Enhanced Bioremediation
Description
Bioremediation is a general term used to
describe the destruction of contaminants by biological mechanisms,
including microorganisms (e.g. yeast, fungi, or bacteria), in contaminated soil
and water. Microorganisms eat and digest organic substances for nutrients and
energy. Certain microorganisms can digest organic substances such as fuels or
solvents into harmless products such as carbon dioxide and water. Once the
contaminants are degraded, the microorganism population dies off, having
consumed their entire food source. Bioremediation may rely on either indigenous
microorganisms (those that are native to the site) or exogenous microorganisms
(those that are imported from other locations). In either case, bioremediation
technologies optimize the environmental conditions so the appropriate
microorganisms will flourish and destroy the maximum amount of contaminants.
Bioremediation
can take place under aerobic or anaerobic conditions. Under aerobic conditions,
microorganisms consume atmospheric oxygen in order to function. Under anaerobic
conditions, no oxygen is present. In this case, the microorganisms break down
chemical compounds in the soil to release the energy they need.
Sometimes,
intermediate products are created as the biological processes break down the
original contaminants. The intermediate products may be less, equally, or more
toxic than the original contaminants. Both in-situ
or ex-situ
bioremediation processes have been developed. In-situ bioremediation treats
the contaminated water or soil where it was found. Ex-situ bioremediation
processes involve removing the contaminated soil or water to another location
before treatment. Enhanced Bioremediation involves the addition of
microorganisms (e.g., fungi, bacteria, and other microbes) or nutrients (e.g.
oxygen, nitrates) to the subsurface environment to accelerate the natural biodegradation process. There are four major
processes, briefly described below.
Gaseous
Nutrient Injection
In this case, nutrients are fed to contaminated groundwater and soil via wells to encourage
and feed naturally occurring microorganisms (see technology descriptions for Air Sparging and Bioventing). Vapor extraction is often used in conjunction with
gaseous nutrient injection. The most common added gas is air. In the presence
of sufficient oxygen, microorganisms convert many organic contaminants to carbon dioxide, water,
and microbial cell mass. In the absence of oxygen, organic contaminants are
metabolized to methane, limited amounts of carbon dioxide, and trace amounts of
hydrogen gas. Another gas that is added is methane. It enhances degradation by cometabolism. That is, as bacteria consume the
methane, they produce enzymes that react with the organic contaminant and
degrade it to harmless minerals. See description of Cometabolism.
Oxygen
Enhancement with Hydrogen Peroxide An alternative to pumping oxygen gas into
groundwater involves injecting a dilute solution of hydrogen peroxide. Its
chemical formula is H2O2, and it easily releases its extra oxygen atom to form water
and free oxygen. This circulates through the contaminated groundwater zone to
enhance the rate of aerobic biodegradation of organic contaminants by naturally
occurring microbes. A solid peroxide product [e.g., oxygen releasing compound
(ORC)] can also be used to increase the rate of biodegradation.
Nitrate
Enhancement
A solution of nitrate is sometimes added to groundwater to enhance anaerobic
biodegradation.
Bio-augmentation Sometimes acclimated
microorganisms are added to soil to increase biological activity. Spray
irrigation is typically used for shallow contaminated soils, and injection
wells are used for deeper contaminated soils.
Limitations
and Concerns
Under
anaerobic conditions, contaminants may be degraded to a product that is more
hazardous than the original contaminant. For example, trichloroethylene
(TCE) frequently biodegrades to the persistent and more toxic vinyl
chloride.
Introducing
cold water or gas may slow the remediation process, as lower temperatures do
not support degradation.
Concentrations
of hydrogen peroxide greater than 100 to 200 parts per million (ppm) in
groundwater inhibit the activity of microorganisms.
Amended
oxygen can be consumed very rapidly near the injection well, which creates two
significant problems: biological growth can be limited to the region near the
injection well, limiting adequate contamination/microorganism contact
throughout the contaminated zone; and bio-fouling of wells can retard the input
of nutrients.
Bioremediation
is not well suited for soils with low permeability (e.g., fine clays). High
permeability is required to allow the nutrients to reach the indigenous
microorganisms.
It
is possible that the subsurface injection of gases below the water table can
induce groundwater flow. It may be necessary to use a pump-and-treat system in conjunction with gas
injection for hydraulic control.
The
circulation of water-based solutions through the soil may increase contaminant
mobility and necessitate treatment of underlying groundwater. If the process is
enhancing groundwater bioremediation, a groundwater circulation system must be
created so that contaminants do not escape from zones of active biodegradation.
See description of Circulating Groundwater Wells.
Nitrate
injection to groundwater is of concern because nitrate is a regulated compound.
Bio-augmentation using non-native microorganisms is also controversial.
Very
high contaminant concentrations may be toxic to microorganisms.
Safety
precautions must be used when handling hydrogen peroxide.
Because
gaseous injection increases pressure in the soil, vapors can build up in
building basements.
Applicability
Enhanced
bioremediation techniques have been successfully used to remediate soils and
groundwater contaminated with fuel, volatile organic compounds (VOCs),
semi-volatile organic compounds (SVOCs), perchlorate,
and pesticides. Pilot-scale studies have
demonstrated microbial degradation of soils contaminated with munitions waste.
While bioremediation cannot degrade inorganic contaminants such as metals, it can
be used to immobilize these contaminants.
Technology
Development Status
Most
forms of bioremediation are commercial. Gaseous Nutrient Injection is currently
being applied, and certain applications are considered commercial. The
development of nitrate enhancement is still at the pilot scale. Techniques for
immobilizing metals are largely experimental.
Web
Links
http://www.frtr.gov/matrix2/section4/4-2.html
http://www.frtr.gov/matrix2/section4/4-31.html
http://clu-in.org/download/citizens/bioremediation.pdf
http://www.estcp.org/Technology/upload/BioaugChlorinatedSol.pdf
(bioaugmentation)
Other
Resources and Demonstrations
See
In Situ Anaerobic Bioremediation, Pinellas Northeast Site, Largo, Florida:
Cost and Performance Report, 1998. D.S. Ingle, M. Hightower, G.W. Sewell, EPA
600-R-98-115, NTIS: PB98-168008. A pilot scale demonstration of nutrient
injection to stimulate in situ bioremediation of chlorinated solvents was
performed at the Pinellas Science, Technology and Research (STAR) Center,
formerly the U.S. DOE Pinellas Plant in Largo, Florida, from January through
June of 1997. The innovative remedy is known as reductive anaerobic biological in
situ
treatment technologies (RABITT). A vertical flow system with two horizontal
wells and a series of infiltration galleries was constructed that allowed
development of an effective ground-water recirculation pattern to enable
continuous nutrient addition and enhance system performance.
See
http://www.itrcweb.org/Documents/ISB-6.pdf
for Technical and Regulatory Guidance and http://www.itrcweb.org/Documents/ISB-8.pdf
for systematic approach to bioremediation. Also see http://www.itrcweb.org/Documents/bioDNPL_Docs/BioDNAPL-2.pdf
and http://www.itrcweb.org/Documents/bioDNPL_Docs/BioDNAPL3.pdf
for approaches to using bioremediation for DNAPL.
See
http://toxics.usgs.gov/bib/bib-Biodegradation.html
for a bibliography of biodegradation and Natural Attenuation.
See
http://www.clu-in.org/download/remed/542r01019.pdf,
ÒUse of Bioremediation at Superfund Sites,Ó EPA 542-R-01-019, September 2001,
48 pages. This document provides site-specific information about 104 Superfund
remedial action sites where bioremediation has been applied, including
available performance data.
See
ÒIn Situ Bioremediation for the Hanford Carbon Tetrachloride Plume: Innovative
Technology Summary Report,Ó 1999. DOE/EM-0418, 22 pp. In situ bioremediation of
the Hanford carbon tetrachloride plume treats ground water contaminated with volatile
organic compounds and nitrates under anaerobic conditions.
See
http://www.clu-in.org/products/newsltrs/gwc/gwc1200.htm#biodegradation
for a description of in situ biodegradation enhanced with the injection of
lactate used to treat the residual source area of a large trichloroethylene
plume. TCE is present in a sludge mixture due to the historical injection of
waste into the basalt aquifer. For eight months lactate was injected 200 to 300
feet below ground surface (bgs). During reductive dechlorination, TCE is
transformed to 1,2-dichloroethylene (DCE), then vinyl chloride (VC), and
finally ethene, the desired end product. Of particular importance was the
appearance of ethene simultaneously with VC, indicating that VC would not
accumulate in the system. The success of the project in a complex fractured
basalt aquifer may be a milestone both for fractured rock remediation and for in
situ
bioremediation of chlorinated solvent source areas.
See http://www.epa.gov/tio/download/remed/engappinsitbio.pdf for engineering approaches to in-situ bioremediation.