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 (HRC¬), 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 (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 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.
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. The development of nitrate enhancement is still at the pilot scale. Techniques for immobilizing metals are largely experimental.
http://costperformance.org/profile.cfm?ID=2&CaseID=2 (molasses injection)
http://www.afcee.af.mil/resources/technologytransfer/programsandinitiatives/enhancedinsituanaerobicbioremediation/substrate/index.asp (lactate, molasses and HRC¬ injection)
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 also http://www.ert2.org/t2bioportal/.