Compost-based reactors offer a controlled in-vessel biological approach to convert biodegradable hazardous materials to harmless and stabilized byproducts by using microorganisms under elevated temperature. In-vessel composting reactors are of two general types: plug flow reactors (vertical and horizontal) and agitated-bed reactors. The increased temperature, typically 122 to 158 deg.F, results from heat released by microorganisms during the degradation of the organic materials. Aerobic composting is used to degrade sewage sludge whereas anaerobic processes are more suitable for hazardous waste treatment. The in-vessel composting of contaminated soil utilizes a bulking agent, such as saw dust or animal waste, to increase the porosity of the media. Conveyors must be used to transport materials in the compost-based reactor system. The efficiency is impacted by moisture content, pH, oxygen, temperature, and carbon-to-nitrogen ratio [1,2].
Composting technology can be applied to municipal sludge, soils, and lagoon sediments contaminated with biodegradable organic compounds. It has been demonstrated that composting is suitable for pentachlorophenol (PCP), refinery sludges, insecticides contained in cannery wastes, explosive-contaminated soil, ethylene glycol contained in landfill sludges, and polycyclic aromatic hydrocarbons (PAHs). High rate composting is conducted in an enclosed reactor and the curing may be conducted in a reactor or an exterior pile [1,2,3].
There are several factors that may limit the applicability and effectiveness of the process. These include: (1) excavation of contaminated soils is required which may cause the release of odorous compounds; (2) the volume of material will increase after composting due to the addition of amendments; (3) it does not allow the degree of flexibility of open systems; and (4) sophisticated mixing equipment must be used in the system [1,2].
Results of a study to investigate in-vessel composting of oil pit sludge mixed with wood chips and manure has been reported . Initial contaminant concentration in the sludge was 10.8% extractable hydrocarbons. After 4 weeks, reduction in hydrocarbon concentration was approximately 92%. The final concentrations of hydrocarbons were low enough for land disposal to be considered.
Specific data required for composting include contaminant concentration, excavation requirements, availability and cost of amendments, space available for treatment, soil type, nutrients, temperature control and heat removal, moisture-holding capacity, and odor potential . A treatability study must be performed to confirm the suitability of mix rates and important process control parameters .
Estimated costs for in-vessel composting of explosive-contaminated soils are more than $190 per cubic yard .
Status of Technology
Because many forms of compost reactors are used in bioremediation applications, the technology is at several levels of development. In-vessel composting to remediate petroleum and oil products currently is being performed at pilot- and demonstration-scales. Composting of other types of compost mixtures is still in the research and developmental stages, with a few applications in the pilot-scale stage .
1. Cookson, John T. Jr., 1995, Bioremediation Engineering Design and Application, McGraw-Hill, Inc., New York, NY.
2. Office of Research and Development, EPA, ATTIC Downloadable Documents, available at http://www.epa.gov/bbsnrmrl/attic/documents.html.
3. Norris, R.D., et al., 1994, Handbook of Bioremediation, CRC Press, Boca Raton, FL.
4. Baker, K.H. and D.S. Herson, 1994, Bioremediation, McGraw Hill, New York, NY.
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