The Composting Alternative to Incineration of Explosives Contaminated Soils

By Harry Craig, EPA Region 10 and Wayne Sisk, U.S. Army Environmental Center

EPA's Region 10 has evaluated composting as an ex-situ solid phase biological treatment technology to degrade nitroaromatic and nitramine compounds in soils. Treatability studies at two National priority List sites --the Umatilla Army Depot Activity site in Hermiston, Oregon and the U.S. Naval Submarine Base site in Bangor, Washington -- demonstrate that composting is a treatment alternative to incineration for remediating these compounds. Composting has been selected as the Record of Decision treatment for 11,000 tons of TNT (2,4,6-trinitrotoluene) , RDX (hexahydro-1,3,5-trinitro-1,3,5, 7-tetrazocine) contaminated soils at Umatilla and for 2.200 tons of TNT at Bangor. Previously, composting has been used primarily to treat municipal solid waste, but not hazardous wastes.

Composting mixes natural organic amendments, such as manure, wood chips, alfalfa and vegetable processing wastes with 30% contaminated soil and adds water to 50% of moisture holding capacity. The process utilizes native aerobic thermophilic microorganisms and requires no inoculation. Composting operates under mesophilic [30-35 degrees Centigrade (C)] and thermophilic (50-55 degrees C) conditions, with thermophilic conditions being optimum. Amendments serve as a source of carbon and nitrogen for thermophiles, which degrade explosives under co-metabolic conditions. Composting produces no chemical air emissions and no leachate; and, it does not require dewatering upon completion of treatment. Composting residues will support the growth of vegetation after treatment, unlike incineration ash or soils treated by solidification/stabilization. The final volume increase in soil is approximately 50% to 100%, similar to stabilization/solidification technologies.

At the Umatilla site, the soils were contaminated from the discharge of 85 million gallons of explosives' wastewater into unlined lagoons from 1950 to 1965. During the pilot-scale treatability study, 30 cubic yards of soil were treated in each of two windrow configurations, one with forced aeration and the other unaerated. After 40 days of treatment, composting reduced initial average contaminant concentrations of 1,574 parts per million (ppm) TNT to 4 ppm; 944 ppm RDX to 2 ppm; and 159 ppm HMX to 5 ppm. Destruction and removal efficiencies (DRE) were: 99.7% for TNT; 99.8% for RDS; and 96.9% for HMX. The treatment process also degraded key biodegradation intermediates of TNT--2A-4,6-DNT (2-amino, 4-6-dinitrotoluene) and 4A-2,6DNT (4-amino-2,6-dinitrotoluene).

At Umatilla, toxicology and leachability tests also were performed to compare toxicity and mobility effects of compost residues to those in untreated soils. Toxicity results showed 87% to 92% reduction of leachate toxicity to Ceriodaphnia dubia, and 99.3% to 99.6% reduction in mutagenicity for Ames assays using strains TA-98 and TA-100. A brief oral rat feeding study did not produce mortality from consumption of compost residues. Leachable concentrations were greater than 99.6% for TNT, 98.6% for RDX and 97.3% for HMX, using the EPA Synthetic Precipitation Leach Procedure (SPLP)(SW-846 Method 1312).

At Bangor, soils have been contaminated from open-burn/open-detonation (OB/OD disposal of munitions from 1946 to 1965. Region 10 conducted bench scale treatability studies to evaluate composting treatment of TNT contaminated soils from three areas of the base -- one wastewater disposal lagoon and two ordnance OB/OD sites. Composting reduced the concentration of TNT in one kilogram of soil from 822 ppm to 8 ppm after 60 days of treatment, with a DRE of 99.5%. A pilot scale treatability study of 60 cubic yards of soil is currently in progress. Results will be available in March 1995. For the treatability studies at both sites, an asphalt liner in a temporary building was used to house the biotreatment system. Site specific factors should determine what containment system, if any, should be used.

The treatability studies at Umatilla and Bangor indicate that composting is capable of achieving risk-based cleanup levels of 30 to 33 ppm for TNT and 9 ppm to 30 ppm RDX after 40 days of treatment. The Feasibility Study estimates treatment costs of $206 to $766 per ton, which is 40% to 50% less than on-site incineration for quantities from 1,200 to 30,000 tons. Actual costs will be refined during full-scale remediation, which is scheduled to begin in 1995.

Composting is suitable for soils and sludges. Composting does not appear to be particularly sensitive to soil type. Umatilla soils are sands/gravel; and, Bangor soils are loams and glacial till. A moderate amount of contaminated wastewater can be treated with soil, since the process consumes water at a rate of approximately one gallon per cubic yard per day of treatment. Contaminated rocks and debris can be crushed or shredded and treated with soils.

For more information, call Harry Craig at EPA's Region 10 Oregon Operations Office at 503-326-3689.