Composting
Background
Composting is a controlled biological process to convert biodegradable hazardous materials to harmless and stabilized by-products by using microorganisms under elevated temperature. The increased temperature results from the heat released by microorganisms during the degradation of the organic materials in the waste. Aerobic composting is used to degrade sewage sludge and anaerobic processes are more suitable for hazardous waste treatment. The composting of contaminated soil utilizes a bulking agent, such as saw dust, animal waste, or the like, to increase porosity of the media. The composting efficiency is impacted by moisture content, pH, oxygen, temperature, and carbon-to-nitrogen ratio [1,2].
Applicability
The 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 in cannery wastes, explosive-contaminated soil, ethylene glycol in landfill sludges, and polycyclic aromatic hydrocarbons (PAHs) [1,2,3].
Limitations
Factors that may limit the applicability and effectiveness of the process include: (1) substantial space requirements; (2) the need for excavation of contaminated soils is required, which may cause the release of odorous compounds; and (3) the volume of material will increase after composting due to the addition of amendment agents [2].
Performance
Results of some previous studies are included in the Table 1.Table 1. Performance data for composting systems [1].
Site characteristics
Beginning Levels
Levels Attained
PAH-contaminated soil
500 mg/kg of soil
<20 mg/kg in 7 weeks
Sewage wastewater sludge containing hydrocarbons and other compounds
1 g/kg of mixture
50% to 100% removed
in 35 days depending
on compoundPAH: polycyclic aromatic hydrocarbon
Data Requirements
Specific data required to evaluate 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 control potential [2].
Cost
There are three composting systems: (1) windrow; (2) static pile; and (3) in-vessel composting. Estimated costs for full-scale windrow composting of explosives-contaminated soils are approximately $190 per cubic yard for soil volumes of approximately 20,000 cubic yards [1,2]. Estimated costs of the other two systems are higher.
Status of Technology
Windrow systems have been demonstrated as an effective technology for treatment of explosive-contaminated soil. In field demonstrations, TNT reductions were as high as 99.7% in 40 days of operation. However, the application of composting techniques to the treatment of contaminated soils has not been extensive. Although full-scale applications are few, there have been many successful pilot studies, including the degradation of PAHs [1,2].
Reference
1. Cookson, J.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.