Bioventing is an in situ remediation technology that uses microorganisms to biodegrade organic constituents adsorbed on soils in the unsaturated zone. Bioventing enhances the activity of indigenous bacteria and simulates the natural in situ biodegradation of hydrocarbons in soil by inducing air or oxygen flow into the unsaturated zone and, if necessary, by adding nutrients. During bioventing, oxygen may be supplied through direct air injection into residual contamination in soil. Bioventing primarily assists in the degradation of adsorbed fuel residuals, but also assists in the degradation of volatile organic compounds (VOCs) as vapors move slowly through biologically active soil. The rate of natural degradation is generally limited by the lack of oxygen and other electron acceptor (i.e., a compound that gains electrons during biodegradation) rather than by the lack of nutrients (i.e., electron donors). In conventional bioventing systems, oxygen is delivered by an electric blower to subsurface wells. In contrast to soil vapor vacuum extraction, bioventing uses low airflow rates to provide only enough oxygen to sustain microbial activity.
Passive bioventing systems use natural air exchange to deliver oxygen to the subsurface via bioventing wells. A one-way valve, installed on a vent well, allows air to enter the well when the pressure inside the well is lower than atmospheric pressure. When atmospheric pressure drops (due to a change in barometric pressure) below the subsurface pressure, the valve closes, trapping the air in the well and increasing oxygen to the soil surrounding the well.
Limitations and Concerns
High soil moisture or low permeability soils reduce bioventing performance. Low temperatures may slow remediation. Extremely low soil moisture content may limit biodegradation and the effectiveness of bioventing.
Vapors can build up in basements within the radius of influence of air-injection wells. Extracting air near the structure can alleviate this problem.
Saturated soil lenses are difficult to aerate.
Fluctuating water tables create saturated soil zones of low air permeability.
A water table within several feet of the surface limits the vent wellÕs radius of influence.
Monitoring vapor at the soil surface may be required.
Bioventing remediates soils contaminated with fuel. Bioventing techniques have been successfully used to remediate soils contaminated with non-chlorinated solvents, some pesticides, wood preservatives, and other organic chemicals.
Technology Development Status
Bioventing is considered a commercial technology. Bioventing has been approved in 38 states and all 10 Environmental Protection Agency (EPA) regions.
Other Resources and Demonstrations
See descriptions of Enhanced Bioremediation and Air Sparging. The U.S. Air Force Bioventing Initiative demonstrated this technology under widely varying site conditions. More than 90 pilot systems were operated at 41 USAF installations.