The treatment area will generally be covered or contained with an impermeable liner to minimize the risk of contaminants leaching into an uncontaminated soil. The drainage itself may be treated in a bioreactor before recycling. Vendors have developed proprietary nutrient and additive formulations and methods for incorporating the formulation into the soil to stimulate biodegradation. The formulations are usually modified for site-specific conditions.
Soil piles and cells commonly have an air distribution system buried under the soil to pass air through the soil either by vacuum or by positive pressure. The soil piles in this case can be up to 20 feet high (generally not recommended, 2-3 meters maximum). Soil piles may be covered with plastic to control runoff, evaporation, and volatilization and to promote solar heating. If there are VOCs in the soil that will volatilize into the air stream, the air leaving the soil may be treated to remove or destroy the VOCs before they are discharged to the atmosphere.
Biopile is a short-term technology. Duration of operation and maintenance may last a few weeks to several months. Treatment alternatives include static processes such as: prepared treatment beds, biotreatment cells, soil piles, and composting.
Typical Biopile for Solid Phase Bioremediation
Biopile treatment has been applied to treatment of nonhalogenated VOCs and fuel hydrocarbons. Halogenated VOCs, SVOCs, and pesticides also can be treated, but the process effectiveness will vary and may be applicable only to some compounds within these contaminant groups.
Factors that may limit the applicability and effectiveness of the process include:
Excavation of contaminated soils is required.
Treatability testing should be conducted to determine the biodegradability of contaminants and appropriate oxygenation and nutrient loading rates.
Solid phase processes have questionable effectiveness for halogenated compounds and may not be very effective in degrading transformation products of explosives.
Similar batch sizes require more time to complete cleanup than slurry phase processes.
Static treatment processes may result in less uniform treatment than processes that involve periodic mixing.
The first steps in preparing a sound design for biotreatment of contaminated soil include:
Soil sampling and characterization.
Laboratory and/or field treatability studies.
Pilot testing and/or field demonstrations.
Site, soil, and contaminant characterizations will be used to:
Identify and quantify contaminants.
Determine requirements for organic and inorganic amendments.
Identify potential safety issues.
Determine requirements for excavation, staging, and movement of contaminated soil.
Determine availability and location of utilities (electricity and water).
Laboratory or field treatability studies are needed to identify:
Amendment mixtures that best promote microbial activity.
Potential toxic degradation byproducts.
Percent reduction and lower concentration limit of contaminant achievable.
The potential degradation rate.
Biopile treatment has been demonstrated for fuel-contaminated sites. Specific site information is contained in the following site information table.
Costs are dependent on the contaminant, procedure to be used, need for additional pre- and post-treatment, and need for air emission control equipment. Biopiles are relatively simple and require few personnel for operation and maintenance. Typical costs with a prepared bed and liner are $130 to $260 per cubic meter ($100 to $200 per cubic yard).