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USE OF COMPOSTING IN HAZARDOUS WASTE TREATMENT.

Adeleke, Olukunle Francis (奥陆克) kadeleke@yahoo.com

1. BACKGROUND AND INTRODUCTION The main methods of treating and safe disposal of hazardous waste can be generally classified into:

Physical and chemical methods (e.g. stripping, carbon adsorption, oxidation, SCF/SCWO, membrane process, etc.)

Biological methods Incineration and pyrolysis Solidification and stabilization Landfilling

Composting is one of the biological treatment or bioremediation methods which also includes land spreading, land treatment (land farming), soil piles (biopiles), bioventing and biosparging, prepared bed reactor, phytoremediation and ex situ bioreactors. The compounds that may be biologically degraded includes oil and oil products, gasoline and constituents, polycyclic aromatic hydrocarbons (PAHs), chlorinated aliphatics (TCE, PCE), and chlorinated aromatics [1]. No matter which method of biological treatment or bioremediation is employed, the following conditions are necessary [1]:

1. microorganism with needed catabolic activity and capacity to transform the compounds at reasonable rate must exist

2. the target compounds must be available to microorganisms 3. they must not generate toxic products at high concentration during remediation 4. the site must not contain concentration or combination of chemicals that are

inhibitory to biodegrading species 5. Conditions on site or in a bioreactor must be conducive to microbial growth or

activity e.g. nutrient, oxygen, moisture content and temperature. 6. The cost of technology must not be more expensive than other techniques that

can also destroy the chemicals.

2. BIOLOGICAL TREATMENT METHODS A brief description of the various biological treatment methods is given below:

o Landspreading and Landfarming. They are traditional methods of petroleum sludge disposal [2]. In landspreading, the sludge is evenly dispersed over a plot of land where it can be degraded by native microbial flora over a period of months or years. In landfarming, the sludge is blended into the soil with tilling equipment, often with the addition of fertilizer to

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increase the rate of degradation. The tilling improves aeration and contact of the organisms with the waste. Landfarming can also be used to biodegrade creosote-containing soils, food processing, pulp and paper, and leather and tanning industries wastes. The major advantage of landfarming and landspreading is the low cost-equipment construction and operation. However, the methods require a large land area may be environmentally unacceptable due to the possibility of groundwater contamination, volatile emissions and the long-term accumulation of heavy metals in the soil [2].

o Prepared bed reactor. Is similar to land farming but includes irrigation

water systems, nutrient addition, a liner at the bottom of the soil and leachate collection system. Clay or synthetic material is used as liner. The method is often used for contaminants like PAHs, and BTEX (benzene, toluene, ethylbenzene and xylene).

o Soil piles or Biopiles. The soil containing the contaminants is dug up and

placed on an impermeable layer (asphalt or concrete) that retains the contaminated leachate. A perforated piping system is placed in the pile, and air or oxygen is introduced or a vacuum is pulled to enhance aerobic decomposition of the pollutant. A solution containing nutrient and microorganisms may be provided through pipes. Leachate may be collected and recycled through the pile. The pile is covered to contain VOCs, to stabilize the microorganisms’ environment, and to control soil erosion. Volatile compounds and gaseous emissions are also collected and treated. Soil piles have been used for the bioremediation of soils contaminated with hydrocarbons, PCP, and the destruction of RDX and HMX in munitions-contaminated soil.

o Composting. In composting, the polluted material is mixed together in a pile

with solid organic matter that is readily degraded and supplemented with nutrient, air and microorganisms (inoculums) if required. Heat is normally generated during the composting which is favorable to biodegradation. The three major types of composting are open windrow, static windrow and reactor system. It has been used to reduce pollutant concentration in soil contaminated with petroleum hydrocarbons, chlorophenols and explosive-contaminated sites (containing TNT, RDX and HMX) [1]. It has also been demonstrated to be suitable for treating toxic heavy oil sludge [2].

o Bioventing and Biosparging. Bioventing involve introducing air into

contaminated soil above the water table, thereby providing the oxygen needed for the aerobic bacteria to biodegrade the pollutant. The air is

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introduced by vacuum extraction method, air injection wells, etc. Biosparging is similar to bioventing, but the air is introduced below water table (i.e. saturation zone). The purpose is to use the air to transfer the volatile pollutants into overlying unsaturated zone with higher microorganism population. Also, some biodegradation will occur in the aquifer [2]. Bioventing is attractive because it operates in situ and because little equipment is required. It has been used for hydrocarbon remediation. It is however not suitable for compounds with high volatility, and soils of low permeability. Biosparging with vapor extraction has been used in sites contaminated with JP-4 jet fuel, and BTEX in soil and groundwater. Ozone (O3) or steam may also be used in some modification of biosparging to replace oxygen.

o Phytoremediation.This is the use plants in the removal or degradation of

organic pollutants. This is achieved by the uptake of the contaminants by the plant, or by biodegradation by microorganisms in or near the root system of the plant in the rhizosphere (immediate surface of the root and adjacent soil). The reasons for the enhanced biodegradation in the rhizosphere is not yet known but may be due to the larger bacterial mass near the root zone than farther down in the soil. It has the advantage of low cost, but it is only suitable where contaminants are near the surface (1-2m deep). Its use may also be limited where pollutants are strongly sorbed or have become aged or sequestrated, where phytotoxicity prevents the plant from rooting extensively, where contaminants leach quickly out of rooting zone, or if the site is oxygen deficient.

o Bioslurry reactors. In a slurry-phase treatment system, the contaminated

solids (soil, sludge or sediments) are mixed constantly with liquid. The operation is similar to activated–sludge process and the biodegradation is extensive because microorganisms’ population, temperature, nutrient, oxygen and other conditions can be controlled. It is normally ex situ and often more expensive than in situ processes. It has been used for the biodegradation of creosotes, 2, 3, and 4 rings PAHs, PCP, dichloromethane, etc. It may be aerobic or anaerobic. Anaerobic bioreactor has been used to treat TNT-contaminated soil. Slurry-phase procedures may be combined with washing technique to treat contaminated soils. The soil is first washed to remove the pollutant and the wash solution introduced to the bioreactor. Fixed film bioreactors, rotating disk bioreactors, fixed-bed reactors, fluidized bed reactors and SBRs have also been used [1].

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3. COMPOSTING

3.1 Definition.

The definition of composting as widely accepted in literature [2] is: The biological decomposition and stabilization of organic substrates, under conditions that allow development of thermophilic temperatures as a result of biologically produced heat, to produce a final product that is stable, free of pathogens and plant seeds, and can be beneficially applied to land.

From the definition, it can be seen that the composting process is viewed primarily as a waste management method to stabilize organic waste such as manure, yard trimmings, municipal biosolids (sewage sludge) and organic urban wastes to provide end-products that is relatively stable, reduced in quantity, and free from offensive odors. The stabilized end-product (compost) is widely used as a soil amendment to improve soil structure, provide plant nutrient, and facilitate the revegetation of disturbed or eroded soil [8]. Composts are said to have reached their stable level when microbial activity drops to a "low" value (oxygen consumption rate (CO2 evolution rate) drops to about 25% of its peak value). Other peoples also suggested that a final compost material should have an absolute oxygen demand of 20-100 mg/(kg(dry).h) [2].

Recent laboratory-, greenhouse-, and pilot-scale research has indicated that the composting process and the use of mature compost also provide an inexpensive and technologically straightforward solution for managing hazardous industrial waste streams (solid, air, or liquid) and for remediating soil contaminated with toxic organic compounds (such as solvents and pesticides) and inorganic compounds (such as toxic metals) . For example, a large number of hydrocarbons, which are common industrial contaminants found in soil and exhaust gas, degrade rapidly during the composting process or in other compost-based processes. Furthermore, the addition of mature compost to contaminated soil accelerates plant and microbial degradation of organic contaminants and improves plant growth and establishment in toxic soils [8]. When mature compost is added to contaminated soils, remediation costs are quite modest in comparison to conventionally used methods. Mature compost also controls several plant diseases without the use of synthetic fungicides or fumigants [8].

Co-composting refers to an aerobic process where putrescible and recalcitrant organic materials are digested together under thermophilic conditions. For the purposes of this report however, the term composting is used to describe any of the two conditions above.

3.2 Composting Techniques [8].

Composting can be done on a large or small scale, with the management

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requirements and intensity increasing dramatically as system size increases. In its simplest form, compostable material is arranged in long rows (windrows) and turned periodically to ensure good mixing (Figure 1). This process can handle large quantities of input, such as yard trimmings of up to 100,000 cubic yards per year, on only a few acres of land.

Raw materials that tend to be very odorous during composting, such as municipal waste sludge (biosolids), can be processed in more elaborate systems and in a confined facility where odorous air can be treated. These systems use rotating drums, trenches, or enclosed tunnels for initial processing, followed by a covered curing period (Figures 2, 3, and 4). In addition, the Beltsville Agricultural Research Center in Beltsville, Maryland, developed a composting system of intermediate complexity, between open-air windrows and the sophisticated systems shown in Figure 5. The Beltsville system (aerated static pile) has several desirable features, and its generic design is adaptable to suit specific purposes. As shown in Figure 5, air is drawn through the compostable material and scrubbed of odorous compounds in a soil filter. Mature compost can be substituted for the soil filter. A compost filter has several advantages over a soil filter, including a higher adsorptive capacity for volatile organic compounds (VOCs) and better air permeability properties. Compost filters are currently used in Europe at composting plants to eliminate nearly all volatile emissions. Composting of contaminated materials can be done on a field scale using simple designs, such as those shown in Figures 9 and 10 [8]. The designs are mechanically simple, are inexpensive, and provide full containment of materials while preventing washing away by rain. If volatile compounds are being processed, air flow can be set to draw air into the pile and pass it through a biofilter to remove the volatiles. In this case, the complexity is in the biological component, not the physical components, and the only moving parts are the microbes and the ventilation system. The result is likely to be an effective, fast-acting, and inexpensive remediation system.

3.3 Requirements for Composting

Temperature. Composting takes place in two phases after initiation: (1) a high rate phase during which thermophilic temperatures (50 °C and higher), are reached due to high microbial activity and heat production by the putrescible material in the compost and (2) after the rapidly degradable components are consumed, temperatures gradually fall during the "curing" stage (Figure 6) until stabilized compost is obtained. At the end of the curing stage, the material is no longer self-heating, and the finished compost is ready for use. The thermophilic temperature range is maintained by periodic turning or the use of controlled air flow. Sometimes the high rate phase is conducted in a reactor such as a vertical-flow cylinder or a rotating drum for 1-4 days. The curing stage (which lasts for weeks) usually takes place in windrows with forced aeration through the piles. However,

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it has been determined that the only benefit of high temperature is the destruction of pathogenic organisms that normally will not be present in industrial or toxic waste anyway, and that active biodegradation occurs only at temperatures below 55°C [2].

Substantial changes occur in microbial populations and species abundance during the various temperature stages. Mesophilic bacteria and fungi are dominant in the initial warming period, thermophilic bacteria (especially actinomycetes) during the high temperature phase, and mesophilic bacteria and fungi during the curing phase [8]. The high microbial diversity in the resulting compost makes compost bioremediation to require far less time than natural attenuation systems for toxic materials e.g. land farming. Organic matter content of the stabilized compost ranges from 30 to 50 percent of dry weight with the remainder being minerals. The combination of a high organic content and a variety of minerals make compost an excellent absorbent for organic and inorganic chemicals [8]. Aeration. Aeration is important because it maximizes the biochemical energy production, maximizes degradation rate, and prevents anoxic reactions that can lead to harmful chemical by-products. Proper aeration requires good porosity conditions inside the compost matrix with a free air space of about 30% of the total composting volume. Aeration not only supplies oxygen for aerobic decomposition but also controls the energy buildup in the composter. For normal aerobic composting of municipal wastes, about 2g of O2 per gram of decomposable substrate is required. This results in an aeration rate of about 60 m3 per hour per tonne of compost. However, to keep the temperature below 60°C, aeration rates as high as 300 m3 per hour per tonne of compost may be required.

Based on the means of aeration, the three major types of composting are open windrow, static windrow and reactor system.

Feed. Composting consists of three types of feeds: the feedstock (material being degraded e.g green waste, MSW waste, industrial waste, contaminated soil), amendments (easily degraded organics, nutrients, and microbes), and the bulking agent (such as wood chips, straw, manure) used for moisture control. Moisture control. Compost windrows are usually situated on an impermeable substrate designed to prevent liquids from soaking into the ground and endangering groundwater. Precise moisture control is important in composting, as too much moisture can impede thermophilic conditions and too little moisture can seriously reduce reaction rates. Good moisture concentrations are normally around 60-65% of the solids/water portion of the compost, which would be 45% of the total composting volume if the free air space is 30%. This leaves about 25% of the total compost volume for the solids fraction of the process.

It is obvious that when really wet feeds such as industrial sludge are to be composted, it is necessary to reduce the moisture content by adding a large amount of dry bulking

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solid. The bulking material should be highly adsorbent and highly porous. Thus, to compost an industrial sludge, a solids bulking agent is a key feed material. Types of bulking materials include wood chips, sawdust, straw, clippings, peat, and manure. To provide the necessary "bulk," particle size must be large, with a typical criterion being that at least 50% of the particles have diameters greater than 2.3mm and 5% have diameters greater than 12.5mm [2]. Nutrients. Other ingredients to be added to the compost pile are nutrients. The feed and bulking agents normally provide sources of carbon, but frequently not enough nitrogen is present. A carbon/nitrogen mass ratio of 30 or less is normally recommended in a composting project, so nitrogen fertilizers may be applied. Other important nutrients that often have to be added are phosphorus and potassium usually at the 100:1 mass ratio. These can also be provided with chemical fertilizers [2].

Microorganisms. The source of microbes and their initial concentrations can be crucial to the success of a composting project. Research has shown that indigenous microbes were better than commercial inocula for remediating oil-polluted sites and that the only benefit of commercial inocula was that nutrients were included with the microbes [2].

3.4 Waste biodegradability.

The biodegradability of the waste is crucial to the successful design of a composter. To measure degradability, standard biological oxygen demand (BOD) and chemical oxygen demand (COD) tests are normally performed prior to attempts to operate a composter. The addition of putrescible organic matter and the bulking agent used will affect the rate and extent of biodegradation.

The dynamics of compost degradation requires the solid (or nonaqueous) waste to be first hydrolyzed, usually by enzymes, or acids released by the microbial cells; dissolution of the hydrolyzed solids into the aqueous phase; and then consumption of the dissolved solids by microbial cells. This underlines the importance of adequate moisture content in the composter, as the solubilization process is usually the rate-controlling step and adequate contact between water and the solid substrate is imperative. The simplest dynamic model degradation is a first-order model:

dS = kS (1) dt

where k depends on temperature according to

k = 0.0126 X (1.066)T - 20 (2) where

T= temperature, °C k=day-1

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S = degradable substrate, g t = time, days

This model is for common municipal waste but could be used with industrial toxic sludge with lower values of k. In general, most continuous composting systems are designed for a feed residence time (equivalent to the actual residence time if there is no recycling) of 60-180 days to allow for complete curing. When recycling is used, the actual residence time can drop to 15 days. 3.5 Applications of Composting. Remediation of Soils Contaminated With Toxic Organic Compounds. On average, bioremediation is among the lowest cost methods for detoxification of soils contaminated with organic compounds (Figure7) and composting is intermediate in cost among the bioremediation technologies (Figure 8) [8].

A wide range of common environmental contaminants have been found to degrade rapidly in compost including; petroleum hydrocarbons (TPH) (gasoline, diesel fuel, jet fuel, oil, and grease); polycyclic aromatic hydrocarbon (PAH) (wood preservatives, coal gasification wastes, refinery wastes); pesticides (insecticides and herbicides); and explosives (TNT, RDX, nitrocellulose) [8].

Studies on the degradation of TNT and RDX have found that a mixture of 30 percent contaminated soil with 70 percent initial compost feedstock provides the best results [8]. However, this addition leads to a higher volume in the final, decontaminated mix which may be about twice the volume of contaminated soil. The volume increase may be limited however if mature compost is added instead of the feedstock to contaminated soil, since a mixture of 40 percent (by weight) compost and 60 percent contaminated soil provided good degradation of several pesticides [8]. In addition to the direct use of composting or mature compost to accelerate contaminant degradation, microorganisms also can be isolated from compost for both basic biochemical studies and as inoculants in remediation projects.

The composting process or addition of matured compost to biopile-type operation may speed up the biodegradation time with similar or even greater contaminant degradation level. The high temperatures achieved during composting also accelerate the relatively slow chemical reactions in soil, where temperatures are only 15 oC to 30 oC in most temperate climates. By comparison, typical temperatures during composting are 50 oC or higher. No remedial technology is appropriate for all contaminants and situations. Guidelines for the best use of composting or addition of mature compost for remediation include [8]:

Contaminants less than 20 feet deep Contaminants that are biodegradable and/or strongly adsorbed to the compost. e.g.

polychlorinated biphenyls (PCB), and 5-rings PAH have low biodegradability. Soil that is toxic to plants and microbes

Limitations to the use of the composting process or the addition of compost for

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biodegradation are: The degradation rate of specific contaminants is affected by the materials being

composted. i.e. the feed component materials determine the rate of degradation. A critical issue is whether the lack of full degradation and formation of

nonextractable metabolites (bound residue) is a satisfactory endpoint of remediation. e.g. aromatic compounds in compost undergo partial degradation, followed by covalent coupling of the hydoxylated metabolite to humic substances. If the metabolite is actually incorporated into the core structure of the humic substance, a relatively long-term stabilization of the metabolite in a form of low bioavailability occurs.

A number of studies on xenobiotic degradation in compost were conducted by measuring the loss of only the parent compound, but these studies did not adequately measure volatilization or adsorption of compounds to vessel components, such as plastics, so the obtained results will not reflect the actual effectiveness of the composter biodegradability.

The outcome of remediation experiments may vary depending on the scale of the experiment. For example, one study found relatively poor degradation of the explosive TNT in laboratory reactors, whereas other studies indicate good degradation of TNT in pilot-scale studies. This is probably as a result of the inability to generate typical and authentic composting conditions in small laboratory containers. For pilot-scale composting studies, a volume of at least 10 to 20 cubic meters of material is required to achieve the typical thermal profiles seen in large windrows.

Inconsistency in the results obtained for different experiments is also a case of concern. While some experiments may discover compost to enhance biodegradation of contaminants in some cases, it may not in others. For example, a research carried out by Moorman, T.B. et. al. (2000) investigated the ability of compost, corn stalks, corn fermentation byproduct, peat, manure, and sawdust at rates of 0.5% and 5% (w/w) to stimulate biodegradation of the herbicides atrazine, metolachlor, and trifluralin added as a mixture to soil. Unlike manure, peat and cornstalk, compost appeared to stimulate microorganisms, but did not enhance the degradation of the herbicides.

The actual mechanism by which compost enhances biodegradation is still unresolved. A research by Kastner, M. and Mahro, B. (1996) describes the degradation of naphthalene, phenanthrene, anthracene, fluoranthene, and pyrene in soil and soil/compost mixtures. Their experiments demonstrated that the decrease of PAH concentrations after compost addition was not caused by a sorption to organic matter; the release of fertilizing substances from the compost and the shift of soil pH brought about by the compost did not cause the stimulatory effect; the microorganisms inherent to the compost were also not necessary for the enhanced degradation; but that it is the presence of the solid organic matrix of the compost that seemed to be

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essential for the enhanced degradation. These results does not agree with the thinking of many other researchers.

Compost biofilters [8]. Compost can be used as the filter medium for the treatment of industrial (and other) wastewater and air streams to prevent the release of toxic or harmful chemicals into the environment. Granular activated carbon (GAC) is widely used for this purpose and as a polishing step in wastewater treatment; however, it is expensive and not very effective under conditions of high air humidity or with liquid wastestreams. GAC's maintenance costs and time requirements can be high. In addition, when toxic materials are trapped in GAC, it may require disposal as a hazardous waste. Sand and gravel biofilters which have been used to treat wastewater for decades are not very satisfactory as biomass supports. Particularly for air streams, compost biofilters provides high porosity, high adsorptive capacity for organic and inorganic compounds, good moisture retention, and the ability to support high degradation rates. They also have the further advantage of relatively long life spans: 1 to 1.5 years of satisfactory performance before bed materials need to be changed unlike GAC filters which might need to be changed more frequently, often daily or monthly, depending on the pollutant content of the incoming air or water stream. Commercial-scale compost biofilters have been used in Europe to treat exhaust gases from composting plants. The number of VOCs removed is substantial, and removal efficiencies are generally high [8].

According to an investigation carried out by Sukesman and Watwood (1997), enhancing the compost with hydrocarbon enrichment can substantially improve its capability as an effective biofilter for trichloroethylene (TCE) removal. The hydrocarbon used (propane and methane) were mainly to induce the appropriate enzyme system, but later, the microorganisms can be sustained on the rich organic and inorganic compounds in the compost. Also, the two different types of compost materials were used gave different results. While the compost derived from deciduous leaf debris recorded over 95% TCE removal, compost mainly derived from woodchips and bark produced poor results. The TCE removal rate was generally less than 15%. The reason could be that the indigenous microbial community in the first compost is more capable of TCE degradation than the second type of compost. The experiment thus shows that the capacity of compost as effective biofilters is not universal but depends on the compost source, and the specific hydrocarbon used for enrichment and its concentration.

A successful compost biofilter generally has the following characteristics: 1. High porosity and water-holding capacity are required. 2. Performance improves with increased time in service. This benefit results from the

selection of microorganisms tolerant to shock loads and other organisms with a high growth rate

3. Additional nutrients are required. Although composts typically have 1 to 2 percent w/w nitrogen, most of that nitrogen is not rapidly bioavailable.

4. Moisture content must remain between 50 to 70 percent to ensure high microbial

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activity. High moisture content also increases the capture of water-soluble VOCs when compared to a drier filter.

5. Operating temperatures must remain between 20 oC and 35 oC. 6. Residence time of the gas phase going through the filter should be at least 30

seconds. 7. Typical depth of the filter bed should be 1 meter. Shorter depths provide poor

performance, except at very low flow rates. Filter beds greater than 1 meter in depth have a tendency to compact, thereby increasing air pressure requirements.

8. The system must be designed to ensure uniform air distribution upon entering the filter, and the filter medium must be dimensionally stable so that crack formation and channeling of airflow does not occur.

Potential for Reclamation of Mine Spoils and Brownfields with Compost. Mine spoils and abandoned industrial sites (brownfields) poses a great threat to the environment. In addition to being unattractive, these sites can present a significant environmental hazard from the leaching of acid and toxic metals into groundwater, as well as erosional transport of hazardous constituents and spoil materials into surface waters. Natural revegetation is often prevented in these areas because of low pH, phytotoxic concentrations of metals, poor physical structure for plant growth, and slopes too steep for plant establishment. Mine spoils and brownfields share a number of problems, including:

Soil compaction or poor physical structure. This results in poor or no plant development and contributes to offsite contamination via soil that erodes from the barren site.

The presence of pyrite. Pyrite minerals are very common associates of ore-bearing minerals. When exposed to air and water, pyrite is converted to soluble iron and sulfuric acid, resulting in soil acidification and acid drainage.

Metal contamination of industrial sites and abandoned mine spoils is common. Transfer of solid toxic metals by wind and water erosion and by leaching of water-soluble metals is a serious threat to surface and ground waters. If soils are contaminated with toxic metals, the only available options for remediation

are removal of the soil and burial in a suitable landfill, chemical immobilization, or use of chemical extractants to remove the metals from soil. All of these options are expensive and impractical for the large volumes of material present at abandoned mine sites.

Biosolids (also known as municipal sewage sludge) were used to enhance plant growth on mine spoils in the eastern United States and on spent oil shale in the western United States. Plants did not absorb the potentially toxic metals in the biosolids, nor were the metals accumulated by pheasants or swine that were fed grain grown in sludge-amended soils [8]. The toxic metals remain in a low-bioavailability form for at least 20 years after biosolids application. These results and others indicate that organic-rich materials, such as compost, are likely to be a useful remediation aid to assist

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revegetation and to immobilize toxic metals in mine spoils and brownfields. Very little literature is available on soil reclamation using compost to enhance plant

growth and to immobilize toxic metals in soil. Because of similarities in composition between compost and the products formed by degradation of waste materials in soil however, the existing literature suggests compost may be a useful material for remediation activities. Compost has a number of advantages over commonly used organic wastes: 1. Compost is rich in humic materials, which have residence times in soil of decades to

centuries. Because of this long residence time, improvement in soil structure will be relatively persistent.

2. Improving the structure of compacted soil may require up to 20 percent by weight of organic materials, which if raw wastes are used, this high rate of application may provide excess nutrients, such as nitrogen, that pose a pollution problem and promote anaerobic soil conditions under which plants will not thrive. In contrast, nutrient release from composted materials is quite slow, therefore, high application rates can be used without producing a nutrient excess.

3. Compost is more effective for revegetation of steep slopes than raw waste materials or biosolids. Mature compost tends to be self-adhesive and forms a flexible, non eroding blanket when applied to the soil surface. It also provides a good growth medium for plant establishment, because the organic matter is stabilized and releases nutrients slowly. Compost, when added to acidified soils, increases pH into a range satisfactory for

plant growth, reduces the content of water-soluble metal ions, and maintains these improved conditions over time. Compost-Enhanced Phytoremediation of Contaminated Soil. Phytoremediation of metal-contaminated soil relies on the ability of plants to accumulate metals at concentrations substantially above those found in the soil in which they grow. Difficulties in establishing plants in toxic, contaminated matrices, and in compacted and barren materials that are not conducive to plant growth can be overcome by the addition of compost. A small body of research indicates that compost can reduce toxicity of contaminated soil (probably through the adsorption of the toxic compounds to organic matter in the compost) [8]. Also, compost addition will normally speed up weed growth and subsequently lead to accelerated decontamination when compared with soil without compost addition. The amount of compost needed to achieve beneficial effects varies with the project goals. For example, 20 percent w/w compost is sufficient to maximize plant growth in a particular herbicide-contaminated soil, but 40 percent compost is needed to accelerate herbicide degradation in the same soil [8]. Removal of Heavy oil sludge contamination. Because heavy oil contain heavier than conventional crude oils, they tend to produce heavier waste products, which are of

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environmental concern. An investigation was carried out by Headly et. al. (2000) to evaluate the suitability of composting for the bioremediation of petroleum sludges produced during the refining of western Canadian heavy oil and to determine the best conditions for scale-up to a pilot-scaled composter. Preliminary experiments using a range of bulking agents, moisture contents, nutrients, cosubstrates, and inocula were undertaken to determine suitable ingredients for the successful composting of the sludge; which serves as the compost evaluation study. The best ingredients and conditions were then chosen for a detailed study of the CO2 evolution rate and apparent loss TPH and other contaminants; this was the compost kinetic study. The experiment was carried out in an environmentally controlled chamber; the bulking agents were used to achieve an initial moisture content of 70%; different type of innocula were used; and a mixture of two fertilizers was applied to give an initial mass C:N:P ratio of 100:3:1. In the investigation, the prime variables measured were (1) TPH, (2) specific PAHs, (3) the metal components in the sludge, (4) moisture content, (5) cell count, and (6) toxicity. Based on SARA (saturates, aromatics, resins, asphaltenes) analysis, the oil was found to contain 36% saturates, 31% aromatics, 18% resins and 6% asphaltenes, by weight. The majority of the biodegradable components are found in the saturates and aromatic fractions, which constitute 67% of the oil portion of the oil portion of the sludge.

For the evaluation study, 24 individual composting experiments were performed in 12 months. Among the bulking agents used (wheat straw, manure, barley straw and a commercial composting product, Solv-II), HTPM gave the best result and manure the least result. It was also found out that the sludge became toxic when present in concentrations greater than 250,000 ppm, below which approximately the same percentage of initial concentrations was consumed. However, it was also observed that at concentrations above 100,000 ppm, the microbial activity was significantly slower. For the inoculum source, the results showed that the microorganisms taken directly from the refinery site were most active in attacking the refinery sludge as against those from manure which showed a negligible contribution to microbial activity. The total bacterial count data also indicated a high level of bacterial activity in all the composting reactors ranging from 105 CFU/g to 1010 CFU/g. This shows that bacteria can be grown in the presence of petroleum sludge.

For the kinetics study, three composting mass formulations were operated at 30 oC for 30 days with sludge concentration of 50,000ppm. The apparent TPH loss of 55% was observed with the Solv-II compost, compared with 30% for the barley straw or HTPM. Most of the n-alkanes were found to be still available on day 11 but have largely disappeared by day 16. The remaining nonaromatic hydrocarbons were predominantly highly branched and cyclic alkanes. As for the apparent PAH loss, naphthalene achieved a loss of 87% in Sol-II mix and averaged 50% in the other two samples. Some of the

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three- to five-ring PAH also recorded some significant loss particularly in the Solv-II. The large losses observed for naphthalene may be due in part to volatilization. Composting of heavy oil sludge was always found to reduce the toxicity of the initial compost mixture. Heavy oil sludge compost mixtures always commence with a “very” toxic rating but finished with a “moderate” or “slight” rating at the end of the composting experiments.

4. CONCLUSIONS AND RECOMMENDATIONS From the review of the various researches on the use of composting and compost-based methods for the treatment of hazardous wastes, the following conclusions can be drawn: 1. Whenever composting is to be used, there is need to first carry out a bench-scale

study which may take a period of three months to over twelve months before actual implementation.

2. There is need for more study on the specific microorganisms required for the degradation of each type of pollutants. This might help save a lot of time in practical applications and also lead to standardization.

3. More research is needed on the relationship between the types or sources of the compost and their biodegradability of specific pollutants. This may have contributed to the wide variations obtained in different investigations.

4. There is still uncertainty in the actual mechanism involved in the degradation by compost. While some researchers claim it is the microorganisms and the available organic and inorganic compounds in the matured compost, others believe it might actually be the structure of the humic materials. The resolution of these uncertainties will help compost plant operators to determine what condition needs to be optimized.

5. The results obtained in the bench-scale experiment and actual field studies are very different. Generally, field-scale studies tend to give higher results. More studies might also help in determining means of simulating and modeling field conditions in the laboratory.

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REFERENCES 1. Alexander, M. 1999. “Biodegradation and Bioremediation.” 2nd. Ed. Academic Press.

N.Y. USA. Chapters 16 and 17. 2. Headley, J.V. et al. “Removal of Heavy oil Sludge Contamination by Composting.”

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