Bioremediation and Enhancements
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
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.
The most common added gas is air (see technology descriptions for Air Sparging
and Bioventing).
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.
Organic Liquid Nutrient Injection. There are also many
organic liquids that can be naturally degraded and fermented in the subsurface
to result in the generation of hydrogen. The most commonly added for enhanced
anaerobic bioremediation include lactate, molasses, Hydrogen Release Compound
(HRCå), and vegetable oils.
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 (ORCå)] 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 and groundwater 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
contaminant-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 and liquids 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 contaminants do not escape
from zones of active biodegradation. See the 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 and radionuclides,
it can be used to immobilize these contaminants.
Technology Development Status
Most forms of bioremediation are 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://costperformance.org/profile.cfm?ID=2&CaseID=2 (molasses injection)
http://www.clu-in.org/techfocus/default.focus/sec/Bioremediation/cat/Overview/
https://ert2.navfac.navy.mil/printfriendly.aspx?tool=BioportalOverview
http://t2.serdp-estcp.org/t2template.html#tool=BioportalOverview&page=intro
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 groundwater 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 a 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 Dense Non-Aqueous Phase Liquids (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,
"Use of Bioremediation at Superfund Sites," 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 "In Situ Bioremediation for the Hanford
Carbon Tetrachloride Plume: Innovative Technology Summary Report," 1999.
DOE/EM-0418, 22 pp. In-situ bioremediation of the Hanford carbon
tetrachloride plume treats groundwater 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. 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 the 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.
See
http://ec.europa.eu/environment/life/project/Projects/files/laymanReport/LIFE03_ENV_B_000018_LAYMAN.pdf
for a description of molasses injection to reduce TCE and to reduce hexavalent
chromium to trivalent chromium.
See
http://t2.serdp-estcp.org/t2template.html#tool=DCE&page=b2.1