge compost is an excellent organic soil conditioner and low-analysis fer-tilizer that has been stabilized. It decomposes slowly and thus remains effective over a longer period of time.The addition of compost to soils improves their physical properties, as evidenced by increased water con-tent, increased water retention, enhanced aggregation, increased soil aeration, increased permeability, in-creased water infiltration, and decreased surface crusting.

Table 8.59 lists the various potential users. However, the main users are the private nonfood plant grow-ers, public agencies, and those involved in land reclamation. Use in agriculture or for food-chain crops canbe limited as a result of industrial contamination of the sludge. The private nonfood users are primarily in

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TABLE 8.58 Comparison of Compost Market Value and Cost of Composting

Estimated cost of composting,Market value of compost, $/yd3 $/dry ton sludge

Free 1151 1043 825 607 38

10 5

Note: 1 yd3 = 0.76 m3 1 dry ton = 0.9 t.

TABLE 8.59 Major Compost Uses by User Type

1. Private residential 4. a. Garden application for food 4. b. Nonfood applications 2. Private food 4. a. Field crops for food and feed 4. b. Garden crops for food and feed 4. c. Fruit trees 3. Private nonfood 4. a. Greenhouses 4. b. Nurseries 4. c. Golf courses 4. d. Landscape contractors 4. e. Turfgrass farmers 4. f. Industrial park grounds 4. g. Cemeteries 4. Public agencies 4. a. Public parks 4. b. Playgrounds 4. c. Roadsides and median strips 4. d. Military installations 4. e. Public grounds 5. Land reclamation 4. a. Landfill cover 4. b. Strip-mined lands 4. c. Sand and gravel pits

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the horticultural sector. Compost is excellent for revegetation of disturbed lands. Table 8.60 shows data fromthe analysis of some composts from sewage sludge.

Distribution of the compost is an essential phase of the marketing program. Identification of distributionalternatives can assure the municipality that the compost will be removed from the site in a timely fashionwithout interfering with treatment plant or composting operations. The distribution system also helps to as-sure users of obtaining material when needed.

Incineration

Disposal of solid waste is one of the most troublesome problems facing urbanized areas at this time.Changes in packaging practices and improvements in the general standard of living have resulted in signifi-cant increases in volumes of solid waste generated over the past 40 years. Additionally, disposal of haz-ardous chemicals from residential areas and commercial establishments has created concerns regarding dis-posal of the as-collected waste in landfills.

One method of alleviating these problems is to process the solid waste by incineration prior to land dis-posal of the residuals. In the incineration process, when properly designed and operated, the combustibleportion of the solid waste is burned, producing a residue essentially free of putrescible organic material.Benefits from this process include reduction of the volume of the solid waste and reduction in the potentialfor groundwater pollution from organic and hazardous constituents. Further, the potential exists for extrac-tion and reuse of mineral constituents in the residue, use of remaining material from the residue as a fill or

8.142 CHAPTER EIGHT

TABLE 8.60 Elemental Composition of Composted Sludge from Different States

Concentration*

NewMassachusetts Maryland Hampshire Utah Connecticut

Percent

Nitrogen, total (N) 0.8 1.6 0.6 1.7 1.2Phosphorus (P) 0.04 1.0 0.6 0.00 —Potassium (K) 0.5 0.2 0.1 0.00 —Calcium (Ca) 1.4 1.4 0.6 0.2 —Magnesium (Mg) 0.3 0.4 0.2 0.1 —Sodium (Na) 0.4 — 0.01 — —Sulfur(S) — — 0— — —Carbon, total(C) 26 23 0— — —

Parts per million

Boron (B) — — 14 — —Cadmium (Cd) 4.0 7.6 — 0.9 5.0 Copper (Cu) 146 300 23 3 96 Lead(Pb) 77 290 — 0.1 11.3 Manganese (Mn) 135 480 180 — —Nickel(Ni) 11 55 — — 7 Strontium (Sr) — — 30 — —Zinc (Zn) 107 770 60 13 162

*Values expressed on dry-weight basis.

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road base, and beneficial use of the energy generated in the combustion process. Problems that must be ad-dressed in the design and operation of such facilities are maintenance of good combustion conditions andachieving proper treatment of emissions so as to limit potential adverse effects on the public to the greatestextent possible.

Nature of the Fuel. Residential and commercial refuse is composed of combustible and noncombustiblematerial and moisture. Combustible waste is made up largely of paper, together with some wood, vegetableand animal waste, cloth, leather, rubber, and plastics. The noncombustible fraction is composed of metals,glass, dirt and stones, and other miscellaneous materials. Larger materials, normally classified as rubbish,are frequently found in municipal solid waste (MSW). Table 8.61 illustrates the type of variation that hasbeen observed in studies defining site-specific and average solid waste composition in the United States.

The table shows a 28.55 to 53.33% variation in paper content, and a 77.53 to 89.14% variation in com-bustible content. Moisture content has been found to vary from 20 to 50%. With little or no regulation of thehandling of refuse by the homeowner, extreme variations in moisture content (and, so also, heat content)may be observed in the solid waste. Thus, after a heavy rain, the moisture content of the solid waste may beso high that it may be difficult to sustain combustion.

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TABLE 8.61 Waste Characteristics as Percentages (63, 109, 110)

Central Wayne United States averageOceanside, N.Y. County, Mich. __________________________________

Component 1966–1967 plant, 1979 1977 1984 1993

Paper materials 3272–53 33 28 55–35 65 35.0 42.1 37.6Plastics 245–8 82 3 39–6.31 3.8 6.5 9.3Rubber and leather — — — 2.3 3.0Textiles 224–397 1.59-5.51 4.3 1.9 2.9Garbage or organics 7.23–16.70 0.35–5.15 14.9 7.3 6.7Wood and lumber 1.22–6.58 0.56–1.50 3.8 3.4 6.6Yard wastes* 0.26–33.33 36.1–45.4 16.3 16.1 15.9Noncombustibles 22.47–14.36 18.18–10.86 21.9 20.4 18Total — — 100 100 100Percent recycled — — 6.6 10.2 21.7

*Includes grass, dirt, and leaves.

TABLE 8.62 Variation in Heat Content of MSW

Noncombustible, %

10 25___________________________ _____________________________

Heat content, Heat content,Moisture, % Combined % Btu/lb Combined % Btu/lb

20 70 6580 55 517030 60 5640 45 423040 50 4700 35 329050 40 3760 25 2350

Note: 2.32 Btu/lb 1 kJ/kg.

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Such studies of the average composition of solid waste are useful to develop a general understanding ofthe nature of the material that must be handled in an incineration facility. However, for the design of the to-tal plant facility, it is important to understand the extent of variation in waste composition that will be expe-rienced at an incineration facility. The combustion system must be able to handle the extremes in solid wasteheat content, both high and low.

As indicated above, most of the combustible fraction of MSW is cellulose. The remainder of the com-bustible content is composed of various fats, oils, waxes, rubbers, and plastics. The heat released by burningcellulose is approximately 8000 Btu/lb (18.6 MJ/kg), whereas that released by virtually all of the other com-bustible components is significantly higher on a per pound basis. In recent years, it has been found that thehigher heating value (HHV) of the combustible portion only of MSW (moisture and ash free) averages about9400 Btu/lb (21.8 MJ/kg). Taking that figure as the moisture- and ash-free heat content of MSW, Table 8.62illustrates the variation in as-received heat content that one would expect in solid waste with moisture con-tent ranging from 20 to 50% by weight and a noncombustible content of 10 and 25% by weight.

For the design of feeding and residue-handling systems, one must have some information on the variabil-ity and extremes of the physical size and shape of the solid waste, together with the variation in noncom-bustible content. These have been troublesome areas of plant operation. If materials-handling facilities forfeeding and residue handling are not dimensionally large enough to pass the largest bulky items in the MSW,or large enough and rugged enough to handle the quantities of materials required to meet plant design ca-pacity, the plants will suffer continued expensive periods of downtime and might have to be derated.

The problems noted above may be compounded by inclusion of industrial and/or hazardous waste withthe MSW. Because of the potential impact of large quantities of very high heat content industrial wastes onheat generation and air emissions, such materials should be specifically identified and quantified prior to fa-cility design. Likewise, hazardous industrial wastes would potentially impose serious design constraints onsuch a facility, both from the standpoint of operator safety and public health. Public relations problems relat-ed to public health concerns probably would rule out accepting hazardous industrial wastes at most, if notall, facilities intended to combust MSW.

Plant Design. The capacity to be provided in an incineration plant is a function of (1) the area and popula-tion to be served; (2) the number of shifts (one, two, or three) the plant is to operate; and (3) the rate ofrefuse production for the population served. If records of collections have been kept, especially by weight,forecasts for determining required plant capacity can be made with reasonable accuracy. If records are notavailable, refuse quantities for establishing plant size may be approximated by assuming refuse generationrates of 4 lb (1.8 kg) per capita per day, when there is little or no waste from industry, to 5 lb (2.3 kg) percapita per day when there is some waste from industry (64). If substantial quantities of industrial wastes areto be handled in an incineration plant, they should be specifically identified as to quantity and suitability fordisposal in the planned facility. Those wastes not suitable for disposal (highly flammable, low heat content,and hazardous wastes, etc.) should be specifically excluded. A small plant [100 tons/day (90 metrictons/day)] will probably operate one shift per day. For capacities above 400 tons/day (360 metric tons/day),or any size plant incorporating boilers, economic and/or equipment operating considerations usually willdictate three-shift operation.

An isolated site is preferred for any such plant to avoid, as much as possible, objections of neighbors.However, well-designed and well-operated incinerators which do not present a nuisance may also be in-stalled in light industrial and commercial areas, thereby avoiding the economic burden of extended collec-tion truck routes or extensive refuse transfer operations. Since considerable vertical distance is involved inpassing refuse through an incinerator, there is an advantage in a sloping or hillside site. Collection and trans-fer trucks can then deliver refuse at the higher elevation while residue trucks can operate at the lower eleva-tion with a minimum of site grading.

Refuse Receipt and Storage. Scales, preferably integrated into an automated record-keeping system,should be provided to record the weight of solid waste delivered to the plant. Sufficient length of entrance

8.144 CHAPTER EIGHT

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road and tipping floor area should be provided so that refuse delivery trucks do not back up onto publichighways during peak delivery times. Either the tipping area or the individual tipping positions should beenclosed so as to prevent nuisance conditions in the vicinity of the plant caused by blowing papers, dust,and/or odors. The tipping area should be large enough to permit more than one truck at a time to maneuverto and from the dumping position.

Since collections usually are limited to one 8-b daily shift five days per week (sometimes with partialweekend collection operations), while burning will usually be continuous, ample storage must be provided.This usually requires two to three days of refuse storage at most energy-from-waste plants. Seasonal andcyclic variations should also be considered in establishing plant storage requirements.

When computing the dimensions required for refuse storage facilities, the required volume should becomputed on the basis of an MSW bulk density of from 300 to 400 lb/yd3 (180 to 240 kg/m3)(65). Other fac-tors to consider in sizing and laying out the refuse storage facilities are that refuse flows very poorly and canmaintain an angle of repose of greater than 90°. Thus, refuse is commonly stacked in the storage facilities tomaximize storage capability. Refuse storage in larger plants is normally in long, narrow, and deep pits eitherextending along the front of the furnaces or split in two halves extending from either side of the front end ofthe furnaces. If the storage pit is over 25 ft in width, it will generally be necessary to rehandle the refusedumped from the trucks. In smaller plants, floor dumping and storage of refuse is common practice.

Refuse Processing. With the increasing interest in utilizing the heat generated in the combustion of MSW,in the late 1960s and early 1970s, a number of people proposed that the refuse be processed to produce acombustible-rich fraction. The first such processing plant was built in St. Louis, Mo., as an EPA demonstra-tion plant in the early 1970s. The combustible-rich fraction produced was called refuse-derived fuel, or RDF.The process consisted basically of shredding the as-received refuse, air classification, and separation of non-combustible materials into recyclable fractions. In some plants, the combustible fraction was shredded a sec-ond time, following air classification, to produce a smaller particle size. It was originally proposed that RDFbe cofired in coal-fired utility boilers with the material being completely combusted while in suspension.

A number of problems were identified in the early operation of these plants. With the initial shredding ofas-received waste, glass was shattered and fine shards were embedded in paper and other combustible mate-rial. The glass, once embedded, could not be effectively removed. This increased the ash content of the com-bustible material and increased the abrasion of the pneumatic RDF conveying equipment.

Further, although the heat content per pound of RDF reportedly was increased by 10 to 15% over the heatcontent of as-received MSW (66), there was a net loss of energy from the system. Thus, from Table 8.63, itcan be observed that with a 10% increase in heat content and 70% capture of combustibles in the processingsystem, the resulting RDF contains only 77% of the heat of the original quantity of as-received MSW. Witha 15% increase in heat content and 80% capture of combustibles, slightly more than 91% of the heat in theoriginal MSW would be contained in the RDF. A much more extensive comparison of the efficiency of var-ious waste-processing systems in extracting the energy contained in MSW is presented in Ref. 67.

Another problem noted (68) from operation of early RDF combustion facilities was caused by incom-

SOLID WASTE 8.145

TABLE 8.63 Input Heat Capture in RDF

Heat content

Percent originalMaterial Weight, lb Per lb Total MSW

As-received MSW 2000 4500 9,000,000 100.0 RDF (70% of MSW) 1400 4950 6,930,000 77.0 RDF (80% of MSW) 1600 5175 8,280,000 91.3

Note: 1 lb = 0.454 kg.

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plete combustion of the RDF in suspension requiring use of dump grates to allow completion of combustionand reduce the load on ash-handling facilities. Also, there has been some evidence, although not entirelyconclusive, of increases in slagging in the boilers, and some problems have been reported with metalwastage of boiler tubes, particularly in the lower waterwall areas.

Later RDF processing plants have used flail mills, or trommels with bagbreaking blades, to break apartbags containing the waste, allowing initial separation of glass and metal. The material passing through thetrommel, primarily the light combustible fraction, is then shredded. Removing the glass prior to shreddingalleviates the problem of contamination of combustible with glass shards. Most plants now anticipate burn-ing RDF in boilers with spreader stokers rather than in suspension-fired utility boilers. Potential problems ofslagging and boiler tube metal wastage must be considered when selecting boilers for RDF combustion.

Processes to produce powdered fuel or RDF fuel pellets, while interesting, have not been developed to astate of commercial availability. Some limited work is still being undertaken to improve the economics andoperability of such systems. However, it appears as if commercial availability is many years in the future.Other processes such as pyrolysis have not been successfully applied to this field (69).

Refuse Feeding. Batch feeding of MSW and/or batch discharge of residue is undesirable because of vari-ations in furnace temperatures of several hundred degrees that usually occur due to air leakage into the fur-nace, resulting in adverse impact on refractory materials, and increased air emissions. In smaller plants withfloor dump and storage of MSW, feeding is accomplished on a semibatch basis by rams that push materialdirectly into the furnace on approximately 6- to 10-min cycles.

In larger plants utilizing pits for refuse storage, the solid waste is normally moved from the pit to a charg-ing hopper (70) by a traveling bridge crane and a grapple or organic-peel type of bucket (see Figure 8.44).

8.146 CHAPTER EIGHT

FIGURE 8.44 Bridge crane for municipal incinerator (70).

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The grapple or bucket size is established by a duty-cycle analysis, taking into account the quantity of mater-ial that must be moved from the pit to the furnaces, the distances over that the material must be moved, al-lowable crane speeds, and the need to rehandle (mixing and/or stacking) material in the pit. Buckets or grap-ples used to date have ranged in capacity from 1.5 to 8 yd3 (1 to 6 m3).

The crane used in this service should be capable of meeting the severest of duty requirements (71). Theload-lifting capability is established by adding to the bucket or grapple weight, 1.5 times the volumetric ca-pacity of the bucket times a density of MSW of 600 to 800 lb/yd3 (360 to 480 kg/m3) (65). In the past, thecrane has been operated from an air-conditioned cab mounted on the bridge. However, in many Europeanplants, and with increasing frequency in the United States, crane operation is being centralized in a fixedcontrol room usually located at the charging floor elevation and either over the tipping positions opposite thecharging hoppers, or in the vicinity of the charging hoppers.

In modern furnaces burning as-received MSW on mechanized grates, the crane-mounted grapple orbucket is used to lift the refuse from the pit to deposit it in a charging hopper. The charging hopper, which isbuilt large enough to prevent spillage on the charging floor and with slopes steep enough to prevent bridg-ing, is placed on top of a vertical feed chute that discharges the MSW into the furnace. The feed chute isnormally constructed of water-cooled steel plates or steel plates lined with smooth refractory material. Thechute is normally at least 4 ft (1.2 m) wide, to pass large objects with a minimum of bridging, and 12 to 14 ft(3.6 to 4.2 m) long. It is normally kept full of refuse to prevent uncontrolled admission of air into the fur-nace. The refuse is fed from the bottom of the feed chute into the furnace by a portion of the mechanicalgrate, or by a ram. The ram generally provides better control of the rate of feed into the furnace than the old-er technique of using a portion of the mechanical grate for refuse feed.

In other plants, particularly those burning RDF, conveyors, live-bottom bins, and shredding and pneumat-ic handling of combustible material have been utilized. Problems related largely to properly sizing the equip-ment and higher than expected maintenance have been experienced to date with these facilities.

Residue Handling. The residue from a well-designed, well-operated mass-fired incinerator burning as-received refuse will include the noncombustible material in the MSW plus somewhat less than 5% of thecombustibles. The nature of this material will vary from relatively fine, light ash, burned tin cans, and partlymelted glass, to large bulky items such as 55-gal drums. The material may be discharged from the furnacethrough manually operated dump gates, or directly from the mechanical grate into a hopper where it isquenched and then discharged to a truck or container positioned below the hopper, through a bottom gate.The residue may also be discharged through a chute into a trough filled with water. Removal from the troughmay be either by a ram discharger onto a conveyor or by a flight conveyor to an elevated storage hopper fromwhich it is discharged to a truck. If a water-filled trough with a flight conveyor is used, normally two troughsare provided, arranged so that the residue can be discharged through either trough. The second trough servesas a standby.

A key feature in the design of ash-discharge facilities is provision for sealing the discharge end of the fur-nace to prevent uncontrolled admission of air. This seal is usually provided by carrying the ash-dischargechute at least 6 in (15 cm) below the water surface in the receiving trough. In the design of the conveyormechanism, the proportions should be large because the material frequently contains bulky metal items andwire causing relatively frequent jamming, and it tends to be extremely abrasive.

Residue is taken to a landfill for final disposal. The volume of material remaining for ultimate disposalwill range from 5 to 15% of that received at the plant. Many plants currently operating in the United Statesthat weigh MSW received at the plant and residue discharged from the furnaces indicate that the weight ofMSW is only reduced from 50 to 60%. However, as much as one-third of the residue weight in these plantsmay be attributed to incomplete drainage of the material prior to its discharge into the final transportationcontainer. The ram-type ash discharger used in European and some of the new, large U.S. plants generallyachieves much better dewatering of residues than older water-filled trough, ash drag residue-handling sys-tems.

SOLID WASTE 8.147

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The residue from incinerators is generally inert, relatively sterile (with combustible content below 5%)and makes good landfill material, particularly if it is well dewatered at the plant. There is some indicationthat heavy metals tend to concentrate in fly ash collected in such plants, rather than in bottom ash. If moreextensive data confirm these preliminary indications, and recognizing the increasing concern regardinggroundwater quality, it may be necessary to handle plant residues separately prior to disposal in the future.

Furnace Design. The principal aspects of furnace design are capacity (grate area and furnace volume),mechanical grates, construction refractories, and combustion air supply.

Capacity. The basic design factors that determine furnace capacity are grate area and furnace volume.Also, the available capacity and method of introducing both underfire and overfire air will influence, to alesser extent, furnace capacity. Required grate area, in a conservative design, is normally determined by lim-iting the burning rate to between 60 and 70 lb/ft2/h (290 and 340 kg/m2/h) of grate area (72). This is basedon limiting the heat release rate loading on the grate to 250,000 and 300,000 Btu/ft2 of grate per hour (2.8 ×106 to 3.4 × 106 kJ/m2/h).

Furnace volume required is established by the rate of heat release from the fuel. Thus, furnace volume isgenerally established by using heat release rates ranging from 12,500 to 20,000 Btu/ft3/h (4.6 × l05 to 7.4 ×l05 kJ/m3/h) with the lower heat release rate being more desirable from the standpoint of developing a con-servative design. A conservative approach to design in this area is desirable because of probable periodic op-eration at above design capacity to meet short-term, higher than normal refuse collections, and possible re-ceipt of high heat content waste.

Waterwall units burning as-received MSW have been built as small as 75 to 100 tons/day (68 to 91 t/day)capacity. However, the cost per ton of rated capacity of such units is relatively high. A more common unitsize is 250 to 300 tons/day (225 to 270 t/day), while waterwall mass-fired units have been built as large as750 to 1200 tons/day (675 to 1090 t/day) capacity (73).

Grates. The primary objective of a mechanical grate is to convey the refuse from the point of feedthrough the burning zone to the point of residue discharge with a proper depth of fuel and sufficient reten-tion time to achieve complete combustion. The refuse bed should be gently agitated so as to enhance com-bustion. However, the agitation should not be so pronounced that particulate emissions are unreasonably in-creased. The rate of movement of the grate or its parts should be adjustable to meet varying conditions orneeds in the furnace.

In the United States over the past 20 years, several types of mechanical grates have been used in continu-ous-feed furnaces. These include traveling grates, reciprocating grates, rocking grates, and a proprietarywater-cooled rotary combustor. The traveling grate conveys the refuse through the furnace on the grate sur-face. Stirring is accomplished by building the grate in two or more sections, with a drop between sections toagitate the material. The reciprocating and rocking grates both agitate and move the refuse material throughthe furnace by the movement of the grate elements and the incline of the grate bed. Additional agitation isobtained, particularly in the reciprocating grate, by substantial drops in elevation between grate sections.The rotary combustor slowly rotates to tumble the refuse material that is conveyed through the inside of thecylinder. The combustor is inclined from the horizontal so that gravity assists in moving the materialthrough the unit.

The Europeans have developed other grate systems, some of which are currently being utilized in plantsbeing constructed or in operation in the United States. The Volund incinerator (Danish) uses a slowly rotat-ing, refractory-lined cylinder or kiln, which is fed by a two-section (drying and ignition) reciprocating grate.Refuse passes through the kiln and residue is discharged to a water quench when combustion is completed.The so-called Dusseldorf or VKW (German) incinerator uses a series of six rotating cylindrical grates, ordrums, placed at a slope of about 30° (74). The refuse is conveyed by the surface of the drums, which rotatein the direction of refuse flow, and is agitated as it tumbles from drum to drum. Underfire air is introducedthrough the surface of the drums. Both the Von Roll and the Martin grates use a reciprocating motion topush the refuse material through the furnace. However, in the Martin grate, the grate surface slope is greater

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and the grate sections push the refuse uphill against the flow of waste causing a gentle tumbling and agita-tion of the fuel bed.

Another variable feature in the various grate designs is the percentage of open area to allow for passageof underfire air (75). These air openings vary from approximately 2 to over 30% of the grate surface area.The smaller air openings tend to limit the quantity of siftings dropping through the grates and creates a pres-sure drop that assists in controlling the point of introduction of underfire air. Larger air openings make con-trol of underfire air more difficult but allow for continuous removal of fine material, which could interferewith the combustion process, from the fuel bed.

Furnace configuration is largely dictated by the type of grate used. In the continuous-feed mechanicalgrate system, the furnace is rectangular in plan and the height is dependent upon the volume required by thelimiting rate of heat release cited earlier. An optimum furnace configuration would provide sufficient vol-ume for retention of gases in the high-temperature zone of maximum fuel volatilization for a sufficientlength of time to ensure complete combustion, and would be arranged so that the entire volume is effective-ly utilized. Temperatures are usually high enough with present-day refuse for proper combustion. Turbu-lence should be provided by a properly designed overfire air system. Figure 8.45 shows an example of amass-fired waterwall boiler system.

SOLID WASTE 8.149

FIGURE 8.45 Refuse furnace, waste-heat boiler, and electrofilter sections (76).

SOLID WASTE



Refractories. With present-day mass-fired waterwall furnaces, the use of refractories in furnace construc-tion has been minimized but not eliminated. Refractory materials may be used to line charging chutes, pro-vide a transition enclosure between the top of the grates and the bottom of the waterwalls, a protective coat-ing on the waterwall tubes, and an insulating layer between the hot gases and the metal walls of fluesdownstream of the primary combustion chamber. Refractory brick used in a charging chute must be able towithstand abrasion and provide some insulation in the lower portions for protection from moderate tempera-tures. The construction above the grates must be able to withstand high temperatures, flame impingement,thermal shock, slagging, spalling, and abrasion. The protective coating on the waterwall tubes must be a rel-atively dense castable material with a relatively high heat conductivity (76). Insulating refractories used influes downstream from the boilers, on the other hand, should have a low heat conductivity.

Refractories are generally classified according to their physical and chemical properties, such as resis-tance to chemical attack, hardness, strength, heat conductivity, porosity, and thermal expansion (77). Thematerial may be cast in brick in a variety of shapes and laid up with air-setting or thermal-setting mortar, ormay be used in a moldable or plastic form. Material used in incinerator construction includes “high-duty”and “superduty” fireclay brick, phosphate-bonded alumina material, and silicon carbide, among others. Inselecting the proper materials for application in this type of service, because the selection of materials is sogreat and the conditions of service so varied and severe (78), advice of a recognized manufacturer should besought.

Combustion Calculations. Factors directly affecting furnace design are moisture and combustible contentof the solid waste being burned, the volatility of the material being burned, and the means for temperaturecontrol. The design of furnaces and boilers and sizing of flues and other plant elements should be based ondesign parameters that result in large sizes. Controls should provide satisfactory operation for loads belowthe maximum.

The combustion portion of MSW is composed largely of cellulose and similar materials originating fromwood, mixed with appreciable amounts of fats, oils, waxes, rubbers, and plastics. The heat released by burn-ing cellulose is approximately 8000 Btu/lb (18.6 MJ/kg) while that released by fats, oils, etc., is approxi-mately 17,000 Btu/lb (39.5 MJ/kg). If cellulose and oil and fat exist in the ratio of 6:1 in MSW, the heat con-tent of the combustible matter in MSW would be approximately 9290 Btu/lb (21.6 MJ/kg).

The heat released during combustion may be determined in a bomb calorimeter, a device with a metalcontainer (bomb) immersed in a water jacket. The heat absorbed by the water is the heat of combustion. Theheat of combustion of a number of materials is shown in Table 8.64.

Another method for determining the approximate heat value for solid fuels is to perform an ultimateanalysis and then apply Dulong’s formula. This formula may be stated as

8.150 CHAPTER EIGHT

TABLE 8.64 Heat of Combustion

Material Heat of combustion, Btu/1b

Carbon (to CO2) 14,093Hydrogen 61,100Sulfur 3,983Methane 23,870Ethylene 21,644Oil (#3–#6) 17,500–19,000Coal (bituminous) 12,000–14,500MSW (as received) 3,500–5,500


SOLID WASTE



Btu/lb = 14,544C + 62,028�H2 – � + 4050S

where C, H2, O2, and S represent the decimal proportionate parts by weight of carbon, hydrogen, oxygen,and sulfur in the fuel. The term O2/8 is a correction used to account for hydrogen that is already combinedwith oxygen in the form of water.

For the purposes of illustrating the calculations required to determine refuse heat content using Dulong’sformula, air requirements, and furnace temperature, the ultimate analysis of MSW in Table 8.65 is used. Inthe determination of air requirements, Table 8.66 combustion constants will be used.

With the ultimate analysis shown in Table 8.65 and the combustion constants shown in Table 8.64, a cal-culation can be made of input and output gas quantities for this given refuse composition. In the calcula-tions, the following assumptions are made: base temperature = 80°F (27°C), 23.15% of air is O2 and 76.85%is N2 moisture in the air = 0.0132 lb/lb dry air; and unburned carbon in residue = 4% of carbon input. Gasquantities are tabulated on Tables 8.67 and 8.68 for two conditions: 140% excess air (EA) (refractory fur-nace where temperatures are controlled by adding excess air), and 80% excess air (waterwall furnace whereheat is absorbed from combustion chamber by water circulating in the waterwall furnace enclosure).

Next a material balance can be calculated (see Table 8.69) for both excess air conditions. The followingadditional assumptions are made in performing these calculations: residue quench water evaporated = 0.03lb/lb of MSW, and fly ash = 2% of MSW burned.

A heat balance and check on flue gas temperature assumptions (see Table 8.70) can now be performed by

O2�8

SOLID WASTE 8.151

TABLE 8.65 Ultimate Analysis

Component Weight %, total Weight, % combined

C 25.0 50.0H2 4.2 8.4O2 20.7 41.4S 0.1 0.2

H2O 28.0 —Noncombustible

N2 0.5 —Ash 21.5 —____ ____

100.0 100.0

Heat content, moisture and ash free

Btu/lb = 14,544 × 0.5 + 62,028 �0.084 – � + 4050 × 0.002

Btu/lb = 7272 + 62,028 × 0.03225 + 8

Btu/lb = 7272 + 2000 + 8

Btu/lb = 9280 Btu/lb

Heat content, as received (complete combustion)

Btu/lb = 9280 × 0.5 = 4640 Btu/lb


0.414�

8

SOLID WASTE



8.152

TAB

LE 8

.66

Com

bust

ion

Con

stan

ts (

79)

The

oret

ical

(st

oich

iom

etri

c) a

ir, l

b/lb

com

bine

d

Req

uire

d fo

r co

mbu

stio

nF

lue

prod

ucts

Mol

ecul

ar__

____

____

____

____

____

____

____

____

____

____

____

____

_

Sub

stan

ce*

Form

ula

Wei

ght

lb/ f

t3ft

3 /lb

O2

N2

Air

CO

2H

2ON

2

Car

bon

C12

.016

——

2.66

68.8

611

.53

3.66

—68

.86

Hyd

roge

nH

22.

016

0.00

5318

7.72

37.

9426

.41

34.3

4—

8.94

26.4

1 O

xyge

nO

232

.000

0.08

4611

.819

——

——

——

Nit

roge

n (a

tm.)

N2

28.0

160.

0744

13.4

43—

——

——

—S

ulfu

rS

32.0

66—

—1.

0063

.29

64.2

92.

00—

63.2

9(a

s S

O2)

Wat

er v

apor

H2O

18.0

160.

0476

21.0

17—

——

——

—A

ir—

62.8

960.

0766

13.0

63—

——

——

—

*All

gas

vol

umes

cor

rect

ed to

60°

F a

nd 3

0 in

Hg

dry.

N

ote:

1 lb

/ft3

= 0

.06

kg/m

3 .

SOLID WASTE



8.153

TAB

LE 8

.67

Gas

Qua

ntit

y C

alcu

lati

ons—

Sto

ichi

omet

ric

or T

heor

etic

al

O2

Air

____

____

____

____

____

____

____

___

____

____

____

____

____

____

____

Frac

tion

alC

ombu

stio

nC

ombu

stio

nC

ompo

nent

com

posi

tion

cons

tant

Qo,

lb/l

b fu

elco

nsta

ntQ

a, lb

/lb

fuel

Car

bon

Bur

ned

0.24

02.

660.

638

11.5

32.

767

L

ost

0.01

02—

2—2—

2—H

ydro

gen

0.04

27.

940.

333

34.3

41.

442

Oxy

gen

0.20

7 2—

2—2—

2—N

itro

gen

0.00

5 2—

2—2—

2—S

ulfu

r0.

001

1.00

0.00

14.

290.

004

Moi

stur

e0.

280

2—2—

2—2—

Ash

0.21

5

2—2—

2—2—

____

___

___

____

_

Tota

l1.

000

0.97

24.

213

Les

s O

2in

fue

l (de

duct

)–0

.207

×

(1/0

.231

5)

=

–0.8

94

____

___

___

____

_

Req

uire

d at

theo

reti

cal a

ir0.

765

3.31

9 O

2an

d ai

r @

80%

EA

(l.8

× Q

) =

1.37

75.

974

EA

= 5

.974

– 3

.319

2—2.

655

E O

2=

1.3

77 –

0.7

650.

612

2—O

2an

d ai

r @

l50%

EA

(2.

5 ×

Q)

=1.

913

8.29

8 E

A =

8.2

98 –

3.3

192—

4.97

9 E

O2

= 1

.913

– 0

.765

1.14

8 2—

SOLID WASTE



8.154

TAB

LE 8

.68

Pro

duct

s of

Com

bust

ion

@ 8

0% e

xces

s ai

r

Com

pone

ntC

alcu

lati

onS

ubto

tal,

lb/l

b fu

elQ

uant

ity,

lb/l

b fu

el

CO

20.

24 ×

3.6

60.

878

H2O fr

om M

SW

com

bust

ion

0.04

2 ×

8.9

40.

375

fr

om M

SW

0.28

0.28

0fr

om c

ombu

stio

n ai

r0.

0132

× 5

.974

0.07

90.

734

____

SO

20.

001

× 2

.00.

002

O2

(exc

ess)

0.61

2N

2 from

MS

W0.

005

0.00

5

from

com

bust

ion

air

0.76

85 ×

5.9

744.

591

4.59

6__

___

____

_

Tota

l wei

ght,

wet

6.82

2To

tal w

eigh

t, dr

y, 6

.822

– 0

.734

= 6

.088

@ 1

50%

exc

ess

air

CO

20.

24 ×

3.6

60.

878

H2O fr

om M

SW

com

bust

ion

0.04

2 ×

8.9

40.

375

fr

om M

SW

0.28

0.28

0fr

om c

ombu

stio

n ai

r0.

0132

× 8

.298

0.11

00.

765

____

_

SO

20.

001

× 2

.00.

002

O2

(exc

ess)

1.14

8N

2 from

MS

W0.

005

0.00

5

from

com

bust

ion

air

0.76

85 ×

8.2

986.

377

6.38

2__

___

____

_

Tota

l wei

ght,

wet

9.17

5To

tal w

eigh

t, dr

y, 9

.175

– 0

.765

= 8

.410

SOLID WASTE



use of Figure 8.46 developed from similar figures in Ref. 78, and the following assumptions: the specificheat of both fly ash and residue = 0.25; and the temperature of the residue = 180°F (82°C).

Since the computation of heat input and heat output balances with minimal unaccounted for losses, theassumed temperatures are satisfactory. If the unaccounted for losses are greater than the minimum necessaryto make the heat calculations balance, the assumed temperature should be adjusted upward or downward tobring the calculations more in balance.

The calculations for the 80% excess air calculations may be carried one step further to estimate steam-generating capability and anticipated boiler efficiency for different assumed conditions. Thus, assumingtemperature of the gases leaving the boiler was 500°F (260°C), steam is generated at 125 lb/in2 gage (860kN/m2), and 400°F (205°C) [hf = 1221 Btu/lb (2.8 MJ/kg)], and the heat content of the boiler feedwater at181°F (83°C) was hs = 181 Btu/lb (0.42 MJ/kg), the calculation in Table 8.71 would be made.

Detailed calculations for boiler design are beyond the scope of the material presented in this section. Sev-

SOLID WASTE 8.155

TABLE 8.69 Materials Balance

@ 80% excess air @ 150% excess air__________________________ ____________________________

Subtotal, Total Subtotal, TotalInput lb/lb lb/lb lb/lb lb/lb

RefuseCombustible material

Complete combustion 0.49 0.49Unburned C 0.01 0.01

Moisture 0.28 0.28Noncombustible 0.22 1.00 0.22 1.00 ____ ____

Total airO2 1.377 1.913N2 4.591 5.97 6.377 8.29 ____ ____

Moisture in air 0.08 0.11 Residue quench water 0.03 0.03 ____ ____

Total 7.08 9.43

CO2 0.88 0.88 Air

O2 0.612 1.148N2 4.591 5.20 6.377 7.52 ____ ____

Moisturein MSW 0.280 0.280from combustion 0.375 0.375from combustion air 0.079 0.110from residue quench water 0.030 0.76 0.030 0.80 ____ ____

Noncombusted materialNoncombustible 0.220 0.220Unburned C 0.010 0.23 0.010 0.23 ____ ____

Unaccounted for 0.01 —____ ____

Total 7.08 9.43

SOLID WASTE



8.156

TAB

LE 8

.70

Hea

t Bal

ance

@ 8

0% e

xces

s ai

r@

150

% e

xces

s ai

r__

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

___

Tota

l, B

tu/

Tota

l, B

tu/

Cal

cula

tion

lb/f

uel

Cal

cula

tion

lb/f

uel

Hea

t inp

utR

efus

e46

4046

40M

oist

ure

in a

ir0.

08 ×

1.0

4884

0.11

× 1

.048

115

____

____

Tota

l47

2447

55

Hea

t out

put

Gas

tem

p. =

186

0°F

Gas

tem

p. =

146

0°F

Dry

gas

(0.8

8 +

5.2

0) ×

476

2894

(0.8

8 +

7.4

2) ×

360

3024

Wat

er v

apor

0.76

× 2

009

(ste

am ta

b.)

1527

0.80

× 1

789

(ste

am ta

b.)

1431

Fly

ash

0.02

× 0

.25

× (

1860

– 8

0)9

0.02

× 0

.25

× (

1460

– 8

0)7

Res

idue

0.22

× 0

.25

× (

180

– 80

)6

0.22

× 0

.25

× (

180

– 80

)6

Unb

urne

d ca

rbon

0.01

× 1

4,09

314

10.

01 ×

14,

093

141

Los

s th

roug

h fu

rnac

e en

clos

ure

0.03

× 4

724

142

0.03

× 4

,755

143

Una

ccou

nted

for

53

____

____

Tota

l47

2447

55

Not

e:1

Btu

/lb

= 0

.43

kJ/k

g.

SOLID WASTE



eral of the major boiler manufacturers in this country have written comprehensive texts on this subject (79,80) and the reader is referred to this material and the boiler manufacturers for detailed information on thissubject.

Combustion Air Supply. As indicated in the section on combustion calculations, the combustion processrequires oxygen to complete the reactions involved in the burning process. The air that must be delivered tothe furnace to supply the exact amount of oxygen required for completion of combustion is called the stoi-chiometric air requirement. Additional air supplied to the furnace is called excess air and is usually ex-pressed as a percentage of the stoichiometric requirements.

The total air supply capacity in an incinerator must be more than the stoichiometric requirement for com-bustion because of imperfect mixing and to assist in controlling temperatures, particularly with dry, high-heat-content refuse. The total combustion air requirements can range to 10 lb of air per pound of refuse forrefractory wall furnaces and from 6 to 8 lb of air per pound of refuse for mass-fired waterwall furnaces.

In the modern mechanical grate furnace chamber, at least two blower systems should be provided to sup-ply combustion air to the furnace—one for underfire or undergrate air and the other for overfire air. Under-fire air, admitted to the furnace from under the grates and through the fuel bed, is used to supply primary airto the combustion process and, secondarily, to cool the grates.

Overfire air may be introduced in two levels. Air introduced at the first level, immediately above the fuelbed, is used to promote turbulence and mixing and to complete the combustion of volatile gases driven off

SOLID WASTE 8.157

FIGURE 8.46 Enthalpy of flue gas (64).

SOLID WASTE



the bed of burning solid waste. The second row of nozzles, which are higher in the furnace wall, allows sec-ondary overfire air to be introduced into the furnace to promote additional mixing of the gases and for tem-perature control.

Blower capacities should be divided so that the underfire blower is capable of furnishing half or more ofthe total calculated combustion air requirements, while the overfire blower would have a capacity of some-what less than half of the total calculated air requirements. Setting these capacities requires some judgmentrelated to assessing how great a variation is anticipated in refuse heat contents during plant operation.Dampers should be provided on fan inlets and on air distribution ducts for control purposes.

Pressures on underfire air systems for most U.S. types of grates will normally range from 2 to 5 in (5 to12.7 cm) of water. European grate systems frequently require a higher pressure. The pressure on the overfireair should be high enough so that the air, when introduced into the furnace, produces adequate turbulencewithout impinging on the opposite wall. This is normally accomplished by the use of numerous relativelysmall 1.5 to 3 in (3.8 to 7.6 cm) diameter nozzles at pressures of 20 in (51 cm) of water and higher.

Boilers. Substantial quantities of heat energy may be recovered during the thermal destruction of the com-bustible portions of MSW. Systems that have been successfully used to recover this energy include mass-fired refractory combustion chambers followed by a convection boiler section; an RDF semisuspensionfired spreader–stoker–boiler unit; and an RDF suspension-burning utility type of boiler. Each system hasapparent advantages and disadvantages.

Mass-Fired. In a refractory furnace waste-heat boiler unit, energy extraction efficiencies are generallylower, assuming the same boiler outlet temperatures, than with the other systems. Approximately 50 to 60%

8.158 CHAPTER EIGHT

TABLE 8.71 Boiler Calculations

Useful heat output

Dry gas 2894Water vapor 1527____

Total 4421 Btu/lb fuel

Heat in gases at boiler outlet at 500°F (260°C)

Dry gas 6.08 × 103 = 626Water vapor 0.76 × 1287 = 978____

Total 1604 Btu/lb fuel

Heat required for 1 lb steam

1221 – 181 = 1040 Btu/lb steam

Steam produced per pound of fuel

= = 2.71

Boiler efficiency

× 100 = × 100 = 59.6%

Note: 1 Btu/lb = 0.43 kJ/kg; 1 lb = 0.454 kg.

2817�4724

(4421 – 1604)��

4724

2817�1040

(4421 – 1604)��

1040

SOLID WASTE



of the heat generated in the combustion process may be recovered with such systems. These units can pro-duce approximately 2 lb steam per pound of normal MSW [heat content = 4500 Btu/lb (10.5 MJ/kg)], versus3 lb/lb MSW in the other units described above. This lower efficiency of steam generation is caused by larg-er heat losses due to higher combustion air quantities needed with such units to control furnace temperaturesso that furnace refractories are not damaged. However, the boilers in such units, if properly designed and op-erated, generally are less susceptible to boiler tube metal wastage problems than the other systems listedabove.

Mass-fired waterwall units are perhaps the most widely utilized type of heat recovery unit in the field to-day. In this type of unit, the primary combustion chamber is fabricated from closely spaced steel tubesthrough which water circulates. This waterwall-lined primary combustion chamber is followed by a convec-tion type of boiler surface. It has been found desirable in these plants to coat a substantial height of the pri-mary combustion chamber, subject to higher temperatures and flame impingement, with a thin coating of asilicon carbide type of refractory material, and to limit average gas velocities to under 15 ft/s (4.5 m/s). Gasvelocities entering the boiler convection bank should be less than 30 ft/s (9.0 m/s) (76). Efficiency of heatrecovery in such units has been found to range generally from 65 to 70%, with steam production usuallyabout 3 lb of steam per pound of normal MSW. Water table studies have been found to be very useful in thelarger units to check on combinations of furnace configuration and location of overfire combustion air noz-zles.

Semi-Suspension-Fired. In an RDF-fired spreader-stoker type of unit, the combustible material is gener-ally introduced through several air-swept spouts in the front waterwall, is partially burned in suspension, andthen falls onto a grate on which combustion is completed as the partially burned material is conveyed to theresidue discharge under the front waterwall face of the furnace. These units can generally handle a coarserRDF than the so-called full-suspension burning units. Densified RDF can also be burned in such units. TheRDF can furnish all the combustible input to the system, or it can be cofired with a fossil fuel, generallycoal. While it was originally felt that such units could achieve more controllable combustion conditions thanthe mass-fired units, experience to date has not proven this concept.

Suspension-Fired. The so-called full-suspension combustion concept was originally proposed so thatfinely shredded combustible material from MSW could be burned in existing utility boilers. In this way, theexpense of constructing a boiler would be mitigated and 10 to 15% of the fossil fuel normally consumed bythe utility would be displaced (saved) by burning the RDF. This has been the least successful of the systemtypes due to problems related to additional handling and greater power requirements to achieve a finershred. Also, some utility boilers seemed to experience a greater tendency for slag formation in the boiler.While the concept initially anticipated that the RDF would completely burn in suspension, experience todate indicates that this does not occur. Accordingly, dump grates are now considered a necessity in suchboiler units to allow for completion of combustion prior to water quenching of the residue.

Efficiencies of both RDF-fired types of boiler units will generally range from 65 to 75%. Steam produc-tion would normally be expected to be somewhat greater than 3 lb steam per pound of RDF.

Limitations. If the energy recovered from the combustion of as-received MSW or RDF is to be used toproduce electricity, some superheat is at least desirable, if not necessary. Since boiler tube metal wastage inthese plants is, at least partially, a function of tube metal temperature (83), and steam is a less efficient cool-ing medium than water, superheater surface is more prone to metal wastage problems than other areas ofboiler tubing. Tube metal temperatures, above which metal wastage can be a significant operational prob-lem, are generally thought to range from 650 to 750°F (395 to 450°C). These temperatures are lower thandesirable for maximum efficiency of electrical generation by steam-driven turbines. However, this limitationdoes not rule out consideration of this form of energy utilization.

Air Pollution Control. Potential emissions from the burning of MSW may be broadly classified into par-ticulates, gaseous emissions, incompletely burned products (primarily hydrocarbon) from the combustionprocess, and trace emissions. Particulates have been a matter of concern, and regulatory agency attention,

SOLID WASTE 8.159

SOLID WASTE



for some time. The initial concern was from the standpoint of reducing gross emissions that were both anaesthetic and a potential public health problem. Current interest and concern, since the initial problem haslargely been solved, is directed toward better control of submicron-size particles (63).

Gaseous emissions, such as CO2, SO2, and NOx, are not generally felt to be a major problem in an incin-eration plant. However, control measures may be required at a specific site if it is located in an area desig-nated by EPA as nonattainment for some or all of the above pollutants. Incompletely burned products fromthe combustion process, such as CO and hydrocarbons, can be a problem if emission levels are not closelycontrolled. Thus, CO levels at poorly controlled MSW incinerators have been observed at well over 1000ppm, an indication of the potential presence of unburned hydrocarbons in the stack gases and much higherthan allowed in those jurisdictions that have established emission limits. Unburned hydrocarbons can causeodor problems, and, depending on the specific compounds, can be toxic. Of course, the most direct methodof control for both CO and hydrocarbons is to achieve better control of the combustion process.

The existence, identification, and quantification of trace metal and gaseous (particularly chlorinated hy-drocarbon) substances has been of increasing interest and concern over the last 15 to 20 years as increasing-ly sophisticated and complicated analytical equipment and procedures have been developed. Trace metalemissions can generally be controlled by better particulate control (84). Some extremely toxic gaseous mate-rials have been identified in the emissions from MSW incineration plants over the past 15 years in the partsper billion and parts per trillion level. However, while the substances are extremely toxic, leading to greatconcern on the part of the public, sampling and analysis methods are still under development, raising ques-tions as to the actual emission levels reported. Emission levels are usually so low that sampling and labora-tory analysis procedures are frequently brought into question; the source or mechanism of formation of thetoxins is still the subject of investigation, and projections of health risks are very approximate. Until betterscientific investigations into this problem produce more rigorous information, concern for potential healthproblems from these emissions will continue.

Regulatory Requirements. Emission standards issued by the U.S. government in 1971, to date, are basedon provisions of the Clean Air Act Amendments of 1970 and as subsequently amended. Different levels ofallowable emissions of particulates and gases have been established for different processes and for the ambi-ent atmosphere. The EPA has also established standard methods for testing and analyzing for these pollu-tants.

Over the past 15 years, allowable emission levels for particulates have been reduced significantly. Figure8.47 illustrates the rather dramatic reduction in allowable particulate emission levels. Current permit levelsusually are one-tenth the 1971 Federal New Source Performance Standards. Figure 8.48 illustrates the typesof different air pollution control equipment available and its collection efficiency relative to particle size.The relatively rapid decrease in allowable emissions, and the current concern for submicron-size particles,has caused an evolution in applied control technology from settling chambers, to wetted-wall collectors, towet scrubbers (tray type, venturi, etc.), to electrostatic precipitators, to baghouse collectors.

Flue Gas Tempering. Flue gases exiting the primary combustion chamber generally will range in temper-ature from 1500 to 1800°F (815 to 982°C) and will contain appreciable quantities of fly ash (approximately15 to 20 lb per 1000 lb) (85). The fly ash loading must be reduced by over 99% to meet most current air pol-lution code requirements. This in turn indicates that equipment, such as electrostatic precipitators or bag-houses, must be used for particulate control. Such equipment requires that entering gas temperatures be heldto 500 to 600°F (260 to 315°C). Thus, flue gas temperatures must be reduced by approximately 1000°F(555°C). This may be accomplished either by use of boilers or by evaporation of water directly by the fluegases.

Calculations for absorption of heat by boilers have been covered earlier. Table 8.72 gives the requisiteheat balance calculations for a spray chamber where flue gas (entering at 1680°F (915°C) and at 847 lb/h(385 kg/h) flow rate for dry gas + 73.1 lb/h (33 kg/h) moisture is cooled to 600°F (315°C) by water evapo-rating from sprays. The calculation results are expressed per 100 lb of 5000-Btu/lb MSW. There will besome heat loss through the furnace walls, estimated at 3% of the heat input. Air leakage into the furnace and

8.160 CHAPTER EIGHT

SOLID WASTE



8.161

FIG

UR

E 8

.47

Em

issi

on c

ontr

ol s

tand

ards

(85

).

SOLID WASTE



flues is estimated as 10% of the dry flue gas, or 85 lbs (38.6 kg) of dry air. With this air, there will be 1.12 lb(0.5 kg) of moisture (85 × 0.0132 lb H2O/lb air). Some minor losses will also occur in sluicing the fly ash,estimated at 1% of the heat in the dry gas.

The computation in Table 8.72 shows that there will be total heat energy of 358,929 Btu that must be ab-sorbed by moisture in the flue gas leaving the furnace and total moisture = 358,929/(1335 – 48) = 278.9 lb.Since the quantity of moisture in the flue gas was 73.1 lb, the theoretical spray water required would be278.9 – 73.1 = 205.8 lb, or 205.8/8.34 = 24.7 gal. In practice, the actual quantity of spray water required willdepend on the manner of introducing the water into the gas stream (fine or coarse sprays, evaporation from awetted surface, etc.). An evaporation efficiency of 50% or less is conservative, or a requirement of at least50 gal per 100 lb of MSW for this particular analysis.

Control Devices. Over the past 20 to 30 years, a number of different approaches have been used to con-trol particulate emissions from incinerators. Settling chambers were probably the earliest means used toabate the pollution impact of gross particulates from incinerators. These chambers, which depended ongravity settling of fly ash were essentially ineffective in removing particles smaller than 10 �m in size.Overall collection efficiencies were on the order of 10%. Cyclones, which depend on centrifugal force to re-move the particulate matter, were also used in some of the earlier plants. While these devices were much

8.162 CHAPTER EIGHT

FIGURE 8.48 Particle classification chart ((70).

SOLID WASTE



more efficient than settling chambers, reaching efficiencies of 50 to 70%, they are not capable of meetingcurrent emission standards. Small-diameter [less than 9 in (23 cm)] cyclones arranged in banks of multiplecyclones can achieve higher efficiencies of removal. Experience has shown that there are serious operationalproblems related to plugging of these small-diameter cyclones due to characteristics of the particulate andmoisture in the flue gases. These problems indicate that this equipment is not suitable for application to thisfield.

Wet scrubbers, particularly venturi scrubbers and tray-type scrubbers, have also been used to control par-ticulate emissions from incinerators. Although medium- to high-energy scrubbers [pressure drop of 7 in (18cm) of water and higher, three-tray scrubbers or venturi scrubbers] should theoretically be able to meet stan-dards, experience to date indicates this is not the case. As a matter of fact, it may be seen in Table 8.73 thatthere is relatively little improvement in emission control with significantly increasing energy inputs usingwet collection equipment.

Several other disadvantages, in addition to the problem of low removal efficiencies noted above, that areinherent in application of wet scrubbers have essentially eliminated this equipment from consideration forapplication to this field. Thus, these units require the use of large quantities of water resulting in problems ofwater cleanup and significantly increasing the probability of corrosion problems (87). Further, stack ex-hausts from such plants either exhibit a white steam plume or the exhaust gases must be reheated, at addi-tional expense, so that the plume separates from the stack.

SOLID WASTE 8.163

TABLE 8.72 Heat Balance for Spray Chamber (in Btu)

Input at 1680°FHeat of dry gas: 847 × 424 = 359,128Heat in water vapor: 73.1 (1900 – 48) = 135,381Heat in fly ash carryover (assume 2.0 lb/100 lb MSW) and = 800

specified heat of 0.25): 2.0 × 0.25 × (1680 – 80)Heat unaccounted for = 400________

Total 495,709Output at 600°F

Heat in dry gas: (847 + 85) × 128 = 119,296Heat in air leakage moisture: 1.12 × (1335 – 48) = 1,441Heat loss through walls @ 3% input = 14,850Minor losses from sluicing = 1,193Heat in vapor from furnace and spray water = 358,929________

Total 495,709

Note: 1055 Btu = 1 J; °C = (5/9) (°F – 32)

TABLE 8.73 Range in Test Results Using Wet Collection (86)

Range in pressure drop, Corrected emissions,Type of facility in wet collection g/ft3 @ 12% CO2

Water sprays less than ¼ 0.19–0.72 Wetted baffle walls ¼–4 0.10–0.9 Tray scrubbers 3–11 0.025–0.6 Venturi scrubbers 8–20 0.01–0.61

SOLID WASTE



Electrostatic precipitators have been a widely used particulate collection device in incinerator plants be-cause they have given consistently successful results. All of the more than 50 units that have been installedat plants on this continent since the early 1970s have successfully passed their acceptance tests, while manyhave achieved emission test results well below the Federal New Source Performance Standards. They arealso very effective in collecting submicron-size particles.

A basic electrostatic precipitator consists of a negatively charged discharge electrode that places a chargeon the particulate matter in the gas stream, and a series of collecting electrodes, generally grounded plates,that provide the surface on which the particulate matter collects and to which it adheres. The collection effi-ciency is dependent on a number of factors, including the strength of the electric field, the gas temperature[usually around 500°F (260°C) for this service], the moisture content of the gas, the resistivity of the dust it-self, the effectiveness of cleaning the electrodes and collection plates, and proper gas flow distribution (88).Achievement of acceptable collection efficiencies requires that the particulate matter have a resistivity of 1× l05 to 2 × 1010 ohm-cm. Other factors that impact collection efficiency are the collecting surface area pro-vided, the velocity of gases in the precipitator [generally 2 to 4 ft/s (0.6 to 1.2 m/s)], and the retention time inthe precipitator (generally from 5 to 10 seconds to meet current enforcement code levels).

Precipitators are generally cleaned by vibrating or rapping the collecting plates. One key to maximumprecipitator collection efficiency is proper rapping or cleaning of the plates. If the rapping is too violent or isnot sequenced properly, the collected dust will be disturbed, reentrained, and carried out of the precipitatorrather than sliding down the plate into the collection hopper below.

Fabric filters are currently the preferred method of particulate control in incinerator plants because theyare known to be high-efficiency particulate collection devices. They are particularly effective in controllingthe emission of small submicron-size particles. The filtering process, occurring as the gases pass throughbags, similar to a household vacuum cleaner, is affected by interception, impingement, and agglomeration.

The choice of woven fabric material for use in a baghouse is based on the required efficiency of particu-late removal, the pressure drop across the unit, the allowable gas throughput, and the gas temperature (89).Normally, the efficiency of removal and the pressure drop across the bags are closely related because theyare both a function of the tightness of the weave or permeability of the filter. The usual values of permeabil-ity for bag materials for this service range from 10 to 70 ft3/min/ft2 (3 to 21 m3/min/m2). Some filter fabricsthat may have application in this service, with some of their characteristics, are listed in Table 8.74. Newerfabrics reportedly have been developed that can withstand temperatures in excess of 750°F (400°C), whichis higher than the temperature one would normally expect in the gases exiting the boiler in an energy-from-waste plant.

With the increasing concern about trace chlorinated hydrocarbon, heavy metal, and acid gas emissionsfrom incineration plants, most modern plants utilize scrubbers and baghouses or dry scrubbers and electro-static precipitators to meet permit requirements for particulate and acid gas control. In this type of gascleanup system, pioneered in Europe, a lime slurry is introduced into the gas stream prior to the particulatecollection device (baghouse or electrostatic precipitator). The lime dosage is set at a multiple of the stoi-chiometric ratio required for complete reaction with the acid component of the effluent gases. After evapo-rating the slurry to dryness, the gas-lime dust mixture is passed through the particulate collection device.

Data on performance testing of such units indicate that such systems, if properly controlled and operated,effectively remove over 90% of HCl, up to 85% of SO2, over 99% of particulates and most of the heavy met-als because of chemical reactions in the gas stream and the generally higher collection efficiencies requiredto remove the lime reagent added to the gas stream. The additional capital and operating costs of such sys-tems are substantial.

Operational Experience. Literally hundreds of waste-to-energy plants have been constructed around theworld since the development of the current basic technologies in the early to mid-1960s. In the process, anumber of operational problems have surfaced, most of which have been solved. While there is room for fur-ther refinement and development of the mass-fired technologies, primarily to improve efficiencies and in-

8.164 CHAPTER EIGHT

SOLID WASTE



8.165

TAB

LE 8

.74

Pro

pert

ies

of F

iber

Mat

eria

ls (

89)

Phy

sica

l cha

ract

eris

tics

Rel

ativ

e re

sist

ance

to a

ttac

k by

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

____

__

Max

imum

Nor

mal

usab

lem

oist

ure

tem

pera

-R

elat

ive

Spe

cifi

cco

nten

t,tu

re, °

FO

rgan

icFi

ber

stre

ngth

grav

ity

%(°

C)

Aci

dB

ase

solv

ent

Oth

er a

ttri

bute

Poly

este

r (D

acro

n)S

tron

g1.

40.

428

0 (1

38)

Goo

dM

ediu

mG

ood*

—

Acr

ylon

itri

le (

Orl

on)

Med

ium

1.2

125

0 (1

21)

Goo

dM

ediu

mG

ood

—Po

lyet

hyle

neS

tron

g1.

00

250

(121

)M

ediu

mM

ediu

mM

ediu

m—

Tetr

aflu

oroe

thyl

ene

Med

ium

2.3

050

0 (2

60)

Goo

dG

ood

Goo

dE

xpen

sive

Po

lyvi

nyl a

ceta

teS

tron

g1.

35

250

(121

)M

ediu

mG

ood

Poor

—

Gla

ssS

tron

g2.

50

550

(288

)M

ediu

mM

ediu

mG

ood

Poor

res

ista

nce

to a

bras

ion

Gra

phit

ized

fib

erW

eak

2.0

1050

0 (2

60)

Med

ium

Goo

dG

ood

Exp

ensi

ve

Asb

esto

sW

eak

3.0

150

0 (2

60)

Med

ium

*M

ediu

mG

ood

—“N

omex

” ny

lon

Str

ong

1.4

545

0 (2

32)

Goo

dM

ediu

mG

ood

Poor

res

ista

nce

to m

oist

ure

*Exc

ept p

heno

l.† E

xcep

t hea

ted

acet

one.

‡ Exc

ept S

O2.

SOLID WASTE



crease plant availability and reliability, this technology is proven and available for application. RDF process-ing and combustion technologies are not as far along in development but, if conservatively designed, thistype of plant also has a place for application in this field. Other technologies that have been tried, in thiscountry and overseas, are not at a stage of development such that they are ready for commercial applicationat this time (90).

A summary of the experience in this country is presented in the following tables. Table 8.75 summarizesthe status of energy-from-waste projects in the United States as of the end of 1984.

Table 8.76 presents a summary of operational MSW processing and resource recovery facility capacitiesin plant sizes larger than laboratory or bench-scale in 1984 and in 1991. Several additional comments are inorder to further qualify the information summarized in this table. Of the 29 operating modular plants in1984, 11 were of 100 ton/day (91-t/day) capacity and larger while the other 18 averaged 40-ton/day (36.4-t/day) capacity. Thus, while there were a great number of plants in this category, they were of relativelysmall size. In the RDF technology, many of the 10 operating plants in 1984 were in startup for extended pe-riods (years) prior to achieving operational status. In many instances, the plants were substantially modified,and in several instances they were derated. Most, if not all, of the eight RDF plants that were shut down by1984 were closed without achieving full-capacity operation. This underscores the fact that utilization of thistechnology should be done very carefully.

As the solid waste disposal crisis deepens and this nation’s energy needs become more critical, in the fu-ture, the available energy in MSW will be tapped more frequently. The technology is available now for suc-cessful application of these techniques if provision is made for adequate funding to purchase high-qualityequipment suitable for the intended service, and to hire properly trained operating staffs.

8.166 CHAPTER EIGHT

TABLE 8.75 Status of U.S. Waste-to-Energy Facilities in 1984 (69)

Rated dailyStatus Number of facilities capacity, tons/day

Operational 52 23,450Under construction 14 9,300Closed 20 12,350

Note: 1 ton = 0.9 t.

TABLE 8.76 Types of Technology at U.S. Waste-to-Energy Facilities

Operational 1984 Operational 1991_________________________________ ________________________________

Rated capacity, Rated capacity,Technology tons/day Percent tons/day Percent

Mass burn 11,000 46.9 69,3300 75.1Modular systems 3,125 13.3 2,700 2.9RDF 9,325 39.8 20,245 22.0Pyrolysis 0 0 0 0Total 23,450 100 92,275 100

SOLID WASTE



Pyrolysis and Gasification Processes

Incineration is the dominant method for the thermal destruction of municipal solid waste (MSW) and istreated separately in this handbook. This section deals with other processes that use high temperatures to al-ter the physical and chemical character of MSW. The thermal processes discussed provide for the conversionof MSW to a variety of alternate fuels that can be substituted for fossil fuels in a wide variety of combustionsystems.

Two processes, pyrolysis and gasification, are presented. Pyrolysis is an old process. Prebiblical Egyp-tians prepared embalming fluid by pyrolysis. Pyrolysis of wood to produce chemicals (acetic acid, acetone,methanol) was widely practiced through World War II. Pyrolysis is the process of heating an organic materi-al in the absence of oxygen. No other reacting material is introduced into the reactor system. Large organicmolecules, as a result of the high temperature, break into smaller and simpler molecules.

Gasification processes introduce reacting gases into the reactor to encourage the formation of gaseousproducts and/or release heat within the reactor. Some of the common gases introduced include oxygen,steam, or hydrogen that cause reactions such as

C + O2 � CO2

C + H2O � CO + H2

C + ½O2 � CO

C + 2H2 � CH4

Gasification reactions usually take place at higher temperatures than required for pyrolysis. For completegasification, all of the solid is converted to a gas.

In many discussions, the terms pyrolysis and gasification are used interchangeably. There is no universal-ly accepted definitions for these terms in modern literature, which has led to confusion. For this discussion,whenever a gas is added to the reactor to promote the gas yield, the system is classified as a gasificationprocess.

The discussion is limited to the organic portion of MSW, referred to as refuse-derived waste (RDF). RDFconsists largely of cellulose based materials (primarily paper) and behaves similarly to wood. The large bodyof knowledge developed for wood is helpful in the prediction of the behavior of RDF.

Cellulose represents a polymeric material as shown in Figure 8.49. This is a polymer material, and when

SOLID WASTE 8.167

FIGURE 8.49 The cellulose molecule.

SOLID WASTE



subject to thermal stress (elevated temperature), it breaks down. The pyrolysis process may be selective ornonselective. In the selective process, a given chemical bond is broken; this may provide a high yield of asingle product. The pyrolysis of RDF is a highly nonselective process. The feed itself is heterogeneous insize, chemical composition, and structure, and there is little chance to select a particular bond to be broken.Raising the temperature of pyrolysis has a major effect on the destruction of the RDF.

Figure 8.50 shows general reaction scheme for cellulose. When cellulose is heated very slowly, the poly-mer is slowly broken down, with the weaker bonds being broken first. The products are largely noncom-bustible gases, a large fraction of nonreactive char, and a wide variety of medium- to high-molecular-weighthydrocarbons. Typical yields from destructive distillation, wherein the wood is heated for many hours at amoderate temperature are

Gas 14–17 wt % Methanol 1.5–2.5 wt % Acetic acid 3.5–8 wt % Tar 12–l6 wt % Char 31–37 wt %

Most of the heating value is retained in the char, which may be stored and used as a fuel.When the cellulose is heated rapidly to a high temperature, the cellulose molecule is shattered into small

molecules having a small tar, liquid, and char yield. Most of the newer thermal systems for treatment ofMSW to produce a fuel product use high temperatures to provide high gas yields. In these applications,since the original structure of the feed material is almost completely destroyed by the high temperature(thermal hammer), the products are largely independent on the physical nature of the feed. The effects oftemperature on product yield are shown in Figure 8.51.

The primary difference between RDF and wood when pyrolyzed to a high temperature is that RDF has a

8.168 CHAPTER EIGHT

FIGURE 8.50 Breakdown of cellulose with heat.

SOLID WASTE



8.169

FIG

UR

E 8

.51

Yie

ld o

f ga

s, c

har,

and

tar

at v

ario

us te

mpe

ratu

res.

SOLID WASTE



lower char yield. The plastic materials in RDF provide little char and a gas with a higher heating value. Theimpurities found in RDF apparently have a catalytic effect that leads to lower char fraction.

The products from pyrolysis include a tar, liquid, and char fraction along with gas. In gasification sys-tems, a reactive gas is introduced to react with these products and increase gaseous products. To achievehigh gas conversion rates, high temperatures are required in gasification systems. The reactive gases mostoften used are oxygen, steam, and sometimes hydrogen. Even when there are no reactive gases added to thesystem, the pyrolysis gases produced will react with char and tar to produce additional gas if the temperatureis high.

Energy/Fuel Recovery. Figure 8.52 shows how energy is recovered (chemical, sensible, or heat) from var-ious thermal treatment systems. The values shown in Figure 8.52 would be similar to those obtained fromthe organic portion of MSW when processed in a thermal process at 1500°F (815°C). In order to maintainthe process at 1500°F, heat (represented by q) may be added to or removed from the process.

At point 1 in Figure 8.52, no air (oxygen) or other reactive gas is introduced. To obtain 1500°F, tempera-ture heat (q1) is required. This represents pyrolysis. Most of the energy leaves the system as chemical energyassociated with the pyrolysis products (represented by �Hi) and as sensible heat of the pyrolysis products

8.170 CHAPTER EIGHT

FIGURE 8.52 Generalized energy relationships for thermal treatment of cellulose.

SOLID WASTE



(represented by �H1). Most of the chemical energy available in the MSW has been converted to chemicalenergy in the pyrolysis products.

To the right of point 1 in Figure 8.52, air has been introduced into the system. The oxygen reacts and re-leases heat. This reduces the chemical energy available in the products and reduces the amount of heatadded. At point 2, sufficient heat is released within the reactor to eliminate the need to add heat. This condi-tion is identified as gasification, since a reactive gas (oxygen) was introduced. It is often referred to as par-tial combustion, starved air combustion, or low-Btu pyrolysis. Much of the energy available in the MSW isremoved as chemical energy, �H2 of the products formed. A major portion of the energy available in theMSW continues to leave as chemical energy. A larger portion of the energy is carried away as sensible heat.If pure oxygen replaces air, the fraction of energy carried away as sensible heat is much less. This is the re-sult of reducing the nitrogen heated to 1500°F (815°C).

To the right of point 2, additional air (oxygen) is introduced, resulting in more combustion and more heatreleased in the reactor. Provisions must be made to remove this energy from the reactor. At point 3, suffi-cient air (oxygen) is introduced to combust all of the MSW. There is no chemical energy in the reactor prod-ucts. If the heat removed, q3, is used to generate steam, this represents the maximum amount of steam thatcan be generated. This is a theoretical condition of zero percent excess air. Practical systems require a sig-nificant amount of excess air to obtain complete combustion.

To the right of point 3, excess air is introduced, passes through the reactor unreacted, and carries out en-ergy as sensible heat, reducing the heat to be removed from the reactor. At point 4, no heat is added to or re-moved from the process (adiabatic system). All of the energy leaves the process as sensible heat and may berecovered in a waste heat recovery system.

The regions to the left of point 3 serve only as suggestive values because the products produced are notknown and depend upon many factors, including the type of reactor selected for the process as well as phys-ical characteristics of the feed.

The major difference between operations to the right of point 3 and those to the left is that those to theleft remove a portion of the energy as chemical energy (fuels). Those fuels may be removed from the processand transported to a user where it may be burned in the user’s combustion equipment. It is this potential torecover useful fuel that may replace an existing fossil fuel that makes alternate thermal treatment systemsother than incineration attractive.

The region to the right of point 3 is termed combustion, which is a special case of gasification where allcarbon is oxidized to CO2 and all hydrogen to H2O. As the products are known, precise energy and materialbalances can be made if the composition and heating value of the fuel are known. Water-walled incineratorsfall in region 3–4, and most other incinerators fall at point 4.

The volume of gas leaving the reactor system is also shown in Figure 8.52 for the case where the majorportion of the products are gaseous. This is important when it is necessary to clean the gases to protect theenvironment. Cost of gas cleanup is strongly dependent upon the volume of gas and concentration of pollu-tant. The ratios of gas volumes for pyrolysis, adiabatic gasification, stoichiometric combustion, and adiabat-ic combustion are 1/2/4/11.

In comparing combustion systems to other thermal destruction processes (OTP), combustion results inhigh gas volumes while OTP results in energy recovered as chemical energy and lowered gas volumes.

Reactor Systems. Systems used for thermal destruction of MSW may be classified according to the mech-anisms of solids flow and the manner in which heat is provided.

1. Gravity: Solids flow under the force of gravity. Shaft furnaces (moving beds) belong to the class. 2. Mechanical: Solids flow as a result of mechanical force. Multihearth furnaces, rotary kilns, and auger

kilns belong to this class. 3. Drag: Solids flow as a result of drag force resulting from gas flow past the solid. Fluidized beds and en-

trained beds belong to this class. 4. Combinations of above.

SOLID WASTE 8.171

SOLID WASTE



The published literature on pyrolysis and gasification of MSW and biomass reveals that Europe stressesshaft kilns (gravity flow), the Japanese emphasize fluidized-bed systems (drag flow), and none of the groupsdominate in the United States.

Heat may be added or removed from the reactor by three mechanisms:

1. Indirect: Heat is added to the vessel through the vessel walls. Combustion occurs external to the reactor. 2. Direct. Heat is released within the reacting vessel by gasification reactions (usually combustion). 3. Carrier: Energy is carried into reacting vessel as sensible heat of a nonreacting material. Carrier system

material may be a solid, gas, or liquid.

Representative examples for each type of reactor system will be discussed below. The systems identifieddo not provide a comprehensive list but include most of the major systems that have reached commercial orsemicommercial status.

Gravity Flow Systems. The shaft furnace has been the dominant reactor system considered for the gasifi-cation of MSW. Figure 8.53 shows its important features. Air or oxygen is introduced through the bottomand the solid through the top where it descends toward the bottom through several zones. At the top the sol-id is dried. The dry solid passes into a higher-temperature zone where pyrolysis occurs (no oxygen zone).Leaving the pyrolysis zone is a solid char that passes into a gasification region where some of the char reactswith H2O (or CO2) to produce H2 and CO. The remaining char moves down into a zone containing oxygen,where combustion occurs and oxygen is consumed.

The shaft furnace is almost always refractory-lined and is a simple reactor system. The major differencesbetween systems is found in the manner that the solid residue is removed and the auxiliary units, such asthose used to preheat the entering gas, clean the product gas, and utilize the product gas. Only a smallamount of pretreatment of MSW, such as the removal of oversized objects, is required.

The Andco-Torrax system is an application of the shaft kiln. In this system, the air to the gasifier is pre-heated to provide a sufficiently high temperature in the bottom of the kiln to slag the inorganic materialpresent. The gas leaving is not cleaned but fired directly in a close-coupled combustion, steam generation,and heat-recovery unit. The gas composition from a typical application is shown in Table 8.77. There areseveral commercial units operating overseas.

Union Carbide developed the Purox system in a 200-ton/day (180-t/day) pilot plant in South Charleston,West Virginia. Instead of using air, this unit uses oxygen. The product gas is treated to remove liquids andtars that are recycled to the high-temperature region of the furnace. The clean product gas composition isshown in Table 8.77. It differs from the Anco-Torrax gas, which contains over 50% N2.

Figure 8.54 shows the major features of a vertical retort. The solid is fed to the top, and char is removedfrom the bottom. It is a pyrolysis system, and the heat is supplied through the walls. A portion of the gasproduct is used as fuel to heat the walls. The heat transfer rate is slow, and the solids residence time is sever-al hours. When these systems were used for the thermal destruction of wood at low temperatures, a typicalproduct showed:

Gas 14–17% Methanol 1.5–2% Acetic acid 3.5–8%Tar 12–16% Char 31–37%

The Destrugas system represents a European-developed vertical retort operated in the pyrolysis mode. Itis based upon coke oven techniques. The furnace is externally heated to temperatures of 1650 to 1830°F(900 to 1000°C). Residence time of solid in the reactor is about 20 h. Typical values obtained from pilotunits showed:

Gas 36 wt.% Residue 28 wt.%

8.172 CHAPTER EIGHT

SOLID WASTE



Tar 4wt.% Water 32 wt.%

Pilot and demonstration plants have been run at sizes up to 18 tons/day (16 t/day). These systems are notpractical at a large size because of the volume to surface ratio limits, the area of heat transfer through thewalls, and the increase in thickness of material to be heated.

Mechanical Flow Systems. The most used reactor in this class is the rotary kiln or retort. The rotary kilnis a cylindrical vessel set at an angle of a few degrees from horizontal. This vessel is rotated slowly. The sol-id material enters the elevated end and is carried up the walls for some distance before it tumbles back to-

SOLID WASTE 8.173

FIGURE 8.53 Features of shaft kiln.

SOLID WASTE



8.174

TAB

LE 8

.77

Gas

Com

posi

tion

s an

d C

hara

cter

isti

cs f

or R

epre

sent

ativ

e S

yste

ms,

Per

cent

by

Vol

ume

Hig

h-te

mp.

Com

pone

ntA

ndo-

Torr

axP

urox

Lan

dgar

dB

abco

ckfl

uid

bed

Eba

ra

CO

10.3

476.

68–

158.

935

H2

11.2

336.

415

–25

13.8

23C

H4

1.9

42.

63–

82.

415

C2H

40.

81

1.8

2.7

7O

23.

0—

1.6

CO

210

.514

11.4

15–1

217

.619

N2

62.3

169

.745

–65

55.2

1P

roce

ssP

yrol

ysis

XG

asif

icat

ion

XX

XX

X

Gra

vity

flo

wX

X

Mec

hani

cal f

low

XX

D

rag

flow

XX

Hea

ting

Indi

rect

XD

irec

tX

XX

X

Car

rier

X

SOLID WASTE



ward the bottom of the kiln (as well as toward the lower end of the inclined vessel where it is discharged).Flights (see Figure 8.55) may be added to the vessel walls to help carry the solid close to the top of the kilnbefore it falls away from the wall.

The rotating kiln serves to agitate the solids and move the solid through the system. Only a small fractionof the total volume is occupied by the solid; most of the volume is occupied by the gas. The gas-solid contactis poor. The solid tumbles through the gas stream to the wall where it remains with little contact with the gasuntil it is ready to drop back through the gas stream. The rotary kiln will accept a wide variety of materials.Not all shapes and materials move through the system with the same speed. Some materials “roll” throughthe kiln rapidly and some light materials are caught in the gas and move through the system rapidly.

The greatest problems associated with any mechanical system result from the protection of rotating partsat high temperatures and the seals between the rotating members and rest of the process.

The Monsanto Landgard System is a directly heated rotary kiln gasifier. In this application, fuel oil isburned with a portion of the air to the kiln to provide the temperatures required for the thermal destructionof the MSW. The reported gas compositions are given in Table 8.77.

A plant to treat 900 tons/day (810 t/d) of MSW was built in Baltimore, Maryland, but never performed as

SOLID WASTE 8.175

FIGURE 8.54 Features of a vertical, continuous-feed retort.

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predicted from data obtained in a (30-ton/day) (27-t/d) pilot plant unit. Monsanto has withdrawn from theproject, and the plant is not being operated.

The Pyrecal process of Babcock-Krauss-Jueffer represents an indirectly heated pyrolysis process. Theexternal heating chamber is divided into several zones. Low pyrolysis temperatures between 750 to 1025°F(400 to 550°C) are used. This results in high liquid tar yields. These are sent to an external thermal crackingunit held at 1830°F (1000°C) where they are broken down into low-molecular-weight gases and char. Thesolid residence time is between 30 and 60 mm. About 30 weight percent of the product is gas and 30 weightpercent char. The gas composition is shown in Table 8.77. A 2 to 3 ton/day (1.8 to 2.7-t/day) pilot plant is be-ing built.

Drag Flow Systems. The solids move through the system as the result of drag forces resulting from theflow of a fluid. This includes molten salt, fluidized bed, and entrained-bed systems. In the fluidized-bed sys-tems (shown in Figure 8.56), the solid particles are inert and are suspended by the upward flow of fluid.

8.176 CHAPTER EIGHT

FIGURE 8.55 Features of a rotary kiln gasifier.

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Each particle is surrounded by a gas. There is an upper interface between the top of the fluid bed (solid–gasmixture) and the gas. The volume of the bed is only about 30% greater than would be occupied by the solidalone.

The particles move about rapidly. This rapid mixing results in isothermal operations. Heat transfer to anysolid added to the bed is rapid.

In the fluid bed, the solid particles are not carried out of the reactor system by the gas stream. In the en-trained bed, however, the gas velocity is large enough to carry the solids out of the reactor along with thegas. The entrained-bed reactor system may best be described as a pneumatic solids transfer system wherechemical reactions occur. In this system, the solids are not mixed and the temperature is not constantthroughout the reactor.

The Hitachi process is a low-temperature fluid-bed gasifier. Air is pressed upward to provide fluidization.The fluid bed is made up of inert sand operating at a low temperature [840 to 1025°F (450 to 550°C)]. The

SOLID WASTE 8.177

FIGURE 8.56 Features of a fluid-bed system.

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bed is fed shredded MSW. This low temperature produces a high liquid and char fraction. A typical yieldshows

Gas 30 wt. % Water 37 wt. % Char 13 wt. % Oil 17 wt. % Ash 3wt.%

The high liquid fraction comes mostly from decomposition of the plastic in the waste (12%) and lessfrom decomposition of the cellulose (5%). It is noted that Japanese MSW has a much larger plastic fractionthan does United States MSW. While the Hitachi process has been classified as gasification because reactivegas (air) is introduced, the low temperatures reached result in almost no reaction of char with these gases,and the product yield is more closely related to the pyrolysis yield.

If the temperatures of the fluid bed are raised to 1500°F (815°C), the char and tar fraction are reduced.The gas composition for this high-temperature system is shown in Table 8.77.

Ebara has developed a pyrolysis system using a fluid bed where heat is carried into the bed by a high-temperature inert solid. The fluid bed is composed of sand, which is continuously drawn from the bed, whilehot sand is continuously added at the same rate. This hot sand rapidly loses all sensible heat above the bedtemperature to the bed. The system is fluidized by recycling pyrolysis gas. The sand drawn off goes to a sandheater that is a second fluid bed where the char and liquids separated from the product gas stream areburned. The gas product does not contain nitrogen, since no air is introduced into the reactor. The productgas composition is shown in Table 8.77. This system has been demonstrated in Yokohama, Japan, at 30-ton/day (27-t/day) capacity. A similar plant has recently started up in Funabachi City, Japan, using three 150-ton/day (135-t/day) trains.

Occidental Research developed an entrained-bed pyrolysis system. Recycle gas was used to entrain hotchar at about 1380°F (750°C). Finely divided RDF is introduced at the bottom. The temperature drops to750°F (400°C) at the top. By controlling the gas velocity and the reactor height, the solid residence time iscontrolled (a few seconds). Controlling the temperature to modest levels and providing short residence timesleads to high liquid yields. Typical values are

Gas 27 wt. % Water 13 wt. % Oil/tar 40 wt. % Char 20 wt. %

The liquid product (“garboil”) was tested as a substitute for Bunker C fuel oil. It had several rather ob-noxious qualities that suggest it is a poor substitute. It was corrosive, requiring special storage facilities andfuel nozzles, etc. It was more difficult to pump and smelled bad. These qualities largely resulted from thewide range of highly oxygenated organics (including acids).

A 100-ton/day (90-t/day) demonstration plant was built in California but never ran successfully and wasshut down.

There are many advantages of the fluid-bed and entrained-bed systems. They have a high throughput. Inthe case of the fluid bed, it is an isothermal system, and the temperature is easy to control. It also produceshigh gas yields. There are also some major disadvantages. The most important is the requirement thatMSW be pretreated to reduce the size to less than 1½ in (3.8 cm) and materials that cause slagging in thebed, such as glass and aluminum, must be controlled to a low value. It requires at least a poor-quality RDFas feed. In the case of the entrained bed, the feed must be ground to the consistency of flour or fluff. Sig-nificant energy may be expended in terms of blowers to provide for suspension and movement of particlesin these systems.

The emphasis in pyrolysis and gasification has moved in the direction of producing a gaseous fuel for

8.178 CHAPTER EIGHT

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several reasons. There are over 15,000 boilers in the United States firing natural gas or oil with a capacitybetween 50,000 and 250,000 lb/hr [150 to 750 ton/day (135 to 675 t/day) RDF]. In many cases, these couldbe used to fire low-Btu gas from a close-coupled gasifier. The gasifier would be located adjacent to the boil-er and the necessary breeching provided to move the hot gas to the boiler. Burners and controls would re-quire replacement. Any char and liquid formed should burn along with the gas in the existing combustionchamber. Overall thermal efficiencies of 85% or more would be expected.

If it is necessary to cool and clean the gas prior to combustion in order to protect the environment, theboiler, or because there is not sufficient time to burn tar and char, the efficiency drops to about 55%. Thesensible heat in the pyrolysis gas as well as the energy of the char, tar, and liquids are lost.

Figure 8.57 shows the effect of boiler efficiency gas and air volumes for fuel gas of varying energy val-ues. It shows the efficiency to drop rapidly for heating values less than 200 Btu/ft3 (7450 KJ/m3) indicatingthat a boiler firing a low-Btu gas would have to be derated. This also shows that the amount of flue gas pro-duced for a giver boiler rating rises rapidly for heating values less than 400 Btu/ft3 (14,900 KJ/m3). For boil-ers designed with critical gas velocities (more likely in American design than European), the low-Btu gaswould again require derating.

For cases of pyrolysis gas, a medium gas is produced, and no derating would be required.The nitrogen-free pyrolysis gas may be used as a process synthesis gas for the production of organic

chemicals. The city of Seattle, Washington, performed an extensive feasibility study to consider producingeither methanol or ammonia from MSW. While the study came to a positive conclusion, the program stalledbefore detailed design was carried out. The preliminary plans called for the Purox process to produce amedium-Btu gas. To produce methanol, this synthesis gas is cleaned; reformed (to convert hydrocarbons toH2 and CO); shifted (to provide an H2/CO ratio of 2/1 needed for methanol production); and scrubbed toremove CO2. This gas then goes to a catalytic reactor where methanol is produced according to the reac-tion

CO + 2H2 = CH3OH

Methanol can be used in an internal combustion engine either as an additive to gasoline where it acts as anoctane enhancer or as straight methanol (which requires engine modification).

Advantages over Direct Incineration. There are numerous advantages to using pyrolysis and gasificationsystems for treatment of MSW as a replacement for incineration. Some of these are listed below. In this list,a system that produces a fuel gas by either pyrolysis or gasification followed by a combustion system for thisfuel gas is compared to direct incineration.

1. There is less gas to clean in order to protect the environment. The volume of air used to incinerate wasteis 5 to 10 times that used to pyrolyze the same waste. The cost of gas cleaning is much less.

2. There is less gas to clean for environmental protection even in the case of close-coupled systems wherethe fuel gas products are burned prior to being cleaned. Large excess air quantities are required to burnthe waste directly, whereas the gaseous fuel products require far less excess air.

3. Pyrolysis systems allow for the energy user to be located some distance from the waste processing plant.For cases where char and liquid are the products, they may be stored until needed. User demands neednot match the incinerator output.

4. Investment and operating costs are higher for incinerator. In most cases, the pyrolysis and gasificationsystems are much simpler than the mass burn incinerators where heat is effectively recovered. Table 8.78points out factors relative to these costs.

5. Pyrolysis and gasification systems can handle a variety of wastes, such as rubbers and plastics, that causeproblems in most incinerators as a result of high temperatures resulting from these high-heating-valuefeeds.

6. There is a much wider range of uses for the energy products from pyrolysis and gasification than from in-

SOLID WASTE 8.179

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cineration (steam). The fuel products can be used in boilers, engines, turbines, etc., as well as a chemicalfeedstock.

7. Low-cost packaged boilers or a wide variety of existing equipment may be used to burn the fuel gas.

Pyrolysis is an old technology that is not widely used today. While there are apparent advantages over in-cineration, it is seldom recognized as an alternative to incineration. Although incineration has had majorproblems in the United States, it still remains conditionally acceptable. Unfortunately, there is not sufficientexperience in operating commercial pyrolysis and gasification systems to establish the validity of many ofthe advantages claimed.

8.180 CHAPTER EIGHT

FIGURE 8.57 Combustion efficiency and theoretical air and flue gas volume as a function of fuel gas heating volume.

(1 lb = .4536 kg)

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Wet Oxidation

The wet oxidation process is a type of incineration that uses oxidation in the presence of water in a closed re-actor at moderately high temperatures and at a wide range of pressures. Since solid wastes are generally gen-erated and collected in relatively dry form, the wet oxidation process is not currently common to solid wasteprocessing but is used for the treatment of wastewater sludges and concentrated wastewater streams.

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SOLID WASTE 8.181

TABLE 8.78 Comparison of Pyrolysis and Combustion Furnaces

Aspects Pyrolysis Combustion

Atmosphere Reducing Oxidizing, sometimes alternately reducing and oxidizing

Temperature Mostly lower Especially local high temperatures are possible (on grate)

Construction In most cases only few Mostly mechanically moved gratesmoving parts

Corrosion during heat Small in case of pure Deposition of fly ash; corrosion of the tubesrecovery pyrolysis (gas cleaning of steam boiler (gas cleaning after energy

before energy recovery) recovery)

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109. Franklin Associates, Ltd., Characterization of Municipal Solid Waste in the United States, 1960 to 2000, report prepared forthe Office of Solid Waste and Emergency Response, U.S. Environmental Protection Agency, Prairie Village, Kan., 1986.

110. Franklin Associates, Ltd., Characterization of Municipal Solid Waste in the United States (1994 Update), report prepared forthe Office of Solid Waste and Emergency Response, U.S. Environmental Protection Agency, Prairie Village, Kan., 1994.

8.184 CHAPTER EIGHT

SOLID WASTE



solid waste

Documents

solid wastesolid waste

residential waste generation

solid waste 8

quartering technique

adverse effects

solid wastes

environmental effect

waste characterization