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Microbiology => Genetic Engineering => Bioremediation
Bioremediation
Bioremediation, the use of microorganisms, such as bacteria, to remove environmental pollutants from soil, water, or gases. Pollutants are persistent chemical compounds that are toxic (poisonous) to plants and animals or have the potential to cause birth defects (teratogenicity) or cancer (carcinogenicity) in humans. Bioremediation accounts for 5 to 10 percent of all pollution treatment.
Microorganisms decompose organic compounds using enzymes, protein-based molecules that control metabolism (life-sustaining chemical reactions) in all living cells. Some of these enzymes act to break down, or biodegrade, pollutants, and this natural process can be exploited for the treatment of industrial, agricultural, or municipal wastes. Each enzyme controls one in a series of steps, called a pathway, by which a toxic substance is metabolized into nontoxic products. Different microorganisms may be used to perform different stages of a pathway.
Bioremediation works best on natural carbon-containing substances called hydrocarbons or on chemicals resembling natural substances. Bacteria that metabolize naturally occurring hydrocarbons, such as certain petroleum products, are widespread in the environment. Certain bacteria that biodegrade gasoline, for example, are found in almost all soils; the gasoline-metabolizing bacteria may be isolated from the other bacteria present in the soil by laboratory testing. Some of the bacteria employed in bioremediation include members of the genera Pseudomonas,Flavobacterium,Arthrobacter, and Azotobacter. Some toxic substances that have been successfully bioremediated include the solvent toluene, the moth repellent naphthalene (mothballs), the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D), and the fungicide and wood preservative pentachlorophenol.
Bioremediation techniques vary widely in their degree of sophistication, but they have some features in common. For example, bioremediation is generally used to treat dispersed, dilute solutions containing one or more pollutants, such as oil or gasoline; more concentrated wastes are usually handled by storage or incineration. Bioremediation has been used successfully in the cleanup of thousands of leaking underground gasoline storage tanks, commonly found at filling stations. A typical treatment process involves digging out and removing the tank, adding growth-promoting nutrients such as nitrogen and phosphorusfertilizers to the surrounding soil, and tilling the soil to introduce oxygen.
Bioremediation was employed on a test basis on Alaskan beaches contaminated with oil released by the Exxon Valdez in 1989. Initial field tests revealed one brand of fertilizer that best stimulated bioremediation of the oil, and that fertilizer was then applied to portions of the oil-coated beaches. In those areas, the beaches appeared dramatically cleaner than in untreated areas, but the complexity of the chemical composition of crude oil makes determining the true effectiveness of the treatment very difficult.
Land farming is a practice whereby pollutants are transported to a field for bioremediation. The pollutants are mixed into the soil using farm equipment. The soils at land-farming sites may contain populations of bacteria with broad metabolic capabilities for handling many different organic compounds.
Industrial wastewater is commonly treated by bioremediation in lagoons or tanks. Wastewater tanks are sometimes seeded with sediments from other industrial treatment ponds, providing a rich source of organisms with the desired metabolic activities. A more sophisticated process involves a bioreactor-a large container through which wastewater is pumped. While in transit through the container, the water contacts microorganisms that are immobilized on the surface of some solid support such as crushed rocks. The bacteria metabolize most of the pollutants in the wastewater before it exits the bioreactor. Bioreactors are sometimes used to treat groundwater that has been pumped to the surface.
Although the use of genetic engineering to produce microorganisms for waste treatment has received much coverage in the popular press, this approach has been little used to date. Naturally occurring organisms often work better in large-scale treatment systems. Also, companies performing bioremediation have been restricted by rules on the release of genetically engineered microorganisms into the environment.
Bioremediation is not always successful; thus, although it is often the most inexpensive method, it is only one of many techniques for dealing with hazardous wastes. For example, waste mixtures may contain heavy metals, which kill the bacteria capable of metabolizing any organic pollutants in the waste. Bioavailability of the pollutant (the amount actually available to be broken down by microorganisms) may also be a problem: Some organic compounds, such as polychlorinated biphenyls (PCBs) and dioxins, may adsorb onto (stick to) some soils too tightly to be metabolized. Finally, some organic compounds are very poorly biodegraded. However, when it is effective, biological waste treatment is desirable because it is inexpensive, can be done at the site of pollution, and causes minimal physical disturbance to the surrounding area compared to other methods.
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