CHAPTER 40EFFLUENT TREATMENT

OPTIMIZATION

In the United States, effluent water quality criteria are no longer matters of localagreements between regional regulatory agencies and industries; they are estab-lished by state and federal statutes. In considering effluent treatment for dischargeinto a receiving stream or lake, each plant outfall must be considered individuallyas each requires a separate discharge permit from the Environmental ProtectionAgency (EPA). This is because the effluent characteristics vary widely; even in thesame industry, plants having the same capacity and processes may produceeffluents of widely different quality and quantity. This may be due to differencesin raw material composition, sources of plant water supply, water use and recycle,geographic location, or age of the facility.

SOURCES OF EFFLUENT

Data on raw water consumption may not provide accurate information as to thequantity of effluent that will require treatment. Instantaneous plant effluent flowscan be as small as 10% or over 200% of the incoming supply rate. Even on a dailyaverage, the effluent may be only 20% of the raw water supply because of evapo-ration losses, or may exceed the raw water flow on a rainy day (Figure 40.1). Toobtain information regarding type of treatment and equipment size requirements,all sources of water must be examined.

Although normally containing little contamination after the initial flow, stormwater runoff can contribute significant volume to the plant effluent. Along theGulf Coast, where precipitation may exceed 60 in (150 cm) per year, plants mustconsider runoff in their treatment plans. In other areas, climatic conditions mayprovide a means to eliminate storm water discharge by favorable evaporationrates. Storm water may be collected and impounded as a good plant watersource—often lower in dissolved solids than the normal supply; instead of beinga discharge load, the storm water can become a benefit. For example, a refineryon a 500-acre (200-ha) site in an area where annual rainfall is 36 in (90 cm) dis-charges an average storm water flow of 1.5 mgd (2.62 m3/min). A significant por-tion of this may be reclaimable if provisions are made to collect it during the rainyseason.

The quantity of wastewater flow can be influenced by infiltration of ground-water into the sewer system, especially during the rainy season. Sewers located

FIG. 40.1 The effect of storm water on plant effluent flow rate. 1 and 2 are peaks caused byrainfall.

below the groundwater table or near stream beds may have continual infiltrationentering through poorly constructed joints and leaking manholes or manhole cov-ers. Along the seacoast, the rate of infiltration may be influenced by tides. Ratesof infiltration range from 15,000 to 50,000 gal/day/mi (35 to 120 m3/km) of sewer.

PROCESS WA TER USE AND CONTAMINA TION

Discharge patterns are normally determined by examining individual processwater uses. Process waters may be required for washing, rinsing, direct contactcooling, solution makeup, chemical reactions, process condensation, and gasscrubbing operations. Spent process waters normally generate the largest load ofcontamination in plant effluents on a daily average basis. This is usually causedby inadvertent leakage of process liquors into the water system. Individual pro-cess water uses within a plant may also produce contaminant discharges that maybe incompatible in a combined industrial-municipal treatment system. Table 40.1lists some process waters in several major industries where salinity increase andindustry-specific pollutants occur.

Contamination may occur from spills and leaks of process or product liquorsfrom manufacturing operations. The volume from spills is usually low in com-parison to total water used, but owing to high concentration, its loss to the seweroften produces a significant impact on the waste treatment operation. Similarly,storage and transport of raw material or product may require attention as a poten-tial source of contamination. In analyzing wastewaters, it should be assumed that

Le

vel,

in.

Average f low rate : 70.4 gpm, 101,376 gpd

Peak f l ow = 308.0 gpm

Low f l o w = 47.2 gpm

raw materials and products in the process area may somehow find their way intothe sewer, and these should be sought in the analysis if they are objectionablecontaminants.

Maintenance operations from cleanup and repair of equipment, tankage, andtank trucks or cars also produce concentrated wastes.

UTILITY SYSTEMS

Additional wastes are generated in preparing raw water for use in the plant's util-ity system. The water treatment plant must dispose of suspended or dissolvedsolids removed from the supply water as well as chemicals added for treatment.These wastes may discharge on a continuous or batch basis to the plant sewerfrom clarifiers, filters, softeners, and demineralizers.

The plant's utility system discharges less water than it receives. Steam is lostto the atmosphere, but the greatest evaporation loss occurs over cooling towers,when these provide the main source of heat removal. To control the buildup ofundesirable materials in its water systems caused by evaporation, the utilitiesplant removes a small portion of water from the boilers or cooling systems asblowdown. The blowdown may be only 5 to 10% of boiler and cooling towermakeup, accounting for the large "shrinkage" between raw water and effluentflows. This blowdown contributes some salinity to the effluent since it has beenconcentrated by evaporation and also contains chemicals that have been addedto control corrosion, scaling, and fouling. A typical water balance of a refineryhaving an evaporative cooling system shows a 60 to 80% loss of water because ofevaporation in the plant.

Finally, plant washrooms and dining facilities generate sanitary wastes. If theseare not sent to a municipal treatment system, on-site treatment must be providedfor disposal.

PLANTSURVEY

Industrial waste treatment planning begins with a review of plant plot plan andsewer diagram. The plot plan provides general information showing plant size,storage of raw materials, intermediates and finished products, manufacturingoperations, materials flow, water supply, and effluent disposal sites. The sewer

TABLE 40.1 Processes that Increase Effluent Salinity

Industry

Pulp/paperPetroleumSteelCokeTextileFoodMachined partsAll industries

Process

Bleaching, pulp washingCrude-oil desaltingBlast-furnace gas scrubbingCondensation of moisture from coalWashing of fiber or yarnPeeling, washing, cleanupCleaning, paint-spray controlWater treatments producing strong liquid wastes (e.g., ion

exchange, reverse osmosis)

drawings show sewer and manhole locations, floor drains, separated waste collec-tion systems, downspouts and storm drains, and any existing effluent treatmentfacilities. A review of these drawings and available rainfall data will give a pictureof sewer loadings, including storm water and plant wastewater.

A valuable reference to possible contaminant discharges is a review of pur-chase order records, chemical inventories used in conjunction with manufactur-ing operations, and final production. Discrepancies between input and outputprovide insights into the types and probable quantities of contaminants that maybe present in the plant effluent (Table 40.2). In reviewing the list of chemicals usedin the plant and potentially present at certain times in the effluent, the surveyteam should be on the alert for toxic pollutants cited by the EPA as "prioritypollutants." If these are present in the plant, their analysis will be required inplant wastewaters. Any treatment program proposed for the plant will have toreduce these pollutants to acceptable levels—and that may be a level not detect-able by current analytical methods. Table 40.3 lists the EPA priority toxic pollu-tants to be aware of in making a pollution survey.

The potential for disposing of spent chemicals by dumps to the plant sewer orother careless practices must be considered and fail-safe procedures developed toprevent such occurrences.

Inspection of the manufacturing facilities should follow the review of draw-ings. This tour should focus on water use and resulting waste discharges from eachprocess to identify problems not readily evident by data review alone. Excesswater usage, faulty control, poor housekeeping, and inadequate equipment arecommon examples of factors contributing to unnecessary contaminant dis-charges. The inspection offers the opportunity of correcting and preventing con-taminant discharges at the source rather than removing the contaminant "afterthe fact" in the plant effluent. Critical to reducing pollution in a plant is a programto make employees as aware of pollution as they are of safety. As with a successfulplant safety program, employee awareness requires continual communication byposters and training sessions.

TABLE 40.2 Raw Materials List for a Small Plant Producing Textile Chemicals*

Acids (sulfuric, stearic, oleic)Aluminum stearateAlcohols (isopropyl, methyl, butyl)Butyl carbitolChlorinated phenolsFormaldehydeKeroseneMineral oilMonoethanolamineOils (pine, peanut, castor)PolyethyleneSodium chloride, perborate, tripolyphosphate, hydroxideSoap flakesTallowTapioca flourWaxes (petroleum and synthetic)

* These chemicals include detergents, softeners, wetting agents, and sizes. All of them are potentialwastewater pollutants.

* This list is being continually updated and must be checked for current use.

The plant tour should also include an inspection of existing waste treatmentfacilities to obtain operating data and equipment sizes. In some plants, night oper-ations may differ from day operations, so inspection of each shift is usuallyrequired.

ASSESSING WASTE FLOWS AND QUALITY

Of prime importance is the record of volumes of water requiring treatment. Flowof total plant discharge as well as individual process wastes must be measured.Many plants have permanent flow monitoring devices to meter total planteffluent. However, individual process area flow measurements are also necessary.To provide realistic data on flows to be treated, measurements should be takenover as long a period as practical to show what flow variations must be accom-modated in a waste treatment unit (Figure 40.2). (See Chapter 7, FlowMeasurement).

Chemical characteristics of the wastewater and quantity of contaminants pres-ent must be established. These are dependent on the raw water supply itself, the

TABLE 40.3 EPA Priority Toxic Pollutants*

1 . Elements and their compounds (organicand inorganic)

AntimonyArsenicBerylliumCadmiumChromiumCopperLeadMercuryNickelSeleniumSilverThalliumZinc

2. Inorganic compoundsAsbestos (mineral)Cyanides

3: Organic compoundsAcenaphtheneAcroleinAcrylonitrileAldrin/dieldrinBenzene, chlorinated benzene, and

nitrobenzenesBenzideneCarbon tetrachlorideChlordane and metabolitesChlorinated ethanesChloroalkyl ethersChloroform

DDT and metabolitesDichlorobenzinineDichloroethylenesDichloropropane and propeneDiphenyl hydrazineEndosulfan and metabolitesEndrin and metabolitesEthylbenzeneFluorantheneHaloethersHalomethanesHeptachlor and metabolitesHexachlorobutadieneHexachlorocyclohexaneHexachlorocyclopentadieneIsophoroneNaphthalene and chlorinated

naphthaleneNitrosaminesPhenols, chlorinated phenols,

nitrophenols, and 2,4-dimethylphenolPhthalate estersPolychlorinated biphenyls (PCBs)Polynuclear aromatic hydrocarbons2,3,7,8-tetrachlorodibenzo-p-dioxin

(TCDD)TetrachloroethyleneToluene and dinitrotolueneToxapheneTrichloroethyleneVinyl chloride

FIG. 40.2 Effluent flow variations caused by cyclic operations.

water treatment processes, the concentration through utility use, contaminantsadded in the process operation, or chemicals lost from manufacturing and inven-tory. Besides the probable contaminants identified through raw materials use,additional types of contaminants may be present as by-products from the plantprocesses or as trace contamination in raw materials. Each industry has its ownwastewater profile. For example, petrochemical production, petroleum refining,and iron and steel manufacturing produce contaminants such as cyanide, phenols,ammonia, and sulfides at different concentrations and ratios. A guide to deter-mining probable contaminants in industrial wastes may be found in numerousEPA-funded studies that were used to establish criteria for effluent compliance byeach major industry. Some of this data is summarized by Table 40.4.

SAMPLING AND ANALYSIS

After determining the contaminants to be analyzed for, sampling of the plantstreams, both in selected manufacturing areas and in plant outfalls, can proceedto define contaminant loading. The sampling schedule should uncover averagecontaminant discharges as well as peaks from cyclic or batch operations and pro-cess variables. In conjunction with the sampling, continuous monitoring of pH,temperature, and conductivity further define variations in wastewater character-istics (Figure 40.3). Existing waste treatment facilities should also be sampled toestablish unit loadings, operating conditions, and contaminant removalefficiencies.

A summary of the raw data may be prepared to show a breakdown of contam-inants and flows making up the total plant discharge, focusing on contributionsfrom the various processes and manufacturing areas.

Industry*

TABLE 40.4 Raw Wastewater Profiles for Selected Industries

Textile

CCCCCMMMMCIIIVVIIMIIC

Steel

MCCMMVVMIIMVVVIVVVVII

Petroleum

CCGMCMMCICVCCCVVCMCCC

Paper

MCMIMMMCMCVIIIVIVMVCV

Mining

MMMVMCIVVIMVIIVVIIIIV

Food

MVVVMMMCCCIIIIVIVMIIC

Coke

MMMMMMMCMCIMMIIVMMMII

Chemical

CCMMCMMVVCVVVCVVVMVVV

Automotive

MCMMMMMIIICVICCIIMIIM

Aluminum

MCCCMVVCVICIICICCIIIV

Contaminants

Suspended solidsSalinitypH variationsOil and greaseSettlcable solidsBODCODHeatColorOdorHeavy metalsCyanidesThiocyanatesChromatesPhosphatesFluoridesAmmoniaOrganics (general)Phenol icsPesticides, biocidesSurfactants

* M, major factor; C, contributes to the problem; I, insignificant; V, varies in the industry, may contribute.

FIG. 40.3 Conductivity and pH record of a mill outfall, showing discharges at about 1 a.m.and 4 to 5 p.m., events repeated each day. This is an old city sewer passing through the mill,and investigation disclosed dumps of spent alkali by a neighboring plant.

EQUALIZATION

One of the most common practical first steps to take in planning waste treatmentprograms is to provide a large basin for equalization of flow, concentration, andin some cases, temperature. It is clear that no physical plant, especially one usinga biological process, can successfully cope with the storm water surges shown inFigure 40.1 or the cyclical process discharges of Figure 40.2, nor can it easily dealwith the concentration changes represented by Figure 40.3.

After all precautions have been taken to minimize the range of swings in flowand concentration, new data must be obtained to establish the variations stillpresent. Then a holding basin must be designed to receive and blend variouswastes to produce an outflow of strength and rate that subsequent waste treatmentequipment can handle.

Sizing the equalization basin volume requires a knowledge of the tolerancelevels of the process equipment to flow and concentration changes. Solids/liquidseparation devices are subject to upset by rate of change in flow and temperature,while biodigestion devices are more disrupted by change in waste concentrationand temperature. Sometimes maximum flow may occur at the same time as max-imum waste strength, so enough data must be obtained to establish the probabil-ity of this. Then, these data must be plotted, the areas integrated, and average,maximum, and minimum loadings established against time to provide a reason-able time in detention.

In some cases, the equalization of a widely variable tributary can minimize thesize of the mainstream equalization basin. Diversion basins are also valuable todeal with storm water and accidental dumps. But, in every case, the equalizationbasin itself, despite the complications of its design formulas, is a simple, practicalpretreatment structure for any industrial wastewater treatment program.

CONSERVATION TO MINIMIZE EFFLUENT LOADING

At this point if it appears that a relatively large volume of water may requiretreatment, consideration is given to reducing plant water use. The first area forwater conservation is usually process water where water use often exceeds equip-ment requirements. Secondly, waters identified as mildly contaminated butacceptable for direct reuse in a process not requiring high quality should be seg-regated for recovery. The third area considered is substitution of recirculatingwater systems for once-through indirect or direct contact cooling waters. In thesteel and petroleum industries, these can make up as much as 90% of total wateruse. Figure 40.4 is a general schematic of water usage that is typical of manyindustrial plants. The major water sources are storm water plus a conventionalraw water supply (which may be surface or well water). The potential opportu-nities for flow reduction shown by Figure 40.4 are: (1) the elimination of unnec-essary or wasteful flows, (2) water recovery and diversion to secondary uses, and(3) water recycled within the system. Figure 40.5 shows the reduction achieved ingoing from a once-through to an evaporative cooling system.

By simply providing additional rinse tanks with a countercurrent flow of rinsewater, metal plating and finishing plants may reduce rinse water requirements byover 75% (Figures 40.6 and 40.7). Installation of water pretreatment or side-stream treatment to remove objectionable contaminants from recirculating cool-ing systems can extend cooling water use and reduce blowdown.

Evaluation of the survey data may show that a single process or manufacturingarea discharges the bulk of the most significant contaminants present in the totalplant discharge. Individualized in-plant treatment of that particular source maybe warranted. In plating shops this is usually required because treatment of indi-vidual contaminants such as cyanide or hexavalent chromium requires chemicalreactions incompatible with one another. Poor maintenance practices may causeunnecessary contamination; improved practices and installation of containments

Concentrat ion r a t i o (CR)

FIG. 40.5 Reduction of makeup flow by concentration in an evaporative coolingsystem.

FIG. 40.4 Potentials for water conservation.

Impound as a source of makeup

Impound for dilutionStormwater

Once-throughcooling

Put on cooling tower— pros and cons

Use for makeup

Reduce blowdown. Recover more condensate,improve pretreatBlowdown to cooling tower (?) pros and cons

ReducethroughputImprovequalityEqual-

izationWaste

treatment Out

Municipal sewer

Separate treatment

Pre-treat Boiler system

Reduce blowdown

iSidestream

Multi-stage

Pre-treat

Coolingtower system

Upper l imit -once- th rough f l ow

Example - 10,000 gpm at 2O0F temperature r ise

Makeup (M)

Blowdown - M-E

Evapora t ion (E)

Lower l im i t -evapora t ion ra te

Process

Sanitary

Mak

eup

,gp

m

Ratio of rinse water to dragout flow rates

FIG. 40.7 Ratio of rinse water to dragout flow rates.

for spills and leaks can reduce contamination. Figure 40.8 shows a scheme forcontainment of strong wastes for recovery or separate disposal.

If possible, sanitary wastes should be collected and sent to a municipal treat-ment plant. Alternatively, septic tanks for the sanitary waste and discharge into adrainage field could be provided if volume is small and ground conditions permit.The septic tank discharge may also be chlorinated and then mixed with the plant'sprocess wastes.

FIG. 40.6 Counterflow rinsing in a metal finishing shop saves water.

Work in Workout containing dragout

Incoming water

Outgoing water

Rat

io o

f C

O/C

R,

(C0

= D

rago

ut l

iquo

r co

ncen

trat

ion

mg

/l,

CR

=Rin

se d

isch

arge

con

cent

ratio

n, m

g/1)

Example:Calculate rinse water (RW) for dragout (D) =2gpm at 10,000 mg/l ;required CR is 150 mg/1CD/CR=200

For single-stage, RW/D =200, RW = 400gpm

For two -stage, RW/D = 14, RW = 28 gpmFor three-stage, RW/D = 58, RW= 11.6 gpm

FIG. 40.8 Diking for containment of strong wastes for separate treatment.

REVIEW OF EXISTING FACILITIES

Some industrial discharges contain colloidal solids or emulsified oils that are dif-ficult to remove and may carry through the waste treatment unit. Coagulants arerequired to destabilize these suspensions and allow the particles to float or sink.A common problem in waste treatment units is short-circuiting of the wastewaterflow, caused by poor distribution of inflow and discharge, tilted overflow weirs,or other poor design features. Installation of baffles or redesign of distributor andcollector manifolds may alleviate the problem and allow effective use of the totalunit. (See Chapters 8 and 9.)

Not to be overlooked in the plant survey is the adequacy of existing equipmentfor treating the average wastewater flow. If clarifiers are found to have a highhydraulic loading, reduction in water usage, the addition of properly sized equip-ment, or additional equalization volume may be required.

Biological waste treatment units are very sensitive to wastewater conditions.Ineffective contaminant removals can result from one or several factors such asvariable flow; too high or low an organic loading; low dissolved oxygen levels;inadequate mixing; insufficient nitrogen or phosphorus nutrient levels; presenceof toxic compounds; erratic changes in waste composition, pH, or temperature;and growth of undesirable filamentous bacteria or fungi. The ineffective treatmentmay also be related to inadequate design, such as insufficient aeration capacity ordetention time and inadequate solids/liquid separation facilities. A biosystemoperation may be optimized by careful review of all operating variables and insti-tution of proper controls. (See Chapter 23, Biological Digestion.)

Although biological waste treatment can effectively reduce organics, microbialgrowths occurring in other types of waste treatment units may hinder overalltreatment efficiency. These growths can produce nonflocculating solids or odo-rous gases that prevent effective sedimentation and cause fouling or plugging of

•Drainage trenches aroundarea at dikes

Spillcontainmentdike

Reactor

Processarea

Sump with pump

Transferpump

Main sewer

Spillcontainmentholding tank

-Manual bypass valveto sewer, only openduring hosedownafter spill

To scavenger,

incinerator, or recovery

equipment. They may be controlled by preventing accumulations in dead spaces,elimination or control of their source of food, and application of biocides.

Through the plant tour and review of survey data, it is often possible to reduceboth the volume of water and quantity of contaminants that require final treat-ment. Plants having existing waste treatment systems may find that the in-plantcontaminant reduction, segregation, or water reuse studies provide the means forsufficient improvement to achieve current effluent compliance. The improve-ments may be realized by reduction of water use with resultant increase in resi-dence time.

Control or replacement of chemicals at the process source may eliminate theneed for a particular waste treatment process, such as activated carbon forremoval of refractory organics. Segregation or in-plant treatment of certain con-taminants may prevent adverse effects that can occur with mixtures of differentprocess streams, such as the complexing of metals from one stream with cyanidesfrom another, and solids density loss by mixing a turbid stream with an oilystream.

PROCESS MODIFICA TION

One alternative for eliminating or reducing process wastes involves the modifi-cation or elimination of steps producing these wastes. For example, the peelingprocess is one of the greatest sources of waste in most fruit and vegetable pro-cessing plants. Research has been directed toward modifying the peeling processso the peel waste can be removed without excessive use of water. One such pro-cess modification is the "dry" caustic peeling process for potatoes. In the conven-tional steam or hot lye peeling processes, potato peels may contribute up to 80%of the total effluent BOD. The new dry caustic peeling method collects peels andcaustic as a solid residue, preventing their entrance into plant wastewaters.

In a chemical plant, additional stages of separation to improve product yieldfrom evaporators, washers, filters, or crystallizers may reduce loss of product tothe sewer. Often these additional stages are not economical in terms of productrecovery costs, but they pay out in reducing final effluent treatment costs.

If possible, the in-plant or waste treatment improvements should be accom-plished prior to making final plans for additional facilities. With completion ofthis program, the total plant discharge should be recharacterized in terms of bothflow and contaminant concentration to reveal the extent of the improvements aswell as the contaminants still requiring reduction. Since the wastewater is likelyto contain an array of contaminants of various concentrations, a single treatmentprocess usually will not effectively remove each contaminant to the required lev-els for discharge to a receiving stream. If the plant has no existing waste treatmentfacilities, a system of two or more stages may be required.

The purpose of the first stage (referred to as primary) or an intermediate stagetreatment process would be to remove or reduce contaminants that would inter-fere or overload a subsequent treatment unit operation (secondary or advancedwaste treatment stage).

With the final wastewater characteristics and effluent quality requirementsestablished, the selection of the treatment program may be initiated. The first stepis to define the progressive stages of treatment required to obtain the final wastequality goals. The designer should prepare a preliminary schematic flow sheet list-ing each sequential process treatment step. Perhaps available technology may

offer several choices of methods as alternate schematic flow sheets forcomparison.

Preliminary evaluation of the possible treatment schemes in terms of esti-mated installed cost, system limitations, complexity of controls, area and laborrequirements, and flexibility for future needs may decide the final treatment selec-tion or simplify the choices considerably.

PILOT STUDIES ESSENTIAL

A pilot study to simulate the waste process is required to generate the necessarydesign information. This may be performed by laboratory bench tests or pilot-scale equipment. Bench scale studies are quicker and easier to perform, and maybe used to screen and define chemical treatment requirements, and determinecontaminant removal efficiency. Obtaining a representative sample is critical forreliable results. The bench test should be performed on composites and variousgrab samples to determine reliability and establish the ability to obtain repetitiveresults. (See Chapter 7, Sampling.)

Pilot plant studies should be undertaken where influent waste fluctuations arepresent and must be considered in the process performance. The pilot studiesshould include a continuous flow evaluation, preferably on a slipstream of the

FIG. 40.9 Bench-scale biological studies require frequent attention toobservation and recording of data during and following the period ofacclimatization.

actual waste flow. The study should continue for a long enough time to includeall the major variables encountered in the plant and to provide sufficient data fordesign purposes. In some pilot scale studies, such as biological treatment, an accli-matization period is required to obtain reliable, steady-state conditions (Figure40.9).

These studies determine the quantity of solid wastes generated by the proposedtreatment. In many industrial wastewater treatment applications, from simple

operations such as food processing to the complex processes of a petrochemicalplant, solid waste handling and disposal can present a formidable technical andeconomic problem. In some cases a chemical wastewater treatment program maybe modified to reduce solids generation. For example, this could involve replacinginorganic coagulants with organic polyelectrolytes to minimize sludge formation.Sources of excess solids may be located and brought under control. Reduction ofchemical losses in the process operations may significantly reduce sludge produc-tion. This is particularly true in metallic waste treatment where lime is often usedfor neutralization and precipitation.

If solids are difficult to remove in a wastewater clarifier, they will more thanlikely be difficult to dewater. These sludges will often require chemical sludge con-ditioning and specialized processing such as precoat vacuum or pressure filtra-tion. Inorganic sludge conditioning chemicals and precoat materials contribute tooperating cost not only from a raw materials standpoint, but also from rehan-dling, since their use contributes to the final volume, thereby increasing ultimatedisposal or haul-out costs. A process of growing interest is solids stabilization, orconversion to a calcareous, cementlike solid that will resist leaching of its inor-ganic toxic components.

In dealing with in-plant or "end-of-pipe" treatment, consideration must begiven to ultimate disposal of the contaminants removed from the wastewater.Volatile ingredients may cause air pollution, and sludges may contain materialsthat would not be acceptable for landfill because of possible water or soilcontamination.

USING PLANT EFFLUENTS

The question often arises as to whether plant effluent can be considered a sourceof plant water. It may, though probably not without additional treatment. Perhapsit is useful simply because it is water—it may be usable for irrigation, for example.It may be used in the plant strictly for wash-up if separate piping can be installedto accommodate it. But in most plants, it has already become concentrated byevaporation in boilers or in evaporative cooling towers, and it may be even fur-ther concentrated by influx of high-salinity wastes from ion exchange regenerationor from sour water strippers, for example. The effluent probably will have becomeconcentrated beyond the tolerance levels of the cooling system or the boiler sys-tem. Additional treatment of the effluent would then be necessary to upgrade itsquality by reducing critical limiting factors such as water hardness, alkalinity, dis-solved solids, or silica (Figure 40.10).

The location of points of use relative to the waste treatment plant, the cost ofseparate piping to reclaim the water, and the balance between further treatmentof the effluent versus more complete treatment of the raw water source must becarefully studied before a decision can be made on the potential of reusing planteffluent as a water source. Most studies show that improved treatment of a watersupply to a boiler or cooling system is preferred to recycling of the final effluentfrom a waste treatment plant as the best method of optimizing the total waterbalance. Often the effluent liquid contains organic matter that would interferewith the proposed chemical treatment and control.

It is fundamental that where an evaporative system is limited by the concen-tration of salinity, adding wastewater with a higher salinity than the control limitset for the evaporative system is counterproductive, as it actually increases thedemand for low-salinity makeup.

FIG. 40.10 Electrodialysis system processing wastewater for concentration and final dis-posal. (Courtesy of HPD, Inc., Naperville, III.)

Because of this increasing salinity with water use—which is multiplied by recy-cle—and because of both legal and technical limitations, it is apparent that themajor goal of recycle is not minimum water consumption, but optimum waterusage. However, these goals coincide when water is scarce or where zero dischargeis a realistic possibility. In water-short areas of the west and southwest, zero dis-charge may be an EPA-permit requirement attached to the location of a plant.Some utility plants in such areas have installed specially designed evaporators toprocess all wastewaters (cooling tower blowdown, spent regenerants, sluicewaters, etc.). These evaporators produce distillate for recycle plus precipitatedsalts as waste solids. Such equipment is costly and can be justified only becauseof the National Pollutant Discharge Elimination System requirement of zero dis-charge. In some areas, solar evaporation ponds achieve zero discharge at theexpense of land area instead of energy. Underground disposal, where permitted,may compete with evaporation.

SURVEILLANCE OF PROGRAM

To ensure consistent compliance with effluent restrictions, not only a well-designed but also a well-operated facility is necessary. In dealing with chemicalwaste treatment, over- or underfeed of chemicals will result in inefficiency andpoor quality. Chemical feed rates should be verified periodically by jar or labo-ratory tests. Also of importance with chemical feeds is correct application pointand uninterrupted feed.

Most regulatory agencies require routine scheduled analyses for effluent com-pliance purposes. The critical effluent analyses should also be checked on the rawwaste flow to adjust unit operations accordingly. The frequency of such tests isbased on influent waste variation.

To keep waste discharges under control, a schedule of in-plant waste sur-charges, based on BOD, flow, and suspended solids from each operating area (like

municipal surcharges) keeps area supervisors aware of the performance of theirdepartments.

In many instances, on-line monitors are desirable to provide a continuousrecord of effluent quality and to adjust chemical additions and equipment perfor-mance as necessary to maintain optimum conditions. These devices can warn theoperator if a test is out of its control range. The monitoring devices should becleaned and calibrated as directed by the equipment supplier and the scheduleadjusted through experience.

In addition to control analyses normally required on waste treatment effluents,regulatory agencies may require biological monitoring for discharges to receivingstreams or bodies of water designated for wildlife or recreational activities. Oneform of this monitoring may be performance of toxicity measurements. Thesemay expose indigenous fish species to controlled dilutions of waste or, in cages,to the receiving stream below the plant outfall. Total counts of various aquaticorganisms above and below the outfall are sometimes preferable. A change in thevariety of species and the populations of each provides information on the influ-ence of the plant wastewater on the receiving stream.

The industrial discharge may comply with applicable criteria but still containcompounds harmful to the aquatic organisms or wildlife exposed to the receivingstream. Another form of monitoring related to biological activity of dischargesinto surface waters is dissolved oxygen sag measurements downstream of dis-charges. The temperature and residual organic and dissolved solids content mayproduce a reduction in oxygen levels essential to fish life. (See Chapter 5.)

ZERO DISCHARGE—POSSIBILITIESAND REALITIES

As designers of industrial plants and utilities have attempted to minimize the costof wastewater disposal, most have almost completely eliminated once-throughcooling in favor of evaporative cooling. A significant result is the discharge ofwater from the plant as vapor rather than as liquid. For example, a typical plantthat used once-through cooling of 50,000 gal/min of water with a temperature riseof 2O0F (a heat duty of 8.3 million Btu/min), converting to a recirculating, evap-orative system would still require a recirculation rate of 50,000 gal/min with a2O0F temperature rise, but would evaporate about 8300 Ib/min or 1000 gal/min,requiring about 1200 gal/min of makeup water. At these rates, the plant wouldreturn only 200 gal/min to the local environment (as blowdown), with 1000 gal/min lost as vapor to the atmosphere to return to the surface many miles away—perhaps in a different watershed or at sea. So, even though the trend has been toabandon once-through cooling, this may not always have been the best plan forthe local environment, which in this example has lost 1000 gal/min from thewatershed and has received 200 gal/min of concentrated blowdown in place ofthe 1200 gal/min withdrawn for makeup.

In the United States, substantial reductions in the rate of water withdrawal andwastewater discharge have been made in all industrial plants through water reuse.The establishment in each plant of a hierarchy of water uses based on qualityrequirements often results in the spent water from the highest water quality userbeing of adequate quality for users of lower quality. Cascading water from highto low quality uses has achieved high rates of reuse and almost reached a pointof diminishing returns.

Although there has been strong interest in zero water discharge—and this prac-

tice has actually been mandated in certain areas where the environment seemedto require it—the cost is prohibitive in most cases; i.e., the cost would make pro-duction at the specific plant site uneconomical. Zero discharge requires separationof water from its dissolved solids by some combination of freeze concentration,evaporation, and crystallization, preceded by electrodialysis or some such mem-brane process (see Figure 40.10). Solar evaporation may be practical in aridregions, but in most cases energy must be applied, and it is this cost that makeszero discharge uneconomical. Even where zero water discharge is actually prac-ticed, there is the requirement for some adequate disposal of the solids removedfrom the water, most of which are soluble and therefore unsuited to outside stor-age where storm water would redissolve them.

SUGGESTED HEADING

Arbuckle, J. G., and Vanderver, T. A., Jr.: "Water Pollution Control," Chapter 3, in Envi-ronmental Law Handbook, 7th ed., Government Institute, Inc., Rockville, Md., 1983.

Athavaley, A. S., Funk, R. J., Sweet, R. G., and Coffey, W. A.: "Deepwell Injection of Indus-trial Wastes," Industrial Wastes, May/June 1981.

Kemmer, F. N.: "Optimizing Water Supply, Treatment and Recycle Practices," Chem.Eng., October 6, 1980.

Lund, Herbert, F. (ed): "Industrial Pollution Control Handbook," McGraw-Hill, New York,1971.

Rubin, Alan, J.: "Chemistry of Wastewater Technology," Ann Arbor Science, Ann Arbor,Mich., 1979.