Composting is a process in which organic wastes are degraded by microorganisms at elevated temperatures under both aerobic and anaerobic conditions. Soils are excavated and mixed with bulking agents and organic amendments, such as wood chips and plant wastes that enhance porosity. Maintaining proper oxygen and moisture content and closely monitoring temperature helps achieve maximum degradation efficiency. Typical compost temperatures range from 54Á to 65Á Celsius. The increased temperatures result from heat produced by microorganisms during the degradation of the organic material in the waste. It produces a byproduct that is stable and in some circumstances results in complete degradation of the contaminant.
There are three major designs used in composting. The first design is an aerobic static pile. Compost is formed into piles and aerated with blowers or vacuum pumps. The second design uses a vessel similar to a bio-reactor. After being placed in the vessel, the compost is mechanically agitated and aerated. The third method is called windrow composting.
Windrow composting is usually considered the most cost-effective composting alternative. After contaminated soil is excavated, large rocks and debris are removed. Amendments such as straw, alfalfa, manure and agricultural wastes are then added. The material is layered into long piles, known as windrows. The windrow is thoroughly mixed by turning with a commercially available composting machine. Moisture, pH, temperature, and contaminant concentrations are monitored. At the completion of the composting period the windrows are disassembled and the compost is taken to the final disposal area.
Limitations and Concerns
Substantial space and labor costs are required for composting.
Windrow composting has the highest fugitive dust emissions (i.e., windblown dust and particulates). Depending on soil type, these emissions may have to be controlled.
Composting results in an increase in material because of the addition of amendment material. If there are any residuals that are not degraded, the end product may require careful disposal.
Heavy metals are not treated by this method. Also, high levels of heavy metals can be toxic to the microorganisms that break down organic compounds.
Composting, especially windrow composting, has been demonstrated for the treatment of explosives-contaminated soil. During a field demonstration, trinitrotoluene (TNT) reductions were as high as 99.7% during 40 days of operation. Maximum removal efficiencies for Royal Demolition Explosive (RDX) and High Melt Explosive (HMX) were 99.8% and 96.8%, respectively. The composting process may also be used for soils contaminated with biodegradable organic compounds. Pilot and full-scale projects have demonstrated that aerobic composting is also applicable to SVOC-contaminated soil.
Technology Development Status
The process has been widely used for municipal wastes, and it has been used for hazardous and explosive wastes. All materials and equipment used for composting are commercially available.
Other Resources and Demonstrations
The Umatilla Army Depot in Oregon has successfully used composting to convert 15,000 tons of contaminated soil into safe soil. Contaminant byproducts were either destroyed or permanently bound to soil or humus. Other sites using composting for explosives include the U.S. Naval Submarine Base in Bangor, Washington; the Navy Surface Warfare Center in Crane, Indiana; and the Sierra Army Depot in California.
A demonstration at the Radford Army Ammunition Plant (RAAP) in Virginia indicated that biological treatment may be feasible for treating nitrocellulose fine wastes from munitions manufacturing processes, and a field demonstration at the Badger Army Ammunition Plant, Wisconsin determined that composting could successfully biologically degrade the nitrocellulose in soils.
See for part EPA publication analysis of composting as a remediation technology: http://www.epa.gov/epawaste/conserve/rrr/composting/pubs/analpt1.pdf
See http://www.clu-in.org/products/NEWSLTRS/TTREND/ttcmpost.htm for treatability study at Bangor Maine.
See http://www.clu-in.org/characterization/technologies/exp.cfm for a technical description of explosives in different media and the use of some analytical techniques.