Bioventing
Description
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.
Aerobic biodegradation of
many chlorinated
compounds may not be effective unless a cometabolite is present.
Monitoring
vapor at the soil surface may be required.
Applicability
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.
Web
Links
http://www.frtr.gov/matrix2/section4/4_1.html
http://clu-in.org/techfocus/default.focus/sec/Bioventing_and_Biosparging/cat/Overview/
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.