environmental bioengineering || biotreatment of sludge and reuse

5Biotreatment of Sludge and Reuse

Azni Idris, Katayon Saed, and Yung-Tse Hung

CONTENTS

INTRODUCTION

SEWAGE SLUDGE

COMPOSTING OF SLUDGE

TYPES OF COMPOSTING SYSTEMS

FACTORS AFFECTING COMPOSTING PROCESS

SOLID STATE BIOCONVERSION TECHNIQUE

MICROBIAL BASIS OF SSB PROCESSES

CASE STUDIES

NOMENCLATURE

REFERENCES

Abstract Sewage sludge, a by-product of domestic wastewater treatment plant, also knownas “biosolids”, is generated in millions of tons each year. While sewage sludge disposal is aworldwide problem, local conditions dictate the adoption of a variety of treatment and reusemethods. Among them, composting has been practiced extensively in Malaysia. This chapterdiscusses the theory of the process, fundamental factors affecting the process, and the basisof solid state bioconversion technique. Numerous case studies exhibiting the large scale andcontinuous operation of sewage sludge composting and their utilization are also presented inthis chapter.

1. INTRODUCTION

As an organic waste generated from wastewater treatment plants, sewage sludge posesproblems in many parts of the world because of its requirements for special handling anddisposal methods. Large land areas have been utilized to store, treat, and dispose of the sludge,but associated environmental hazards have put tremendous pressure on authorities and wastemanagement agencies to reduce or reuse the unwanted waste sludge. Both safe and economical

From: Handbook of Environmental Engineering, Volume 11: Environmental BioengineeringEdited by: L. K. Wang et al., DOI: 10.1007/978-1-60327-031-1_5 c© Springer Science + Business Media, LLC 2010

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methods must be utilized to dispose of, or use, the sludge materials (1). Since sewage sludgeis organic in nature, biotreatment methods can be applied to sludge to convert it into reusablebyproducts using a composting process. In this way, the wastes can be utilized in a sustainableway and be viewed as a renewable source of raw material to produce natural products such asbiocompost.

Composting is an environmental-friendly waste management method for tackling thedisposal problem of organic wastes such as sewage sludges and municipal solid waste(1). With the appropriate nutrients, carbon source, and moisture content during the com-posting process, microorganisms will be destroyed and the organic matter will be stabi-lized. The stabilized end product (compost) can contribute as a soil amendment to improvesoil fertility and provide plant nutrients. These beneficial uses of compost can improvehealthy plant production, reduce the use of chemical fertilizers, and conserve naturalresources.

Haug (2, 3) regarded composting as the biological decomposition and stabilization oforganic substrates under conditions that allow the development of thermophilic tempera-tures as a result of biologically, produced heat, with a final product sufficiently stable forstorage and application to land without adverse environmental effects. He also added thatcomposting is a form of waste stabilization, but one that requires special conditions ofmoisture and aeration to produce thermophilic temperatures. Hughes (4) stated that com-posting is the decomposition or incomplete degradation of organic waste materials by amixed microbial population, usually under warm, moist, and aerobic conditions. Accord-ing to Bertoldi et al. (5), composting can be defined as a biooxidative process that leadsto a highly stabilized organic product, which could be used directly as soil conditionerand fertilizer. Biddlestone et al. (6) stated that composting is the decomposition of hetero-geneous organic matter by a mixed microbial population in a warm, moist, and aerobicenvironment. Diaz et al. (7) defined composting as the biological decomposition of wastesconsisting of organic substances of plant or animal under controlled conditions to a statesufficiently stable for a nuisance-free storage and utilization. According to Gaur (8), com-posting is a biochemical process in which diverse and mixed groups of microorganismsbreak down organic materials to a humus-like substance that is similar in properties to farmmanure.

Mitchell and Lonsane (9) reported that composting is a process that can be carried outusing low or high technology, but it is basically a socio-economic process since it removesor renders harmless a waste, which might otherwise result in an undesirable and offensivefermentation. In low-technology applications, agricultural wastes are placed in piles andoccasionally turned. A succession of microbes arises from the original microflora. Readilyutilizable substrates are degraded mainly to carbon dioxide and water, leaving a productcontaining substrates that are more difficult for microbes to degrade (especially lignocellu-lose); this product is then suitable for use as a soil conditioner. These biologically stablewastes represent much less of a pollutant to the environment than the original agriculturalby-products.

Biotreatment of Sludge and Reuse 167

2. SEWAGE SLUDGE

2.1. Sewage Sludge Generation

Sewage sludge, also known as biosolids, is what is left behind after wastewater is cleanedin domestic wastewater treatment works. It represents the largest in volume among theby-products of wastewater treatment plants. Sludge handling and disposal is perhaps oneof the most complex environmental problems. This is because the sludge resulting fromthe wastewater treatment operations and processes is usually in the form of a very dilutesuspension, which typically contains from 0.25 to 12% solids, depending on the operationand process used. Apart from that, sludge is composed largely of the substances responsiblefor the offensive, pathogenic, and toxic characteristics of the untreated wastewater. It is knownto have high organic matter and plant nutrients and, in theory, makes good fertilizer. However,most developed countries regulate its use because it contains a multitude of metals, organicpollutants, and pathogens.

In the United States, the application of sewage sludge to land, especially on agriculturallands, has been contentious since the late 1980s, when national and international clean waterregulations prohibiting the ocean dumping of sludge were first enacted. Research scientistsand engineers in many parts of the world working on sludge management and utilizationcontinue to advocate the natural ability of sludge, like soil, to immobilize potentially toxicmetals. They point to cleaner water, as well as higher crop yields for farms that use thematerial.

Treatment plant operators will continue to face the challenge of disposing of millions oftons of sewage sludge generated each year (as shown in Table 5.1) (10). If not applied to land,most sludge must be burned in incinerators or land filled, which may create another form ofenvironmental risk. Table 5.1 shows that large quantities of sludge either go into landfill or areused for agriculture purposes. The total annual US production of sludge is reported to be stableor only growing slowly, exceeding 7 million tons of dry matter; however, in Western Europe,where tougher clean water laws are beginning to take effect, sludge production is growingsignificantly, as small communities build and improve waste treatment plants to comply withregulations. Recent figures quoted the European Union (EU) sludge production as increasingfrom approximately 6 million to 8 million tons of dry matter during 1992 and 2000 (11).

In many developing Asian countries, sludge management and disposal remain relativelyunattended and often receive low priority for development funding. A growing economy suchas China’s will be facing serious sludge production issues due to the installation of many newsewage treatment facilities; the probable estimate is 4 million tons of annual sludge generationwithin the next few years.

2.2. Health Impacts of Sludge Utilization

Sewage is a complex mixture of waterborne wastes of human, domestic and industrialorigin. Environmental issues include a list of health risk components in sewage such aspolluting organic matter, emulsified oil and grease, bacteria and virus, nitrate and phosphate,as well as heavy metals and organochlorines.

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Table 5.1Sewage sludge generation rates (10)

Country Amount (million Disposal method (%)tons dry solids/yr) Application Land Incineration Other

to land filling

Austria 320 13 56 31 0Belgium 75 31 56 9 4Denmark 130 37 33 28 2France 700 50 50 0 0Germany (West) 2, 500 25 63 12 0Greece 15 3 97 0 0Ireland 24 28 18 0 54Italy 800 34 55 11 0Luxembourg 15 81 18 0 1Holland 282 44 53 3 0Portugal 200 80 13 0 7Spain 280 10 50 10 30Sweden 180 45 55 0 0Switzerland 215 50 30 20 0United Kingdom,

19911, 107 55 8 7 30

United States 6, 900 41 17 22 20

It is known that sludge contains toxic metals, although at what level and when such metalsmight cause harmful effects are largely unknown. In most cases, the metals are not a problem,but they could be an issue in the future. Traditionally, many European scientists favor the lowestimate of toxicity, whereas many US scientists favor a higher one. If the high estimate isconsidered, farmers could be facing long-term risks of damaged soil, which would be almostimpossible to remedy.

There is no general agreement concerning the maximum allowable concentrations ofvarious metals in sewage sludge. Table 5.2 shows the limits of heavy metal contaminant insludge, which is adopted by many European countries and the USA (10). Based on the USexperience, the national average level of heavy metal found in sludge is about 20 times highercompared to the national average heavy metal content in the soil. Figure 5.1 illustrates thehigh metal content found in sludge (12).

2.3. Regulatory Issues on Sludge Disposal

Opponents of sludge have focused on the long-term buildup of heavy metals in the soil.They argue that over time, metals such as zinc, lead, copper, and cadmium, may build up tolevels high enough to damage agricultural soils. Some opponents advocate a full-scale banon the use of sludge as fertilizer. But for others, who acknowledge the benefits of sludge,questions still remain regarding the levels at which heavy metals can cause harmful effects.


Table 5.2Heavy metal contaminant standards in sewage sludge for land application (10)

Country Year Cd Cu Cr Ni Pb Zn Hg

EuropeanCommunitya

1986 1–3 50–140 100–150a 30–75 50–300 150–300 1–1.5

France 1988 2 100 150 50 100 300 1Germanyb 1992 1.5 60 100 50 100 200 1Italy 3 100 150 50 100 300 –Spain 1990 1 50 100 30 50 150 1

The Netherlandsc

Clean soilreferencevalues

0.8 36 100 35 85 140 0.3

Intervention values 12 190 380 210 530 720 10United Kingdomd 1989 3 135 400a 75 300 200e 1Denmark 1990 0.5 40 30 15 40 100 0.5Finland 1995 0.5 100 200 60 60 150 0.2Norway 1 50 100 30 50 150 1Sweden 0.5 40 30 15 40 100 0.5United States f 1993 20 750 1,500 210 150 1,400 8

aValues are currently being revised.bValues are for soil pHs > 6. At pH 5–6, the Cd and Zn limits are 1.0 and 150 mg/kg, respectively.cSoil cleanup levels which also apply to agricultural land amended with sewage sludge. Concentrations less

than the clean soil reference are considered clean soil.d Values shown are for soil pHs 6–7. Other values apply at pH 5–6 and > 7 (U.K. DoE, 1989).eChanged following Independent Scientific Committee recommendations (see text).f Calculated from maximum cumulative pollutant loading limits mixed into soil plow layer. Soil background

concentrations are not taken into account.

Regulatory agencies from the EU have begun work on a new sludge directive that will placelower permissible limits for heavy metals (11). Meanwhile, another EU directive sets absolutevalues for contaminants in food, which could also drive down permitted levels of metalsin sewage sludge in the future. Regulations on sludge disposal in the EU include the 1986Sewage Sludge Directive 86/278/EEC, the Organic Farming Regulation (EEC) No. 2092/91,the Landfill Directive 1999/31/EC, and the Commission Decision 2001/688/EC which isrelated to eco-labeling of soil improvers and growth media. EU regulations on sludge disposalare currently under revision; it is foreseeable that sludge disposal will encounter much morestringent standard in the near future. Meanwhile, ocean dumping of sludge in the EU countrieshas been practically forbidden, but the EU Landfill Directive does not prohibit land filling ofsludge.

The EU Directives are set up to safeguard public health and safety and essentially meet thefollowing requirements:

• Pretreatment of sludge to minimize risk• Restriction on the content of heavy metals in soil on which sludge is applied

170 A. Idris et al.

2500

2000

1500

1000

Con

cent

ratio

n of

Tox

icH

eavy

Met

als

(ppm

)500

0National Average in Soil

Source: Environmental Working Group. Based on data from USEPA1988 Sewage Sludge Survey and USEPA 1997.

National Average in Sludge

Fig. 5.1. Comparison of heavy metal content in sludge from USA (12).

• Restriction on the content of heavy metals in sludge• Restriction on the content of micro pollutants• Restriction on the content of nutrients added to soil (N&P)• Restriction on the amount of dry solids/heavy metals spread per unit land area and time• Legislative compliance control

US regulations concerning sewage sludge disposal and application on land are summarizedin 40 CFR Part 503, in 1995 by USEPA (13), which is known as “Standards for the Useor Disposal or Wastewater Sludge.” The US regulations contain very specific directions forsludge treatment before disposal including requirements for pathogen reduction. Under theUS standard, sludge is categorized into two classes: Class A sludge – possible use withoutrestrictions and Class B sludge – to be used under specific site restrictions. Sludge in Class Bcategory has less stringent pre-treatment requirements.

Compared to the EU, the United States has the most relaxed standards for heavy metalscontent in sludge for land application. As shown in Table 5.2, proposed EU standards forheavy metals are up to 100 times higher than in any other country.

2.4. A Sustainable Approach for Sludge Disposal

While sludge disposal is a worldwide problem, local conditions dictate the adoption of avariety of disposal routes. The ultimate resting place of the sludge must be either on land or inthe water. Since many countries have banned ocean dumping, the choice for sludge disposaltends to be restricted to land-based technology.

The widely practiced landfill disposal of sewage sludge is coming under increasing pressureas suitable sites become less available and controls on toxic materials become more stringent.When landfill operations and application to agricultural soil are practiced, the main issuesthat limit their widespread use are related to pathogens, heavy metals, toxic organics, andtransport and application difficulties. A variety of technologies is available to circumvent thesedifficulties however, sometimes, at considerable cost. Current research findings indicate thatorganochlorines at low concentrations in the soil do not transfer to crops but are degraded


by soil microorganisms through a bioremediation pathway. However, heavy metals do accu-mulate if the amount is at high concentration and may have a significant transfer to the foodchain. For this disposal method, and whenever a high level of heavy metals is detected, a cost-effective means for removing the metals from sludge is required. The most common methodemployed for achieving the necessary reduction of metals in sludge is the biotreatment orcomposting process.

Composting method used for reducing heavy metals content in sludge have shown promis-ing results and valuable by-products can be produced without damaging the environment.These methods are very attractive for many waste management operators as the capitalrequirement is significantly much less compared to building incineration plants.

3. COMPOSTING OF SLUDGE

3.1. Historical Background of Composting

For many centuries, farmers in many parts of the world have practiced composting oforganic wastes to some extent. The Chinese living in the river deltas were an outstandingexample, whereby their recycled crop residues, human wastes, and alluvial mud went backto the soil. Using composts in agriculture can minimize organic wastes and can reducethe addition of fertilizers and fungicides in crop production (14). By practicing excellenthorticulture, with high labor input, their lands have remained productive for some 4,000 years.Other noted proponents of composting are the people of the Runza Valley in the Himalayaswho have practiced their agriculture in terraced fields on the mountainside.

Composting, as practiced by the Chinese, has probably changed very little over the cen-turies. The theory of the process has been developed over the years by the Western worldnamely, fundamental reaction and its application to large-scale and continuous operation. Inthe 1930s, the popularity of composting begun as appreciation grew of the physical, chemical,and microbial interactions involved in composting. Development in the mechanization ofcomposting arose in response to the need for a continuous, controlled, and rapid disposal todeal with the large quantities of municipal wastes produced in towns and cities. Over the years,many composting system have been commercialized, but their basic features are very similar.The only major differences have been in the actual fermentation section, which represent thepits, heaps, cells, bins, digesters, silos, and rotating drums.

In the 1970s, composters with a capacity of over 100 tons/day were rare. During the 1990s,there were 500 tons/day units being installed, and nowadays, 1,000 tons/day ones are beingconsidered. Many systems employ fermentation in open elongated heaps (windrows). Recentcomposers utilize the well-known Dano rotating drum followed by maturing in windrows. Afew are starting to employ a high degree of automation in vertical, multifloored silos withcontinuous agitation and control of aeration and moisture (15). The problem with compostinghas been the handling of large volumes of waste materials; the end product is very bulky andhas rather low market value. After an initial upsurge of interest in the 1950s, surprisingly fewfurther composting units have been installed in the West. Most interest has been shown inthe oil-rich Middle East states where finances are more readily available, and there is a gooddemand for compost for reclaiming desert soils.

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Furthermore, with the sharp increase in oil prices in the 1970s and their effect on fertilizercosts, it became apparent that the agricultural systems of many less developed countries couldnot be based solely on mineral fertilizers. Composting of agriculture residues and sewagesludge has gained new impetus and the compost products could easily find a market in thesedeveloping countries.

3.2. Composting Process

Figure 5.2 shows an overview of the composting process. Microorganisms play a vital rolein this process by using nitrogen and carbohydrate as energy sources for their activity whilethey multiply to produce new organisms. Like any other microbial process, the compostingstep requires oxygen and moisture to produce a compost product. Water, carbon dioxide, andheat are generated during the process (16).

In the degradation of organic matter into simpler substances, there are two modes ofdecomposition, aerobic and anaerobic. Fungi, actinomycetes, bacteria, and molds play adominant role in both of those processes. In aerobic decomposition, living organisms uti-lize oxygen, feed upon the organic matter and develop cell protoplasm from the nitrogen,phosphorous, some of the carbon and other required nutrients (17). Anaerobic decompositionis characterized by low temperatures, unless heat is applied from an external source. Theanaerobic process is associated with the production of odorous immediate products, and alsogenerally proceeds at a slower rate than aerobic composting (18).

N-sourceCarbohydrate

NewOrganisms

O2 MoistureMicroorganisms

H2O + CO2 + Heat

NH3 + H2O(NH4OH)

Intermediate MetabolitesHumus (Compost)

Death

Death

Fig. 5.2. General overview of composting.


Microorganisms

New Microorganisms

Organic Waste

O2

H2O

Energy

Humus

CO2

H2O

Heat

Fig. 5.3. Respiration and heat release in composting.

During the decomposition process, three distinct stages are involved:

• The rapid decomposition of some of the constituents by microorganisms• Synthesis of new substances by these organisms• The formation of resistant complexes by various processes of condensation and polymerization.

Bioconversion of organic matter is carried out by different groups of heterotrophic microor-ganisms such as bacteria, fungi, actinomycetes, and protozoa. Microorganisms involved in theprocess derive their energy and carbon requirements from the decomposition of carbonaceousmaterial. The microorganisms take in moisture, O2 from the air and food from the organicmaterial. The organisms give off water, CO2, and energy, and then they reproduce themselvesand eventually die. Some of the energy released is used for growth and movement, while theremainder is given off as heat. The process is shown diagrammatically in Fig. 5.3.

4. TYPES OF COMPOSTING SYSTEMS

The composting system can be divided into two main categories: non reactor and reactorsystems. In non reactor or non mechanical systems, the entire composting process occursoutside a reactor. Typical examples of non reactor systems are static piles and windrows. Instatic piles, wastes are placed into heaps and aeration can be enhanced by the periodic manualturning of the heap. Windrows are used for large quantities of waste and are considered alow-cost method of composting because of their simplicity in design.

In contrast, reactor systems allow the degradation of organic waste to occur in a morecontrolled environment within the wall of a vessel. Reactor systems are normally mechanicalor enclosed systems in which the process can be relatively faster than that of non reactorsystems. The most popular reactor composter is the Dano system developed in Europe. Thiscomposting process can be controlled rather easily as the waste can be fed through the reactormade faster or slower depending on requirement. In this method, the composting temperaturecan be maintained either by using insulation of the vessel or injection of air through thecomposter. Table 5.3 shows the comparison between reactor and non reactor compostingsystem.

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Table 5.3Comparison of reactor and non reactor system

Type of composter Non reactor system Reactor system (in-vessel)

Turned Aerated Forced aeration Forced aerationWindrow pile with agitation without agitation

Capital cost Low Low for smallsystem; high forlarge system

Very high High

Operating cost Low High Low LowControl of air supply Restricted Complete Complete CompleteRequirement for

subsequent dryingNot required Not required –

self-dryingNot required Small drying

requiredSensitivity to

climate changeSensitive No No No

Land requirement Very high High Very low Low

Advantages of a reactor system include the following:

• The compost product can be produced in shorter time• Parameters such as temperatures are easily controlled• Odors are not produced• Flies and rodents are not attracted to the end product• A smaller area is required for operation

Disadvantages of a non-reactor system include the following:

• The system requires a large land area for creating piles or windrows• Labor requirements are relatively high• There is a lack of protection from rain and wind, etc. and consequently difficult to control the

process• Although it is said that this process will not cause any appreciable odors, it is believed to be

almost impossible to decompose heterogeneous material of this nature with a total absenceof generation of odorous gases such as sulfides, ammonia, mercaptans, and similar substancesassociated with anaerobic processes

• Strict pest and vector control measures would have to be employed to eliminate nuisance and toprotect public health.

5. FACTORS AFFECTING COMPOSTING PROCESS

The major environmental parameters that need to be properly controlled in the operationof composting process are C/N ratio, particle size, moisture content, aeration, temperature,and pH. The optimum conditions for rapid composting are summarized in Table 5.4.

5.1. Temperature

Thermophilic temperatures (45–65◦C) are favored for increasing the efficiency of theprocess and are lethal to pathogens. High temperatures are essential for the destruction of


Table 5.4Optimum conditions for rapid composting

Parameters Value References

C/N ratio of feed 25–30:1 (6)Particle size 10–50 mm (22)Moisture content 50–60% (6)Aeration 0.6–2.0 m3 air/day/kg (21)Temperature 50–70◦C (6)Agitation Every 5–10 min (21)pH control pH 6.0–8.0 (19)

pathogenic organisms and undesirable weed seeds. Decomposition also proceeds much morerapidly in the thermophilic temperature range. The optimum temperature range is 50–70◦C,with 60◦C usually being the most satisfactory (2). A prolonged high temperature of 70–75◦Cmay inhibit some of the beneficial microbial actions and increase nitrogen loss due to thevaporization of ammonia. Temperatures should be sufficiently high for a long enough time toaccomplish more a rapid decomposition rate, kill pathogenic organisms, destroy weeds andvegetable seeds, and destroy fly eggs and larvae.

5.2. Time

The quality of a compost product greatly depends on the length of time that the mixture iscomposted. If a high composting temperature (optimum 50–55◦C) is not maintained throughout the material for a sufficient length of time (> 2 days), pathogen destruction will not reachthe required level. Reactor retention times and curing times may vary from system to system(2). Most composting systems are able to produce compost products within 40–60 days.

5.3. pH

Optimum pH levels are required to achieve satisfactory composting and yield neutralcompost. According to Verdonck (19), optimum pH levels are 6.0–8.0 for composting and4.0–7.0 for the end product. Both acidic and basic materials can be successfully compostedto a neutral product. Aerobic bacteria thrive well at a pH range of 6.0–9.0. Actinomycetesgrow at a pH of 5.5–9.5, whereas fungi develop within much wider pH ranges from 3.0 to 9.5.Control of pH is unnecessary for most municipal wastes. Normal digestion follows from anacid pH of 5.0 to a final alkaline pH of 8.0–9.0.

5.4. C/N ratio

Microorganisms use carbon as a source of energy through metabolic oxidation as well as inthe synthesis of cell wall and other cellular structure and protoplasm. Microorganisms cannotlive without nitrogen, as it is a major constituent of protoplasm. Gaur (20) documented 25–30as the satisfactory C/N ratio for initial process of composting and 30–35 C/N ratios for efficientcomposting. The required composting time depended on the initial C/N ratio (16). The time

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Table 5.5Composting time for sewage sludge–solid waste mix based oninitial C/N ratio (21)

Initial C/N ratio Approximate composting time required (days)

27 4532 4352 40

required for co-composting of sludge mixed with organic solid wastes took about 40 days(Table 5.5).

5.5. Moisture Content

Microorganisms need 40–60% moisture to survive. Moisture content in waste should notexceed 80%, for the suffocation (little air availability within the waste matrix) will kill allaerobic microorganisms. In practical aerobic composting, a high moisture content must beavoided because water displaces air from the interstices between the particles, giving riseto anaerobic conditions. Previous studies indicated that the moisture content of solid wastesstudied usually fell in the range of 40–60%, which is the most satisfactory range for aerobiccomposting.

If the amount of moisture in the waste mixture is below 40% (w/w), decomposition willbe aerobic but slow. The optimum moisture level for aerobic composting is 50–60%. Moistconditions are normally applied in a composting system by the use of water sprinklers. Ingeneral, a range of 40–80% may be quite satisfactory depending upon the nature of materialto be composted (20).

5.6. Aeration

Proper aeration is applied to obtain rapid nuisance-free decomposition in the compostingprocess. Aeration also helps to reduce the high initial moisture content in the compostingmaterial. Oxygen is not only necessary for aerobic metabolism and respiration of microorgan-isms but also for oxidizing the various organic molecules present in the moisture. Organiccomposting requires aeration to provide sufficient oxygen for the aerobic microbes. In amechanical unit with continuous aeration, the desirable amount of air was 10–30 ft3/day perpound of volatile solids (in the initial charge) (21). This provides more than twice the amountof oxygen needed for the oxidation of the organic matter but was desirable because loweraeration rate resulted in prolonged composting while higher rates resulted in rapid coolingand drying of the refuse. The oxygen consumption in a composting mass depends on severalfactors:

• The state of process• Temperature• Degree of agitation of the mass• Composition of the composting mass• Particle size of the mass• Moisture content.


5.7. Mixing

Mixing or turning affects the type and rate of composting. Turning provides sufficientoxygen for aerobic activity to take place, so that it will speed up the composting process. Themixing rate depends on the type of composting and special machines for turning/mixing andaerating are used in large-scale composting involving a windrow pile. Constant slow mixingor intermittent mixing every 5–10 min or a combination of forced air and less frequent mixingis recommended (21).

5.8. Size

Composting material that consists of small particles is more readily decomposed thanmaterial with larger particles, as the surface area of contact is greater. On the other hand,if particles are too fine, there will be less oxygen diffusion. Furthermore, very fine materialtends to lose some of its usefulness as a soil amendment. Typical particle sizes of material usedfor composting range from 10 to 50 mm (22). It has been reported that shredding offers ampleopportunity for rapid aerobic decomposition. The optimum size for raw material is about5 cm. Fine grinding is recommended for mechanical composting with constant or intermittentstirring (20).

5.9. Microorganism

Wastes such as garbage and sewage sludge normally contain many types of bacteria,actinomycetes, and fungi. Research indicated that no pure culture of organisms could comparewith a mixed culture in the aerobic composting of organic matter. Many types are necessary forcomposting. The required microorganisms rapidly multiply if the proper environment existsfor them. Thermophilic bacteria play a major role in decomposing protein and other readilybroken down organic material while actinomycetes and fungi decompose cellulose and lignincompounds (21).

5.10. Use of Inocula

Normal microbial treatment of waste requires innoculum, which is carried out by seedingusing active microbes. Composting developments have been accompanied by considerablediscussion of special inocula, supposedly containing several pure strains of laboratory-cultured organisms or other biological factors essential in the decomposition of organicmatter and nitrogen fixation such as enzymes, hormones, preserved living organisms, activatedfactors, bio-catalysts, etc. In fact, several commercial composting processes have been builton the use of special inoculum, often known only to its discoverer and proponent, who claimit to be fundamental to the successful operation of the process. The need and value of suchinocula have always been debatable; most composting studies have strongly indicated thatthey are not absolutely necessary (4).

5.11. Seeding and Reseeding

Some believe that seeding with suitable active microbial consortia is essential for rapidstart-up of a composter. For mechanical digestion, seeding by recycling of actively composting

178 A. Idris et al.

material is essential for efficiency. For the best result, reseeding should be done at thebeginning of each stage of digestion (21).

6. SOLID STATE BIOCONVERSION TECHNIQUE

Similar to composting, solid state bioconversion (SSB) has been popular recently as awaste recycle method. Solid state bioconversion (SSB) can be briefly defined as a processwhereby an insoluble substrate, with sufficient moisture but not free water, can be convertedto compost by different microorganisms (9). The medium consists of an unrefined agriculturalproduct such as wheat bran, wheat straw, and rice straw, which may contain all the nutrientsnecessary for microbial growth. In fact, SSB is a carefully engineered composting processthat utilizes a bioreactor configuration to ensure that control of process can be achieved. InSSB, pretreatment of substrates is carried out simply by moistening or swelling the substrate,or cracking of the substrate surface to increase the accessibility of the internal nutrients, ormilling of large substrate pieces into small particles (23, 24). The low moisture availabilitymay favor the growth and production of fungi, but may not be favorable for the compostingprocess.

The use of small reactor volumes, low capital and operating costs (25) are among theadvantages of the SSB system. High volumetric productivity and yields in SSB reactorshave been well documented. Examples include twofold higher volumetric productivity forprotein production on wheat straw (26) and higher volumetric yields of celluloses from severalthermophilic fungi (27). Vigorous agitation during aeration is not required, since thin films ofwater at the substrate surfaces have a high surface area, allowing rapid oxygen transfer (28).Downstream processing and waste disposal is often simplified or minimized. For productsrecovered by solvent extraction, less solvent is required. Kumar and Lonsane (29) calculated a50–60% saving in downstream processing costs for the recovery of gibberellic acid from SSBcompared to liquid state bioconversion (LSB).

Most of the processes using the SSB technique are commercialized throughout the world.SSB processes offer production of various metabolites of bacterial, fungal, and yeast origin.This trend may lead to extensive industrialization of SSB processes for diverse products.Potentially, many high value products could be produced using SSB. Various enzymes andantibiotics depend on mycelial differentiation, which may be suppressed in LSB. Improve-ments in socio-economic applications of SSB are desirable, as economy is becoming impor-tant for their continued operation without subsidies. The improved processes must use a cheapsubstrate locally available in abundance throughout the year, while the inoculum preparationmethod must be simplified. Hasseltine (30) also advocated the great potential for the use ofmixed cultures in SSB for enhancing the productivity and the rate of bioreactions.

7. MICROBIAL BASIS OF SSB PROCESSES

7.1. Microbial Type

Many bacteria, yeasts, and fungi are capable of growth on solid substrates and thereforefind application in SSB processes (43, 44). Amongst these microorganisms, filamentous fungi


are the best adapted for these processes and dominate in the research presently carried outaround the world.

7.2. Bacteria

Many bacteria, yeasts, and fungi are capable of growth on solid substrates and thereforefind application in SSB processes. Bacterial SSB processes are few in number. In composting,moist solid organic wastes are decomposed by a succession of microorganisms arising fromthe natural flora. The release of metabolic heat during early decomposition of the ligno-cellulosic substrate causes the pH and temperature to rise, resulting in the domination ofthermophilic bacteria when the temperature exceeds 60◦C (31).

7.3. Yeasts

As with bacteria, yeasts generally participate in traditional SSB processes only as minormembers of the microflora (32). Yeasts are found during the early stages of ensiling, butlactobacilli are the dominant microorganisms. Yeasts have also been added to utilize excesssoluble sugars formed during SSB of cellulosic substrates by cellulolytic fungi (33–35).

7.4. Filamentous Fungi

Filamentous fungi are the most important group of microorganisms for SSB processesowing to their physiological capabilities and hyphal mode of growth. Filamentous fungiare also very active in the early and late stages of composting, although they are unable toproliferate at temperatures in excess of 60◦C (36). A second reason for this upsurge was therealization of the ability of many filamentous fungi to degrade macromolecular substrates,especially carbohydrates.

Solid substrates usually consist of complex arrangements involving a number of macro-molecules such as starch, cellulose, hemicellulose, pectin, lignin, protein, and lipid (29). Thesemacromolecules or specific representatives amongst them usually provide the carbon andenergy for microbial growth. Of most importance are the polymers starch and cellulose. Sincestarch and cellulose are the most important nutritional molecules, amylases, and cellulosesare necessary for their utilization. Filamentous fungi-producing amylases include speciesof Mucor, Rhizopus, and Aspergillus, while cellulase producers important in SSB includeTrichoderma reesi, Trichoderma lignorum, Chaetomium cellulolyticum, and the white-rotbasidiomycetes (31). The hyphal mode of growth gives the filamentous fungi a major advan-tage over unicellular microorganisms in the colonization of solid substrates and the utilizationof available nutrients.

8. CASE STUDIES

8.1. Case 1: Utilization of Sewage Sludge as Fertilizer and as Potting Media

A study was carried out to investigate the possibility of using sewage sludge as fertilizer forsweet maize (36). Domestic sewage sludge was collected from oxidation ponds in a tropicalMalaysian climate. The processed sewage sludge was applied on land at rates ranging from

180 A. Idris et al.

Table 5.6Metal concentrations in maize grain plantedusing sewage sludge (31)

Parameter Value (mg kg−1)

Zn 4.95–19.18Cu 0.56–2.60Cd 0.037–0.052Pb 0.034–0.052Mn 1.56–8.53Fe 8.16–24.93Ni 0.66–1.22Cr 0.12–0.44

186 to 746 kg N ha−1. The study was conducted for three corn cycles. Sewage sludge wasapplied about 2 weeks prior to sowing of each crop. Maize was harvested at maturity about75–78 days after sowing. Application of sewage sludge produced a significantly higher yieldof maize than the control. The total yield ranged from 1,009 to 4,068 t ha−1. However, nosignificant difference was observed between the inorganic fertilizer (producing 2,959 t ha−1)and sewage sludge in terms of total yield produced. In addition, there were no statisticaldifference in economic yield (marketable yield) of maize fertilized by sewage sludge andchemical fertilizer. In summary, the sewage sludge performed as well as the inorganicfertilizer.

Results revealed that concentration for all the metals in grain corn treated with sewagesludge increased after third maize cycle (Table 5.6). However, concentrations of these metalswere all below the permitted safety level (37). Metal concentrations in other parts of plants(leaves, stem, sheath and cob) were also still within the safety level in terms of consumption.In general, the distribution of metals in plants followed as: leaves and stems > sheaths andcobs > grains (36).

Furthermore, the possibility of using sewage sludge as potting media for horticulture cropssuch as jasmine and chrysanthemum was also investigated. Sewage sludge consisted of stabi-lized anaerobic sludge originating from old domestic septic tanks. Sewage sludge was mixedwith coconut coir as a peat substitute in potting medium for chrysanthemum. Results showedthat sewage sludge with coconut coir in the ratio of (3:1) could be used in the standard pottingmedia as a peat substitute for chrysanthemums giving similar growth and number of flowersas peat but with only Agroblend, at the recommended rate, or with half the recommended ratesof Agroblend and Agrofos. This revealed that the use of chemical fertilizers could be reducedwith the use of sewage sludge in potting media for chrysanthemums (36).

Different rates of sludge application were used to investigate the possibility of sewagesludge as fertilizer for the jasmine plant. The lowest sludge rate (25%) was able to give goodplant growth. Losses of nutrients were likely minimal as most nutrients in the sludge were inorganic forms and released more slowly as compared to chemical fertilizers, which needed to


Table 5.7List of microorganism utilized for composting (38)

Experiment Inoculum

T1 Organic GroT2 P. chrysosporiumT3 T. harzianumT4 P. chrysosporium + T. harzianumT5 Mucor hiemalisT6 Mucor hiemalis + Mucor hiemalis

be replenished more frequently (36). Results revealed that stabilized sewage sludge could alsobe used as an organic fertilizer for horticultural plants such as jasmine plant.

8.2. Case 2: Reduction of Heavy Metals in Sewage Sludge During Composting

A study was carried out to determine the effect of inoculating various microorganismson the metals concentration during the composting progress. Table 5.7 presents the list ofmicroorganisms used. Sewage sludge was collected from a mechanical dewatering operation.Sawdust was added as an amendment at different ratios (1:1, 1:1.5, 1:1.7, and 1:2) to thedewatered sludge to adjust the water content to 60%. The sludge was mixed together withsawdust and microorganism and then transferred into a horizontal drum bioreactor (HDB) of300 L (38), as shown in Fig. 5.4.

Among the all-experimental runs mixing sewage sludge with 1:1.7, sawdust in the presenceof Phanerochaete chrysosporium and Trichoderma harzianum was found to be the mostsuitable for efficient composting. In general, there was a general reduction in all-metalcontents after composting (Table 5.8). The highest reduction (50%) was recorded for Cd,where combination of P. chrysosporium and T. harzianum were used (38). The highestconcentration of metals in composted sewage sludge was observed for Fe and the lowest forPb. This indicated that Fe was the most loosely bounded to the sewage sludge organic matrixand Pb was the most strongly bounded. A lower concentration of extracted metals in thecomposted sewage sludge revealed that composting renders part of the insoluble metals. Thisresult confirmed that when the sludge is composted, there is lesser risk due to metals in thesludge during application on soils.

8.3. Case 3: Solid State Bioconversion of Oil Palm Empty Fruit Brunches (EFB)into Compost by Selected Microbes

The palm oil industry plays a major role in the economic development of Malaysia. Inprocessing oil palm fruit for oil extraction, palm oil mills produce a considerable amountof solid wastes in the form of fibres, nutshells, and (EFB) empty fruit brunches (Fig. 5.5).For every 100 tons of fresh fruit bunches processed, there will be approximately 20 tonsof nutshells, 7 tons of fibres, and 26 tons of empty bunches discharged from the mill (39).Disposal of the oil palm wastes requires prudent handling and consideration.

182 A. Idris et al.

Air / water supply exhaust

Contact Point for internal control

Thermocouple array

shaft

Drum 300-liter ProbePort

Control

Bush with Seal

1000 mm

700 mm D

315 mm

198 mm107 mm920 mm

60 mm

40 mm

Feed Window

160 m

1000 mm

Fig. 5.4. SSB of sewage sludge in horizontal drum reactor (38).

Table 5.8Metal content in compost produced using different microorganisms (38)

Parameter Untreated Compost T1 Compost T2 Compost T3 Compost T4 Compost T5g/kg sludge

Cd 0.47 0.29 0.26 0.23 0.2 0.27Fe 19.7 12.1 14 11.2 8.98 9.89Cu 0.60 0.49 0.33 0.54 0.22 0.50Zn 1.84 0.27 1.11 0.01 0.98 1.5Cr 0.31 0.25 0.19 0.1 0.08 0.21Pb 0.30 0.28 0.19 0.18 0.17 0.20Ca 16.5 15.2 14.3 13.2 12.3 13.6

Note: the species of microorganisms are indicated in Table 5.7.

Hassan (39) investigated the solid state bioconversion technique (SSB), by selectingmicroorganisms to convert EFB to compost. Shredded and partially dried EFB wereallowed to compost for 4 weeks using ammonium sulfate as a source of nitrogen with theaddition of single and mixed culture inoculum of Aspergillus niger, Trichoderma reesie, andP. chrysosporium.


Fig. 5.5. Palm oil fruit brunches.

In summary, the mixed culture of the three fungi produced better results compared tosingle fungi. The carbon decomposition was 54% for mixture of three followed by 53.4%for P. chrysosporium, 41% for A. niger, and 34.6% for T. reesie. Composting increased thetotal nitrogen content by 92.1% for a mixed culture followed by 77.4, 67.6, and 64.7% forP. chrysosporium, A. niger and T. reesie, respectively. After 4 weeks of composting, the initialC/N ratio of 47 in EFB compost dropped to 26.1 in the control and between 12.3 and 18.6 inthe single cultures. The lowest C/N ratio of 11.3 was achieved by EFB compost inoculatedwith mixed culture. There was a 60% reduction in the C/N ratio over the control. In the EFBcompost, the total phosphorus was greater in the inoculated series than in the control. Themaximum content of 1.44% was recorded with mixed culture followed by P. chrysosporium1.28%, A. niger 0.99, and T. reesie 0.57% (39). This indicated that the organic phosphoruspresent in the organic wastes was mineralized and converted to a form, which could be readilyassimilated by plant.

It appears that compost produced from EFB inoculated by mixed culture contains thehighest percentage of N, P, K, Ca, and Mg followed by P. chrysosporium, A. niger, andT. reesie, respectively. The percentage increase of N, P, K, Ca, and Mg content of the composttreated by mixed culture over the commercial product was 63.7, 37.1, 35.9, 39.8, and 20.2%,respectively. The humus content of the compost was increased significantly by inoculationwith celluloytic cultures. The maximum humus content of 14.8% was noted with a mixedculture followed by 12.2% with P. chrysosporium, 10.5% with A. niger, and 8.7% withT. reesie, which is similar to the control (39). In addition, results revealed that the quality ofthe finished compost will be improved if the EFB were cut or shredded into smaller fractions.

8.4. Case 4: Composting of Selected Organic Sludges Using Rotary Drum

This study was carried out in a 75-L rotary drum modified from a cement mixer (Fig. 5.6) tocompost several organic sludge including food factory sludge, palm oil mill effluent (POME)sludge, landfill leachate sludge, and sewage sludge. The temperature, moisture content, pH,and carbon–nitrogen ratio were controlled and monitored. The rotary drum was insulated with

184 A. Idris et al.

polystyrene to maintain the temperature and was operated in a continuous rotation with asufficient air supply (40).

The pH of composting sludge mixtures was between 5.39 and 7.75, and the moisturecontent for the organic sludges was more than 80%, except for leachate sludge where themoisture was 63.9%. The C/N ratio of these raw organic sludges was low, ranging from 7 to19. The food factory sludge contained a high total microbial count at 2.7 × 1010 cfu/g andfollowed by POME sludge, sewage sludge, and leachate sludge at 1.0 × 108, 2.0 × 107, and7.0 × 106 cfu/g, respectively. The nutrient concentration of P, K, and Mg was high in POMEsludge, which measured at approximately 12,602, 2,118, and 322 ppm, respectively. However,the highest concentration of Mn was found in sewage sludge, measured at 606 ppm. Overall,sewage sources contained the highest concentration of heavy metals in raw sludge (40).

The composting rate was also studied using a mixed ratio of 3:1 (sludge to bulking agent).In the study, it appeared that sludge from sewage, POME, food factory, and leachate underwentthe fermentation phase of approximately 5, 5, 10, and 13 days respectively, while the curingtook about 35, 30, 30, and 17 days, respectively, to achieve completion. In the final stage,decomposition rate measured was recorded to be about 60, 52, 55, and 50% for sewage,POME, food factory, and leachate sludge, respectively. The best achievement for compostingof sewage sludge, POME sludge, food factory sludge, and leachate sludge were approximately40, 35, 40, and 30 days, respectively (40).

Leachate sludge compost product measured the highest pH of 8.03. In terms of P, K, andMg, the highest value was found in POME sludge compost, while the highest Ca and Mn were

Fig. 5.6. Schematic diagram of rotary drum composter (40).


found in leachate sludge compost products. In comparison, the concentration of heavy metalsin final compost products decreased in the following order: POME sludge > sewage sludge >

food factory sludge and leachate sludge. In terms of physical characteristics, the researchcompost products were dark brown and had an earthy smell. The number of total coliformbacteria was recorded to be less than 102 cfu/g. Using a growth study, the germination ratesusing compost from POME sludge, sewage sludge, food factory sludge, and leachate sludgewere 80, 90, 78, and 94%, respectively (40).

The compost product obtained in this study was applied as a biofertilizer for growingspinach. Results showed that spinach grown with sewage sludge compost produced leaveswith a greener color in leaves and promoted superior growth (shown as Pot C in Fig. 5.7).Continuous growth studies after 5 weeks indicated sustained greening of the leaves and goodgrowth especially for sewage compost (shown as Pot A in Fig. 5.8). In conclusion, it appearsthat sewage sludge compost, food factory sludge compost, leachate sludge compost, andPOME sludge compost all showed similar characteristics as commercial composts (40).

In another study, the compost produced using a windrow system (heap method) and a rotarydrum system (composter) were compared. In the windrow system, composting was performedusing different percentage of inoculum with 0.1 and 1.0% Effective Microorganisms (EM)(40). For both systems, pH values were around 6.58–6.85, and moisture content was around65–67%. An important parameter for the composting process that needed attention was the

A B EDC F

A – Growth using leachate sludge compost (LSC)

B – Growth using food factory sludge compost (FFSC) C – Growth using sewage sludge compost (SSC) D – Growth using commercial compost (CC) E – Growth using POME sludge compost (PSC) F – Control

Fig. 5.7. Growth of spinach in different compost products after 3 weeks.

186 A. Idris et al.

A B C D E F

A – Growth using sewage sludge compost (SSC) B – Growth using commercial compost (CC) C – Growth using leachate sludge compost (LSC) D – Growth using POME sludge compost (PSC) E – Growth using food factory sludge compost (FFSC) F – Control (Note: arrangement of pots followed the order of growth)

Fig. 5.8. Growth of spinach in different compost products after 5 weeks.

C/N ratio. For a windrow system, the C/N ratio started around of 22–24, while for therotary drum system, the C/N ratio started at 28 because of the low nitrogen content in mixedsubstrates compared to the nitrogen value in windrow system, which is more than 2%. Thetotal microbial count for windrow (0.1%EM), windrow (10%EM), and rotary drum werearound 1.1 × 107, 1.0 × 108, and 8.6 × 108 cfu/g, respectively. The highest number of totalcoliform bacteria was obtained from windrow system (0.1%), measured at about 1 × 106.

The nutrient content presented a higher value in the rotary drum composter, especiallyfor P, K, Ca, and Mn; the data recorded were 1382, 873, 1011, and 80 ppm respectively.Similarly, the concentration of heavy metals for Fe, Zn, Pb, and Ni were found to be the highestfor the rotary drum system; the values were 2547, 107, 41, and 329 ppm, respectively (40).Generally, the physical, chemical, and biological characteristic showed that compost productswere similar to those of the commercial composts. In terms of the number of pathogens andthe concentration of heavy metals, they all complied with the standards of USEPA and weresuitable for use as biofertilizer and soil conditioner.

8.5. Case 5: Bioreactor Co-composting of Sewage Sludge and Restaurant Waste

Three different types of dewatered sewage sludge, i.e., septic tank, oxidation pond, andactivated sewage sludge were co-composted with municipal solid waste in a two-stage process.


Fig. 5.9. Section view of 200 L – bioreactor composter (41).

The first phase of the co-composting process, known as the fermentation phase of sewagesludge and restaurant waste, was performed in a 200-L bioreactor (Fig. 5.9). Shredded gardenwaste was added as bulking agent. A 2:1 (wt/wt) ratio of municipal solid waste and sewagesludge was found to give the best initial C/N ratio for the composting process. The secondphase of composting process was performed in an open space using a windrow system (heapmethod). The produced compost was characterized and the results were almost identical tocommercial compost and also complied with US EPA standards (41).

A growth study using produced compost to grow spinach showed satisfactory results. Theratio of the compost to the soil was 2:1 based on a volume basis. It was found that thegrowth of spinach using compost produced from the oxidation pond and activated sewagesludge was almost identical to that of commercial compost (Fig. 5.10). The spinach thatgrew in the activated sewage sludge compost product produced more greenish color in theleaves (36).

NOMENCLATURE

C = carbonCa = calciumCC = commercial compostCd = cadmiumcfu = colony forming units (coliform count)cm = centimeterCO2 = carbon dioxideCr = chromium

188 A. Idris et al.

A B C D

E

A-Activated sludge compost B-Septic tank compost C-Oxidation pond compost D-Commercial compost E-No compost

Fig. 5.10. Growth studies with spinach using sewage sludge composts (35).

Cu = copperD = diameterEFB = empty fruit bunchEM = effective microorganismEU = european unionFe = ironFFSC = food factory sludge compostg = gramH2O = waterK = potassiumLSB = liquid state bioconversionLSC = leachate sludge compostmm = millimeterMn = manganeseN = nitrogenNH3 = ammoniaNH4OH = ammonium hydroxideNi = nickelO2 = oxygen◦C = degree celsiusP = phosphorus


Pb = leadPOME = palm oil mill effluentppm = parts per millionPSC = palm oil mill sludge compostSSB = solid state bioconversionSSC = sewage sludge compostUSA = United States of AmericaUS EPA = Unites States environmental protection agencyZn = zinc

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environmental bioengineering || biotreatment of sludge and reuse

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