All types of biodegradation, both in situ or ex situ, can be used to remediate soils: in situ biodegradation, bioventing, composting, bioslurping, biopiles (controlled solid phase), or landfarming. Slurry-phase biological treatment is also applicable but is less widely used. Biodegradation uses indigenous or inoculated microorganisms (e.g., fungi, bacteria, and other microbes) to degrade (i.e., metabolize) organic contaminants found in soil and/or ground water. In the presence of sufficient oxygen (aerobic conditions), microorganisms will ultimately convert many organic contaminants to carbon dioxide, water, and microbial cell mass. In the absence of oxygen (anaerobic conditions), the contaminants will be ultimately metabolized to methane. Sometimes contaminants may not be completely degraded, but only transformed to intermediate products that may be less, equally, or more hazardous than the original contaminant.
In situ bioremediation of soil typically involves the percolation or injection of ground water or uncontaminated water containing dissolved oxygen and nutrients. Ex situ bioremediation typically uses tilling or continuously mixed slurries to apply oxygen and nutrients, and is performed in a prepared bed (liners and aeration) or reactor. Bioventing is a presumptive remedy for fuels. Bioventing is an in situ technique that uses air injection to aerate the soil and enhance biodegradation. The AFCEE Bioventing Initiative currently encompasses more than 145 fuel sites at 56 military installations, including one Marine, three Army, and one Coast Guard facility. Approximately 50% of the current systems are full scale. As of September 1995, approximately 125 are installed and operating. The remainder are to be installed.
Incineration uses high temperatures, 870 to 1,200o C (1,400 to 2,200o F), to volatilize and combust (in the presence of oxygen) organic constituents in hazardous wastes. The destruction and removal efficiency (DRE) for properly operated incinerators exceeds the 99.99% requirement for hazardous waste and can be operated to meet the 99.9999% requirement for PCBs and dioxins. Distinct incinerator designs are rotary kiln, liquid injection, fluidized bed, and infrared units. All types have been used successfully at full scale.
Soil vapor extraction (SVE) is an in situ unsaturated (vadose) zone soil remediation technology in which a vacuum is applied to the soil to induce the controlled flow of air and remove volatile and some semivolatile contaminants from the soil. The gas leaving the soil may be treated to recover or destroy the contaminants, depending on local and state air discharge regulations. Explosion-proof equipment should be used for fuels. Vertical extraction vents are typically used at depths of 1.5 meters (5 feet) or greater and have been successfully applied as deep as 91 meters (300 feet). Horizontal extraction vents (installed in trenches or horizontal borings) can be used as warranted by contaminant zone geometry, drill rig access, or other site-specific factors.
Ground water extraction pumps may be used to reduce ground water upwelling induced by the vacuum or to increase the depth of the vadose zone. Air injection may be effective for facilitating extraction of deep contamination, contamination in low permeability soils, and contamination in the saturated zone (see Treatment Technology Profile 4.34, Air Sparging).
Low temperature thermal desorption (LTTD) systems are physical separation processes and are not designed to destroy organics. Wastes are heated to between 90 and 315o C (200 to 600o F) to volatilize water and organic contaminants. A carrier gas or vacuum system transports volatilized water and organics to the gas treatment system. Ground water treatment concentrates the collected contaminants (e.g., carbon adsorption or condensation). The bed temperatures and residence times designed into these systems will volatilize selected contaminants but will typically not oxidize them. LTTD is a full-scale technology that has been proven successful for remediating petroleum hydrocarbon contamination in all types of soil. Decontaminated soil retains its physical properties and ability to support biological activity.