Lakhasly

Biological treatment involves the use of microorganisms or vegetation (phytoremediation).This measurement is not required, but it is available to resolve regulatory concerns. In addition to fuels, aerobic bioventing has treated a variety of other contaminants, including nonhalogenated solvents, such as benzene, acetone, toluene, and phenol; lightly halogenated solvents, such as 1,2-dichloroethane, dichloromethane, and chlorobenzene; and SVOCs, such as some PAHs (Figure 4).The manual contains information on bioventing principles; site characterization; field treatability testing; system design, operation, and installation; site closure; and techniques to demonstrate the extent and mechanism for contaminant removal.Low oxygen levels (less than five percent) are a good indicator that existing bacteria are capable of degrading the contaminants of concern, because soil gas in uncontaminated soil generally exhibits oxygen concentrations similar to ambient air.For sites where non-fuel contaminants are to be treated by bioventing, other factors may be considered in establishing biological a bioventing system has been
soil gas monitoring to ensure that the site is well oxygenated, in situ respiration testing to monitor the progress of remediation, and operation and maintenance of the bioventing system.Bioventing involves supplying oxygen to contaminated unsaturated soils with low oxygen concentrations to facilitate aerobic microbial biodegradation.For example, laboratory and pilot-scale studies may be needed to evaluate effectiveness, design the bioventing system, estimate treatment times, and demonstrate that biodegradation is the primary mechanism of removal.The injection system should be designed considering soil gas permeability, contaminant diffusion and distribution, and environmental factors, such as moisture content, pH, temperature, and electron acceptor conditions.Performance monitoring, after and amount of contaminant removed, oxygen supply, and carbon dioxide generation, as well as mass balances relating the three amounts, may be useful in establishing bioremediation as the primary mechanism of removal. Many naturally occurring microorganisms (typically, bacteria and fungi) can transform hazardous chemicals to substances that may be less hazardous than the original compounds.When building foundations or similar structures are close to the site, vacuum extraction wells, which draw air through the subsurface, may be used to avoid the buildup of contaminated, and possibly explosive, vapors in the building basements.Microbial bioremediation occurs under both aerobic and anaerobic conditions and at contaminated sites as either intrinsic and/or enhanced biodegradation.Typically, the environment is manipulated by supplying amendments, such as air, organic substrates, nutrients, and other compounds, whose absence limit 15 treatment.A soil gas survey can provide useful information, especially at sites with relatively shallow contamination (depths typically less than 20 ft).In addition, the soil gas survey can be useful in delineating the extent of contamination and identifying locations for vent wells and monitoring points.Microrganisms also have been used to alter the valence of some hazardous metals (e.g., Cr(VI)), thereby making them less hazardous and less mobile.Several plant species have the ability to bioaccumulate heavy metals found in the soil, and some tree species can sequester, destroy, and/or evapotranspire various organic compounds.The U.S. Air Force Bioventing Initiative and the U.S. EPA Bioremediation Field Initiative evaluated bioventing at 125 sites contaminated by petroleum hydrocarbons.At sites using extractive bioventing, the degree of volatilization versus biodegradation may be determined by measuring offgas concentrations.Finally, measurement of stable isotope ratios may be useful in qualitatively validating biodegradation as the mechanism of contaminant removal.Monitored natural attenuation (MNA) often relies on intrinsic bioremediation as an important removal mechanism.Bioventing is typically operated in air injection mode to alleviate low oxygen levels in the subsurface.

Biological treatment involves the use of microorganisms or vegetation (phytoremediation). Many
naturally occurring microorganisms (typically,
bacteria and fungi) can transform hazardous chemicals to substances that may be less hazardous than
the original compounds. Microrganisms also have
been used to alter the valence of some hazardous
metals (e.g., Cr(VI)), thereby making them less
hazardous and less mobile. Several plant species
have the ability to bioaccumulate heavy metals
found in the soil, and some tree species can
sequester, destroy, and/or evapotranspire various
organic compounds.
Microbial bioremediation occurs under both aerobic
and anaerobic conditions and at contaminated sites
as either intrinsic and/or enhanced biodegradation.
Intrinsic bioremediation depends on indigenous
microorganisms to degrade contaminants without
any amendments. Monitored natural attenuation
(MNA) often relies on intrinsic bioremediation as an
important removal mechanism. During enhanced
bioremediation, biodegradation is facilitated by
manipulating the microbial environment. Typically,
the environment is manipulated by supplying
amendments, such as air, organic substrates, nutrients, and other compounds, whose absence limit
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treatment. In some cases, bioremediation has been
enhanced by adding microbial cultures (bioaugmentation).
3.2.1 Bioventing
Bioventing involves the injection of a gas into the
subsurface to enhance the biodegradation of a
contaminant. The gas can be used to keep the subsurface aerobic or anaerobic, or to provide a substrate that enables cometabolic degradation to occur.
Aerobic Bioventing
Aerobic bioventing has a robust track record in
treating aerobically degradable contaminants, such as
fuels. Bioventing involves supplying oxygen to
contaminated unsaturated soils with low oxygen
concentrations to facilitate aerobic microbial biodegradation. Using the supplied oxygen, the microbes
oxidize the contaminants to gain energy and carbon
for growth. Oxygen is typically introduced by air
injection wells that push air into the subsurface.
Aerobically degradable contaminants may be treated
by bioventing, but fuels have received the most
attention. The U.S. Air Force Bioventing Initiative
and the U.S. EPA Bioremediation Field Initiative
evaluated bioventing at 125 sites contaminated by
petroleum hydrocarbons. At sites where initial studies
were positive, pilot-scale bioventing was installed
and operated for one year. The experience from
bioventing demonstrations at these sites was
condensed into a manual (U.S. EPA 1995a & 1995b).
The manual contains information on bioventing
principles; site characterization; field treatability
testing; system design, operation, and installation; site
closure; and techniques to demonstrate the extent and
mechanism for contaminant removal. Based on this
research, bioventing proved to be an economical and
effective method to treat unsaturated soil
contaminated by petroleum products. Regulatory
acceptance of this technology has occurred in 30
states and in all 10 EPA regions. The use of this
technology in the private sector has increased
following the U.S. Air Force Bioventing Initiative
and the U.S. EPA Bioremediation Field Initiative.
Estimated costs range from $10 to $60 per cubic yard
(U.S. EPA 1995b).
In addition to fuels, aerobic bioventing has treated a
variety of other contaminants, including nonhalogenated solvents, such as benzene, acetone, toluene,
and phenol; lightly halogenated solvents, such as
1,2-dichloroethane, dichloromethane, and chlorobenzene; and SVOCs, such as some PAHs (Figure
4). The principles outlined in the manual are also
applicable for aerobically degradable non-fuel
contaminants, but since the experience with these
other types of contaminants is more limited, more
information may be needed. For example, laboratory
and pilot-scale studies may be needed to evaluate
effectiveness, design the bioventing system, estimate treatment times, and demonstrate that biodegradation is the primary mechanism of removal. In
evaluating the feasibility of treating other
contaminants, the key is to understand the volatility
relative to the biodegradability.
Bioventing is typically operated in air injection
mode to alleviate low oxygen levels in the
subsurface. The injection system should be designed
considering soil gas permeability, contaminant
diffusion and distribution, and environmental
factors, such as moisture content, pH, temperature,
and electron acceptor conditions. When building
foundations or similar structures are close to the site,
vacuum extraction wells, which draw air through the
subsurface, may be used to avoid the buildup of
contaminated, and possibly explosive, vapors in the
building basements.
Extracted gases require treatment since volatile compounds may be removed from the ground. In cases
of remote locations without electric power, passive
air delivery systems may be used. These systems use
one-way valves and changes in barometric pressure
to deliver air to the subsurface; however, passive
systems may have longer treatment times depending
on the quantities of air supplied to the subsurface.
Compared to soil vapor extraction, all bioventing
delivery systems employ lower air flow rates that
provide only the amount of oxygen required to
enhance removal. When operated properly, the low
flow rates of air injection do not result in the release
of the contaminants into the atmosphere through
volatilization.
To determine if bioventing is appropriate at a
specific site, existing site data should be evaluated
and, if needed, additional data collected. For
example, information about the types, quantities, and
three-dimensional distribution of contaminants is
needed. This includes the presence and location of
free product and whether there is a chance of
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continuing contamination from leaking pipes or
tanks. Bioventing alone is not sufficient to remediate
sites with large quantities of free product or ongoing
releases. Information about the historical water table
levels and soil characteristics, such as gas
permeability, is also needed. A soil gas survey can
provide useful information, especially at sites with
relatively shallow contamination (depths typically less
than 20 ft). The soil gas
survey is useful to determine
whether oxygen-limited
conditions exist. Low oxygen
levels (less than five percent)
are a good indicator that
existing bacteria are capable
of degrading the contaminants of concern, because
soil gas in uncontaminated
soil generally exhibits oxygen
concentrations similar to
ambient air. In addition, the
soil gas survey can be useful
in delineating the extent of
contamination and identifying
locations for vent wells and
monitoring points. If this
preliminary information looks
promising, more specific information should be gathered,
such as soil contaminant concentrations and distribution
and soil characterization.
Respiration rate, soil gas
permeability, and oxygen
radius of influence will be
needed to properly design the
system.
Performance monitoring, after
and amount of contaminant removed, oxygen
supply, and carbon dioxide generation, as well as
mass balances relating the three amounts, may be
useful in establishing bioremediation as the primary
mechanism of removal. For sites where non-fuel
contaminants are to be treated by bioventing, other
factors may be considered in establishing biological
a bioventing system has been

soil gas monitoring to ensure
that the site is well oxygenated, in situ respiration
testing to monitor the progress of remediation, and
operation and maintenance of the bioventing system.
At some sites, surface emissions sampling may be
needed. At sites using extractive bioventing, the
degree of volatilization versus biodegradation may be
determined by measuring offgas concentrations.
Injection-based systems may be briefly reconfigured
to gain similar information. Measurements of the rate
activity as the primary mechanism of removal.
Finally, measurement of stable isotope ratios may be
useful in qualitatively validating biodegradation as
the mechanism of contaminant removal. This
measurement is not required, but it is available to
resolve regulatory concerns.