BIOREMEDIATION
Bioremediation is the use of living organisms or ecological processes to deal with a given environmental problem. The most common use of bioremediation is the metabolic breakdown or removal of toxic chemicals before or after they have been discharged into the environment. This process takes advantage of the fact that certain microorganisms can utilize toxic chemicals as metabolic substrates and render them into less toxic compounds. Bioremediation is a relatively new and actively developing technology. Increasingly, microorganisms and plants are being genetically engineered to aide in their ability to remove deleterious substances.
In general, bioremediation methodologies focus on one of two approaches. The first approach, bioaugmentation, aims to increase the abundance of certain species or groups of microorganisms that can metabolize toxic chemicals. Bioaugmentation involves the deliberate addition of strains or species of microorganisms that are effective at treating particular toxic chemicals, but are not indigenous to or abundant in the treatment area. Alternatively, environmental conditions may be altered in order to enhance the actions of such organisms that are already present in the environment. This process is known as biostimulation and usually involves fertilization, aeration, or irrigation. Biostimulation focuses on rapidly increasing the abundance of naturally occurring microorganisms capable of dealing with certain types of environmental problems.
Accidental spills of petroleum or other hydrocarbons on land and water are regrettable but frequent occurrences. Once spilled, petroleum and its various refined products can be persistent environmental contaminants. However, these organic chemicals can also be metabolized by certain microorganisms, whose processes transform the toxins into more simple compounds, such as carbon dioxide, water, and other inorganic chemicals. In the past, concentrates of bacteria that are highly efficient at metabolizing hydrocarbons have been "seeded" into spill areas in an attempt to increase the rate of degradation of the spill residues. Although this technique has occasionally been effective, it commonly fails because the large concentrations of hydrocarbons stimulates rapid growth of indigenous microorganisms also capable of utilizing hydrocarbons as metabolic substrates. Consequently, seeding of microorganisms
that are metabolically specific to hydrocarbons often does not affect the overall rate of degradation.
Environmental conditions under which spill residues occur are often sub-optimal for toxin degradation by microorganisms. Most commonly the rate is limited by the availability of oxygen or of certain nutrients such as nitrate and phosphate. Therefore the microbial breakdown of spilled hydrocarbons on land can be greatly enhanced by aeration and fertilization of the soil.
Metals are common pollutants of water and land because they are emitted by many industrial, agricultural, and domestic sources. In some situations organisms can be utilized to concentrate metals that are dispersed in the environment. For example, metal-polluted waste waters can be treated by encouraging the vigorous growth of certain types of vascular plants. This bioremediation system, also known as phytoremediation, works because the growing plants accumulate high levels of metals in their shoots, thereby reducing the concentration in the water to a more tolerable range. The plants can then be harvested to remove the metals from the system.
Many advanced sewage-treatment technologies utilize microbial processes to oxidize organic matter associated with fecal wastes and to decrease concentrations of soluble compounds or ions of metals, pesticides, and other toxic chemicals. Decreasing the aqueous concentrations of toxic chemicals is accomplished by a combination of chemical adsorption as well as microbial biodegradation of complex chemicals into their inorganic constituents.
If successful, bioremediation of contaminated sites can offer a cheaper, less environmentally damaging alternative to traditional clean-up technologies.
