Composting
Description
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 54¡ to 65¡
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.
The
excavation of contaminated soils is required. This may cause the uncontrolled release of volatile organic compounds (VOCs)
and dust.
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.
If
VOC or semi-volatile organic compounds (SVOC)
contaminants are present in soils, off-gas (i.e., air emissions) control may be
required.
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.
Some
compost from industrial wastes reportedly contains high levels of heavy metals and dioxin. These wastes should not be recycled
into compost products (i.e.., soil amendments).
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.
Applicability
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.
Web
Links
http://www.frtr.gov/matrix2/section4/4-12.html
Other
Resources and Demonstrations
See
the descriptions of Bio-Piles and Bio-Reactors.
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
http://www.epa.gov/epawaste/conserve/rrr/composting/pubs/analpt2.pdf
http://www.epa.gov/epawaste/conserve/rrr/composting/pubs/analpt3.pdf
http://www.epa.gov/epawaste/conserve/rrr/composting/pubs/analpt4.pdf
See http://www.clu-in.org/products/NEWSLTRS/TTREND/ttcmpost.htm for treatability study at Bangor Maine.