Bioremediation | Research & Encyclopedia Articles

Bioremediation

Bioremediation is a type of biotechnology in which living organisms or ecological processes are utilized to deal with some environmental problem. The most common use of bioremediation is to metabolically break down or otherwise remove toxic chemicals before or after they have been discharged into the environment. In such uses, bioremediation takes advantage of the fact that certain microorganisms can utilize such toxic chemicals as metabolic substrates, in the process rendering them into simpler, less toxic compounds. Bioremediation is a relatively new and actively developing technology.

In general, bioremediation methodologies focus on: (1) enhancing the abundance of certain species or groups of microorganisms that can metabolize toxic chemicals (this is also known as bioaugmentation) and/or (2) optimizing environmental conditions for the actions of these organisms (also known as biostimulation). Bioaugmentation may involve the deliberate addition of strains or species of microorganisms that are specifically effective at treating particular toxic chemicals, but are not indigenous to or abundant in the treatment area. Biostimulation usually involves fertilization, aeration, or irrigation in order to decrease the importance of environmental factors in limiting the activity of microorganisms. Biostimulation focuses on rapidly increasing the abundance of naturally occurring, ubiquitous microorganisms that are capable of dealing with certain types of environmental problems.

Bioremediation of spilled hydrocarbons

Accidental spills of petroleum or other hydrocarbons on land and water are regrettable but frequent occurrences. Such spills can range in size from a few gallons that may be spilled during refueling to enormous spillages of millions of tons as occurred to both the sea and land during the Gulf War of 1991. Once spilled, petroleum and its various refined products can be persistent environmental contaminants. However, these organic chemicals can also be metabolized by microorganisms which in the process transform them into simpler compounds, ultimately to carbon dioxide, water, and other inorganic chemicals.

Numerous attempts have been made to increase the rates by which microorganisms break down spilled hydrocarbons. In some cases, specially prepared 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 sometimes been effective, it commonly is not. This occurs because the indigenous microbial communities of soils and aquatic sediments contain many species of bacteria and fungi that are capable of utilizing hydrocarbons as a metabolic substrate. After a spill the occurrence of large concentrations of hydrocarbons in soil or sediment stimulates rapid growth of those microorganisms. Consequently, seeding of microorganisms that are metabolically specific to hydrocarbons does not always make much of a difference to the overall rate of degradation.

More important, however, is the fact that the environmental conditions under which spill residues occur are almost always highly sub-optimal for their degradation by microorganisms. Most commonly, the rate of microbial breakdown of spilled hydrocarbons 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 occasionally tilling the soil to keep conditions aerated and by fertilizing with nitrogen and phosphorus while keeping conditions moist but not wet. Therefore, bioremediation systems for dealing with soils contaminated by spilled gasoline or petroleum can be based on simple tillage and fertilization.

Similarly, petroleum refineries may utilize a bioremediation process called landfarming in which oily wastes are spread onto land which is then tilled and fertilized until microbes reduce the residue concentrations to an acceptable level.

After some petroleum spills, more innovative approaches may prove to be useful. For example, it is difficult to fertilize aquatic habitats, because the nutrients simply wash away and are therefore not effective for very long. In the case of the Exxon Valdez spill in Alaska in 1989, research demonstrated that nutrients could be applied to soiled beaches as an oleophilic (that is, oil-seeking), nitrogen and phosphorus-containing fertilizer. Because of its oleophilic nature, the fertilizer adhered to the petroleum residues and was able to significantly enhance the rate of oil degradation by the naturally occurring community of microorganisms. This treatment was applied to about 73 m (118 km) of oiled beach and proved to be successful in speeding up the process of degradation of the residues by increasing the rate of oxidation by about 50%. No attempts were made in this case to "seed" the microbial community with species that are specifically adapted to metabolizing hydrocarbons. It was believed that hydrocarbon-specific microbes were naturally present in the beach sediment and that their activity and that of species with broader substrate tolerances only had to be enhanced by making the ecological conditions more favorable, that is, by fertilizing.

Bioremediation of metal pollution

Metals are common pollutants of water and land when they are emitted by many industrial, agricultural, and domestic sources. In some situations, organisms or ecological processes can be successfully utilized to concentrate metals that are dispersed in the environment, especially in water. The metals can then be removed from the system by harvesting the organisms. For example, metal polluted waste waters can be treated by encouraging the vigorous growth of certain types of algae, fungi, or vascular plants, usually by fertilizing thewater within some sort of constructed lagoon. This bioremediation system works because the growing plants and microorganisms absorb metals from the water (acting as so-called biosorbents), and thereby reduce their concentrations to a more tolerable range. The plants can then be harvested to remove the metals from the bioremediation system. In some cases, the plant biomass may even be processed to yield metal products of economic value.

Bioremediation of acidification

In some situations, artificial wetlands can be engineered to treat acidic waters associated with coal mining or other sources of acidity. Coal mining disturbs soil and fractures rocks and exposes large quantities of pyritic sulfur to atmospheric oxygen. Under such conditions, certain species of bacteria oxidize the sulfide of the mineral pyrites to sulfate generating large quantities of acidity in the process which is known as acid mine drainage. The resulting acidity is often treated by adding large quantities of acid-neutralizing chemicals such as lime or limestone. However, it has also been recently demonstrated that natural, acid-consuming, ecological processes operate in wetlands. These processes can be taken advantage of in constructed wetlands to decrease much of the initial acidity of acid mine drainages and thereby reduce the costs of conventional treatments with acid-neutralizing chemicals. The microbial processes that consume acidity are various, but they include: (1) the chemical reduction of sulfate to sulfide at the oxygen-poor interface between the sediment and the water column and around plant roots, (2) the reduction of ferric iron to ferrous in the same anoxic microhabitats, as well as (3) the primary productivity of phytoplankton, which also consumes some acidity.

A less intensive type of bioremediation can be used to mitigate some of the deleterious ecological effects associated with the acidification of surface waters, such as lakes and ponds. In almost any fresh waters, fertilization with phosphate will greatly increase the primary productivity of algae and vascular plants. In acidic waters, this process can be taken advantage of to reduce the acidity somewhat, but the most important ecological benefit occurs through enhancement of the habitat of certain aquatic animals. Ducks and muskrat, for example, can breed very successfully in fertilized acidic lakes, because their habitat is improved through the vigorous growth of vegetation and of aquatic insects and crustaceans. However, the productive but still acidic habitat remains toxic to fish. In this case, manipulation of the ecosystem by fertilization mitigates some but not all of the negative effects of acidification.

Bioremediation of sewage

Sewage represents a very complex mixture of wastes, usually dominated by fecal materials but also containing toxic chemicals that have been dumped into the disposal system by industries and home owners. Many advanced sewage-treatment technologies utilize microbial processes to both oxidize the organic matter associated with fecal wastes and to decrease the concentrations of soluble compounds or ions of metals, pesticides, and other toxic chemicals. The latter effect, 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 simpler, inorganic constituents. Microbial processes are relied upon in many sewage treatment systems including activated sludges, aerated lagoons, anaerobic digestion, trickling filters, waste stabilization ponds, composting, and disposal on land.