Composting is a controlled biological process by which organic contaminants (e.g., PAHs) are converted by microorganisms (under aerobic and anaerobic conditions) to innocuous, stabilized byproducts. Typically, thermophilic conditions (54 to 65 °C) must be maintained to properly compost soil contaminated with hazardous organic contaminants. The increased temperatures result from heat produced by microorganisms during the degradation of the organic material in the waste. In most cases, this is achieved by the use of indigenous microorganisms. Soils are excavated and mixed with bulking agents and organic amendments, such as wood chips, animal, and vegetative wastes, to enhance the porosity of the mixture to be decomposed. Maximum degradation efficiency is achieved through maintaining oxygenation (e.g., daily windrow turning), irrigation as necessary, and closely monitoring moisture content, and temperature.
There are three process designs used in composting: aerated static pile composting (compost is formed into piles and aerated with blowers or vacuum pumps), mechanically agitated in-vessel composting (compost is placed in a reactor vessel where it is mixed and aerated), and windrow composting (compost is placed in long piles known as windrows and periodically mixed with mobile equipment). Windrow composting is usually considered to be the most cost-effective composting alternative. Meanwhile, it may also have the highest fugitive emissions. If VOC or SVOC contaminants are present in soils, off-gas control may be required.
The composting process may be applied to soils and lagoon sediments contaminated with biodegradable organic compounds. Pilot and full-scale projects have demonstrated that aerobic, thermophilic composting is able to reduce the concentration of explosives (TNT, RDX, and HMX), ammonium picrate (or yellow-D), and associated toxicity to acceptable levels. Aerobic, thermophilic composting is also applicable to PAH-contaminated soil. All materials and equipment used for composting are commercially available.
The following factors may limit the applicability and effectiveness of the process:
- Substantial space is required for composting.
- Excavation of contaminated soils is required and may cause the uncontrolled release of VOCs.
- Composting results in a volumetric increase in material because of the addition of amendment material.
- Although levels of metals may be reduced via dilution, heavy metals are not treated by this method. Also high levels of heavy metals can be toxic to the microorganisms.
- Data Needs
Specific data required to evaluate the compost process include contaminant concentration, excavation requirements, availability and cost of amendments required for compost mixture, space available for treatment, soil type, and amenability of the contaminants to composting.
Windrow composting has been demonstrated as an effective technology for treatment of explosives-contaminated soil. During a field demonstration conducted by USAEC and the Umatilla Depot Activity (UMDA), TNT reductions were as high as 99.7% in 40 days of operation, with the majority of removal occurring in the first 20 days of operation. Maximum removal efficiencies for RDX and HMX were 99.8% and 96.8%, respectively. The relatively simple equipment requirements combined with these performance results make windrow composting economically and technically attractive.
Costs will vary with the amount of soil to be treated, the soil fraction in the compost, availability of amendments, the type of contaminant, and the type of process design employed. Estimated costs for full-scale windrow composting of explosives-contaminated soils are approximately $190 per cubic yard for soil volumes of approximately 20,000 yd3. Estimated costs for static pile composting and mechanically agitated in vessel composting are $236 and $290, respectively. Composting may be an economic alternative to thermal treatment, however, when cleanup criteria and regulatory requirements are suitable.