Bioremediation

Bioremediation of spilled hydrocarbons

Bioremediation of metal pollution

Bioremediation of acidification

Bioremediation of sewage

Bioremediation of soils contaminated with toxins

Resources

Bioremediation is any process that cleans an environment degraded by contaminants with the use of microorganisms or other such organisms. The process has been used for many purposes over the centuries. One major use was the desalination of agricultural lands. In modern times, George M. Robinson is credited with being the first person to use bioremediation to solve a major environmental problem. In the 1960s, while a petroleum engineer in California, Robinson experimented with mixtures of microbes in his garage. In 1968, he successfully applied his microbial mixture onto a commercial oil spill—what is considered the first full-scale use of biorediation.

As a type of biotechnology, bioremediation uses living organisms or ecological processes 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 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 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, for instance, 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 certain microorganisms, whose processes transform the toxins 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 introduced, or 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 frequently 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. Thus, 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 land farming, 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 mi (118 km) of oiled beach. It proved 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 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 because 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 wastewaters can be treated by encouraging the vigorous growth of certain types of algae, fungi, or vascular plants, usually by fertilizing the water 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 coalmining or other sources of acidity. Coal mining disturbs soil and fractures rocks exposing 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 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 is a very complex mixture of wastes, usually dominated by fecal materials but also containing

KEY TERMS

Bioaugmentation— Increasing the abundance of microorganisms that are specifically effective at bioremediation.

Biostimulation— Optimizing environmental conditions for the actions of microorganisms important in bioremediation, usually by fertilizing or aerating.

toxic chemicals that have been dumped into the disposal system by industries and homeowners. 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.

Bioremediation of soils contaminated with toxins

Many of the chemicals commonly used in industry or agriculture, or produced as by-products of industrial processes are persistent poisons, such as DDT (dichlorodiphenyltrichloroethane) and other organo-chlorines that are dangerous to humans and wildlife and that do not readily breakdown under natural processes. In most cases, contaminated soils would be burned and then hauled away for storage as hazardous waste. However, new bioremediation techniques that use bacteria to break down the toxins are being tested around the world. These techniques involve many varieties of bacteria, working in a carefully orchestrated sequence. By controlling environmental conditions around the contaminated site—for example, by enclosing the site in a large tentlike construction and controlling heat and moisture inputs as well as oxygen levels—clean-up experts believe they can encourage the proper series of bacterial relationships required to break down the contaminates. The bacteria used in such projects are not genetically altered or specially bred strains. If successful, this approach to the bioremediation of contaminated sites will offer a cheaper, less environmentally damaging alternative to traditional clean-up technologies.

See also Hazardous wastes; Oil spills.

Resources

BOOKS

Atlas, Ronald M. and Jim Philp, eds. Bioremediation: Applied Microbial Solutions for Real-world Environmental Cleanup. Washington, DC: ASM Press, 2005.

Fingerman, Milton, and Rachakonda Nagabhushanam, eds. Bioremediation of Aquatic and Terrestrial Ecosystems. Enfield, NH: Science Publishers, 2005.

Scragg, Alan H. Environmental Biotechnology. Oxford, UK: Oxford University Press, 2005.

Singh, Ajay, and Owen P. Ward, eds. Biodegradation and Bioremediation. Berlin, Germany, and New York: Springer, 2004.

Bill Freedman

Bioremediation

A number of processes for remediating contaminated soils and groundwater based on the use of microorganisms to convert contaminants to less hazardous substances. Most commercial bioremediation processes are intended to convert organic substances to carbon dioxide and water, although processes for addressing metals are under development. Many bacteria ubiquitously found in soils and groundwater are able to biodegrade a range of organic compounds. Compounds found in nature , and ones similar to those, such as petroleum hydrocarbons , are most readily biodegraded by these bacteria. Bioremediation of chlorinated solvents, polychlorinated biphenyl (PCB)s, pesticides, and many munitions compounds, while of great interest, is more difficult and has thus been much slower to reach commercialization.

In most bioremediation processes the bacteria use the contaminant as a food and energy source and thus survive and grow in numbers at the expense of the contaminant. In order to grow new cells, bacteria, like other biological species , require numerous minerals as well as carbon sources. These minerals are typically present in sufficient amounts except for phosphorus and nitrogen , which are commonly added during bioremediation. If contaminant molecules are to be transformed, species called electron acceptors must also be present. By far the most commonly used electron acceptor is oxygen. Other electron acceptors include nitrate, sulfate, carbon dioxide, and iron. Processes that use oxygen are called aerobic biodegradation. Processes that use other electron acceptors are commonly lumped together as anaerobic biodegradation.

The vast majority of commercial bioremediation processes use aerobic biodegradation and thus include some method for providing oxygen. The amount of oxygen that must be provided depends not only on the mass of contaminant present but also on the extent of conversion of the contaminants to carbon dioxide and water, other sources of oxygen, and the extent to which the contaminants are physically removed from the soils or groundwater. Typically, designs are based on adding two to three pounds of oxygen for each pound of biodegradable contaminant.

The processes generally include the addition of nutrient (nitrogen and phosphorus) sources. The amount of nitrogen and phosphorus that must be provided is quite variable and frequently debated. In general, this amount is less than the 100:10:1 ratio of carbon to nitrogen to phosphorus of average cell compositions. It is also important to maintain the soil or groundwater pH near neutral (pH 6–8.5), moisture levels at or above 50% of field capacity , and temperatures between 39°F (4°C) and 95°F (35°C), preferably between 68°F (20°C) and 86°F (30°C).

Bioremediation can be applied in situ and ex situ by several methods. Each of these processes are basically engineering solutions to providing oxygen (or alternate electron acceptors) and possibly, nutrients to the contaminated soils, which already contain the bacteria. The addition of other bacteria is not typically needed or beneficial.

In situ processes have the advantage of causing minimal disruption to the site and can be used to address contamination under existing structures. In situ bioremediation to remediate aquifers contaminated with petroleum hydrocarbons such as gasoline , was pioneered in the 1970s and early 1980s by Richard L. Raymond and coworkers. These systems used groundwater recovery wells to capture contaminated water which was treated at the surface and reinjected after amendment with nutrients and oxygen. The nutrients consisted of ammonium chloride and phosphate salts and sometimes contained magnesium, manganese, and iron salts. Oxygen was introduced by sparging (bubbling) air into the reinjection water. As the injected water swept through the aquifer , oxygen and nutrients were carried to the contaminated soils and groundwater where the indigenous bacteria converted the hydrocarbons to new cell material, carbon dioxide, and water. Variations of this technology include the use of hydrogen peroxide as a source of oxygen and direct injection of air into the aquifer.

Bioremediation of soils located between the ground surface and the water table is most commonly practiced through bioventing . In this method, oxygen is introduced into the contaminated soils by either injecting air or extracting air from wells. The systems used are virtually the same as for vapor extraction. The major difference is in mode of operation and in the fact that nutrients are sometimes added by percolating nutrient amended water through the soil from the ground surface or buried horizontal pipes. Systems designed for bioremediation operate at low air flow rates to replace oxygen consumed during biodegradation and to minimize physical removal of volatile contaminants.

Bioremediation can be applied to excavated soils by landfarming, soil cell techniques, or in soil slurries. The simplest method is landfarming. In this method soils are spread to a depth of 12–18 inches (30–46 cm). Nutrients, usually commercial fertilizers with high nitrogen and low phosphorous content, are added periodically to the soils which are tilled or plowed frequently. In most instances, the treatment area is prepared by grading, laying down an impervious layer (clay or a synthetic liner), and adding a six-inch layer of clean soil or sand. Provisions for treating rainwater runoff are typically required. The frequent tilling and plowing breaks up soil clumps and exposes the soils and thus bacteria to air. This method is more suitable for treating silty and clayey soils than are most of the other methods. It is not generally appropriate for soils contaminated with volatile contaminants such as gasoline because vapors can not be controlled unless the process is conducted within a closed structure.

Excavated soils can also be treated in cells or piles. A synthetic liner is placed on a graded area and covered with sand or gravel to permit collection of runoff water. The sands or gravel are covered with a permeable fabric and nutrient amended soils are added. Slotted PVC pipe is added as the pile is built. The soils are covered with a synthetic liner and the PVC pipes are connected to a blower. Air is slowly extracted from the soils and, if necessary, treated before being discharged to the atmosphere . This method requires less room than landfarming and less maintenance during operations, and can be used to treat volatile contaminants because the vapors can be controlled.

Excavated soils can also be treated in soil/water slurries in either commercial reactors or in impoundments or lagoons. Soils are separated from oversize materials and mixed with water, nutrients are added, and the slurry is aerated to provide oxygen. In some cases additional sources of bacteria and/or surfactants are added. These systems are usually capable of attaining more rapid rates of biodegradation than other systems but have limited throughput.

Selection and design of a particular bioremediation method requires that the site be carefully investigated to define the lateral and horizontal extent of contamination including the total mass of biodegradable substances. Understanding the soil types and distribution and the site hydrogeology is as important as identifying the contaminants and their distribution in both soils and groundwater. Designing bioremediation systems requires the integration of microbiology, chemistry, hydrogeology, and engineering.

Bioremediation is generally viewed favorably by regulatory agencies and is actively supported by the U. S. Environmental Protection Agency (EPA). The mostly favorable publicity and the perception of bioremediation as a natural process has led to greater acceptance by the public compared to other technologies, such as incineration . It is expected that the use of bioremediation to treat soils and groundwater contaminated with petroleum hydrocarbons and other readily biodegradable compounds will continue to grow. Continued improvements in the design and engineering of bioremediation systems will result from the expanding use of bioremediation in a competitive market. It is anticipated that processes for treating the more recalcitrant organic compounds and metals will become commercial through greater understanding of microbiology and specific developments in the isolation of special bacteria and genetic engineering .

[Robert D. Norris ]

RESOURCES

BOOKS

Chapelle, F. H. Ground-Water Microbiology and Geochemistry. New York: Wiley, 1993.

Hinchee, R. E., and R. F. Olfenbuttel, eds. In Situ Bioreclamation: Applications and Investigation for Hydrocarbons and Contaminated Site Remediation. Butterworth-Heinemann, 1991.

Hinchee, R. E., and R. F. Olfenbuttel, eds. On-Site Bioreclamation: Processes for Xenobiotic and Hydrocarbon Treatment. Butterworth-Heinemann, 1991.

Matthews, J. E., ed. In Situ Bioremediation of Groundwater and Geological Materials: A Review of Technologies. Chelsea, MI: Lewis, 1993.

National Research Council. In Situ Bioremediation: When Does It Work? Washington, DC: National Academy Press, 1993.

Bioremediation

Introduction

Bioremediation comes from two words—bio, which is short for biological, and remediation, meaning to remedy. The term refers to the use of living organisms including bacteria, fungi and plants, or enzymes from these organisms (enzymes are proteins that speed up chemical reactions without being changed by the reaction) to deal with an environmental problem.

The microbes can be added to the contaminated soil or water, which is typically removed from the original site of contamination and transported to another, more controlled site, although bioremediation can be done directly on-site. If successful, the contaminated environment is considerably cleaner, and can sometimes be returned to the state it was prior to the contamination.

Bioremediation exploits the fact that certain microorganisms that have the requisite enzyme(s) are able to use the toxic contaminating compound(s) as food. They degrade the compound to a form that can be used to provide energy for their growth and reproduction. The portions of the pollutant that remain are usually no longer toxic.

Examples of bioremediation include the degradation of chlorinated hydrocarbons including pesticides such as lindane and DDT, polychlorinated biphenyls (PCBs), and toxic compounds such as dioxins and furans. These compounds persistent in the environment for a long time and can accumulate in the food chain. Oil spills can be cleaned up, typically by adding nutrients such as nitrogen and sulfate containing fertilizer that encourages the growth of the microbes that will ultimately degrade the oil.

Although bioremediation can be done using microorganisms that are naturally present in the environment, the process can also be done, sometimes more efficiently and quickly, by deliberately adapting the organism to the presence of high concentrations of the pollutant. As well, organisms can be deliberately manipulated genetically to enhance their ability to degrade the target pollutant.

Historical Background and Scientific Foundations

Since bioremediation is a natural process, it was recognized centuries ago. For example, the first known use of microorganisms to treat wastewater dates back to Roman sewer systems built in 600 BC.

The modern era of bioremediation dates back to March 24, 1989. Then, the oil tanker Exxon Valdez hit a reef off of Prince William Sound in Alaska; the resulting spill of at least 11 million gallons of crude oil fouled an estimated 75 mi (120 km) of coastline in a region rich in sea life. The application of microorganisms and nutrients to oil-contaminated shoreline was successful in helping to remove much of the pollution. Following this success, the use of bioremediation was embraced by the U.S. Environmental Protection Agency (EPA), which had until then been hesitant about the strategy in natural settings.

At about the same time, scientists were gaining expertise in the genetic tools to deliberately insert and delete informational regions (genes) from the genetic material (genome) of bacteria, viruses, and other microorganisms. As well, genes contained in smaller circles of genetic material called plasmids that were capable of making new copies independent of the genome could be added to bacteria. The use of plasmids is particularly advantageous since multiple plasmids can be present in an individual bacterium, enabling more of the gene product to be made.

The ability to genetically modify microorganisms allowed microbes to be adapted more rapidly to a contaminated environment than could naturally occur. One example is the bacterium Deinococcus radiodurans, which is naturally resistant to radioactivity. Genetic engineering has made the organism capable of using the compounds toluene and mercury as nutrients. Since these are pollutants that are typically present in radioactive nuclear waste, D. radiodurans can be used in the bioremediation of radioactive spills.

For natural or engineered bacteria to be used for bioremediation, conditions that are favorable to the survival and, ideally, growth of the bacteria need to be established. This is because living and growing bacteria require nutrients; when the target toxic contaminant is used as a nutrient, biomediation can be successful.

Bioremediation is typically done using one of two strategies. The first approach, which is designed to increase the numbers of the target microbe or groups of microbes that can degrade the pollutant, is termed bioaugmentation. Bioaugmentation involves the deliberate addition of certain microbes that are adept at degrading the particular toxic chemicals, but which are normally not present or present in low and ineffective numbers.

Bioaugmentation was done in the aftermath of the Exxon Valdez spill using bacteria naturally present in seawater. The water was sprayed onto contaminated shoreline along with a mixture of nutrients. More precise bioaugmentation can be done using genetically engineered bacteria; this approach is more common when the contaminated soil or water is physically contained, either by use of an impermeable barrier or by the removal of the contaminated material to another site.

Instead of bioaugmentation, the contaminated site can be changed to enhance the metabolic activity of naturally occurring microbes that are already present in the contaminated soil or water. This process is known as biostimulation. Typical strategies are fertilization (addition of nutrients to the site), increasing the amount of oxygen present in the soil or water (aeration), or the addition of water (irrigation). Aeration encourages the growth and activity of aerobic bacteria, which require or can at least tolerate the presence of oxygen. Anaerobic bacteria, which do not tolerate the presence of oxygen, tend to have a metabolism that is much slower than aerobic bacteria. Although anaerobic bacteria may be capable of bioremediation, the timescale will be too long to be practical or safe in terms of restoring the site. By encouraging the activity of aerobic bacteria, bioremediation can be accomplished faster. Irrigation also increases bacterial activity, since virtually all bacteria require the presence of some moisture.

Bioremediation using microbes is not a solution for every contamination problem, however. For example, contamination involving some heavy metals cannot be easily handled using bacteria, since the metal cannot be easily taken up by the microbes or used as a nutrient.

WORDS TO KNOW

ANABOLISM: The process by which energy is used to build up complex molecules.

BIOAUGMENTATION: The introduction of specific strains of microorganisms to break down pollutants in contaminated soil or water.

CATABOLISM: The process by which large molecules are broken down into smaller ones with the release of energy.

GROUNDWATER: Fresh water that is present in an underground location.

One solution can be to use other living things that can bind the pollutant. The best example is phytoreme-diation—the use of plants. Plant species that have been bred to tolerate the presence of high concentrations of the particular contaminant can be planted on the contaminated site. As the plants grow, the pollutant compounds can accumulate in the stem or leaves. The plants can then be harvested along with the accumulated pollutant, which can be destroyed at very high temperature (incineration) or reclaimed. Successive rounds of planting and harvesting will, over time, greatly reduce the level of the contaminants in the soil.

Impacts and Issues

Bioremediation has and continues to be very successful in dealing with soil and water contamination. If successful, bioremediation of contaminated sites can be less expensive and less damaging to the environment than traditional clean-up technologies.

Although bioaugmentation has been used in the clean-up of contaminated sites since the late 1980s, the use of deliberately engineered bacteria is still not the first choice when the bioremediation is accomplished at the site of the contamination, due to concerns of the potential spread of the engineered organism in the environment. This risk can be engineered to be very low, since the engineered microbes can be genetically crippled to require the presence of a key nutrient that is not normally present in the environment, and so which must be supplied. Without the available nutrient, the engineered microbe dies.

These concerns have been beneficial, since they heightened scrutiny of bioremediation technology and resulted in strict controls over the process.

The use of genetically engineered bacteria is more common when the polluted soil has been removed and taken to another location for treatment in an enclosed container. The on-site use of genetically engineered

IN CONTEXT: BACTERIA AND SOIL REMEDIATION

Soil-modifying bacteria are employed in bioremediation. This process relies on the use of naturally occurring soil bacteria to convert a hazardous compound into a less hazardous or completely nontoxic compound. Some naturally occurring soil bacteria have the ability to break down a wide variety of compounds, including toxic compounds. However, contamination of soil with human-made chemicals, such as gasoline and polychlorinated biphenyls (PCBs, chemicals that used to be used as coolants), can often be most effectively handled by bacteria that have been specifically genetically engineered.

plants is more common and more easily accepted publicly. An example in a different area is the planting of genetically engineered crop plants. Despite ongoing unease by some, as of 2007 genetically engineered soybean, cotton, and corn plants were grown on over 100 million acres in the United States, making these varieties overwhelmingly more dominant than their natural counterparts.

As of 2008, bioremediation has been used to help clean up tens of thousands of sites involving contaminated soil and water around the globe. Included among these are Superfund sites—heavily contaminated sites in the United States that have long since been abandoned, and for which the EPA provides funds for clean up and restoration. Since its inception in 1980, almost 1,300 sites have been designated for remediation under this program.

Research is underway to identify microorganisms that have the genetic capability to be exploited in bioremediation. Much of this research examines microbes recovered from contaminated soil and water, the rationale being that they must have already adapted to the presence of the pollutant(s). The “high throughput” technology that is used can screen thousands of samples daily for the evidence of microbes with superior toxic degradation power.

See Also Chemical Spills; Oil Spills; Spill Remediation

BIBLIOGRAPHY

Books

Atlas, Ronald M., and Jim Philip. Bioremediation: Applied Microbial Solutions for Real-World Environment Cleanup. Washington: ASM Press, 2005.

Pisano, Gary P. Science Business: The Promise, The Reality, and the Future of Biotech. Boston: Harvard Business School Press, 2006.

Walker, Sharon. Biotechnology Demystified. New York: McGraw-Hill Professional, 2006.

Web Sites

Government of Canada. “Biobasics: Bioremediation.” February 9, 2006. http://biobasics.gc.ca/english/View.asp?x=741 (accessed February 7, 2008).

U.S. Geological Survey. “Bioremediation: Nature’s Way to a Cleaner Environment.” June 28, 2006. http://water.usgs.gov/wid/html/bioremed.html (accessed February 7, 2008).

Brian D. Hoyle

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 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 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 certain microorganisms, whose processes transform the toxins 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 is 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. Thus, 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 land farming, 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, oilseeking), 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 mi (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 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 because 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 the water within some sort of constructed lagoon. This bioremediation system works because the growing plants and microorganisms absorb metals from the water (acting as socalled 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 exposing 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 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 is 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.

Bioremediation of soils contaminated with toxins

Many of the chemicals commonly used in industry or agriculture, or produced as by-products of industrial processes are persistent poisons, such as DDT and other organochlorines that are dangerous to humans and wildlife and that do not readily breakdown under natural processes. In most cases, contaminated soils would be burned and then hauled away for storage as hazardous waste. However, new bioremediation techniques that use bacteria to break down the toxins are being tested around the world. These techniques involve many varieties of bacteria, working in a carefully orchestrated sequence. By controlling environmental conditions around the contaminated site—for example, by enclosing the site in a large tent-like construction and controlling heat and moisture inputs as well as oxygen levels—clean-up experts believe they can encourage the proper series of bacterial relationships required to break down the contaminates. The bacteria used in such projects are not genetically altered or specially bred strains. If successful, this approach to the bioremediation of contaminated sites will offer a cheaper, less environmentally damaging alternative to traditional clean-up technologies.

See also Hazardous wastes; Oil spills.

Resources

books

Freedman, B. Environmental Ecology. 2nd ed. San Diego: Academic Press, 1994.

periodicals

Frederick, R.J., and M. Egan. "Environmentally Compatible Applications of Biotechnology." BioScience, 44 (1994): 529-535.

Bill Freedman

KEY TERMS

Bioaugmentation

—Increasing the abundance of microorganisms that are specifically effective at bioremediation.

Biostimulation

—Optimizing environmental conditions for the actions of microorganisms important in bioremediation, usually by fertilizing or aerating.

Bioremediation

Bioremediation means to use a biological remedy to abate or clean up contamination. This makes it different from remedies where contaminated soil or water is removed for chemical treatment or decontamination, incineration, or burial in a landfill. Microbes are often used to remedy environmental problems found in soil, water, and sediments. Plants have also been used to assist bioremediation processes. This is called phytoremediation. Biological processes have been used for some inorganic materials, like metals, to lower radioactivity and to remediate organic contaminants. With metal contamination the usual challenge is to accumulate the metal into harvestable plant parts, which must then be disposed of in a hazardous waste landfill before or after incineration to reduce the plant to ash. Two exceptions are mercury and selenium, which can be released as volatile elements directly from plants to atmosphere. The concept and practice of using plants and microorganisms to remediate contaminated soil have developed over the past thirty years.

The idea of bioremediation has become popular with the onset of the twenty-first century. In principle, genetically engineered plants and microorganisms

can greatly enhance the potential range of bioremediation. For example, bacterial enzymes engineered into plants can speed up the breakdown of TNT and other explosives. With transgenic poplar trees carrying a bacterial gene, methyl mercury may be converted to elemental mercury, which is released to the atmosphere at extreme dilution. However, concern about release of such organisms into the environment has limited actual field applications.

Natural Bioremediation

Natural bioremediation has been occurring for millions of years. Biodegradation of dead vegetation and dead animals is a kind of bioremediation. It is a natural part of the carbon, nitrogen, and sulfur cycles. Chemical energy present in waste materials is used by microorganisms to grow while they convert organic carbon and hydrogen to carbon dioxide and water.

Managed Bioremediation

When bioremediation is applied by people, microbial biodegradation processes are said to be managed. However, bioremediation takes place naturally and often it occurs prior to efforts to manage the process. One of the first examples of managed bioremediation was land farming (refers to the managed biodegradation of organic compounds that are distributed onto the soil surface, fertilized, and then tilled). Many petroleum companies have used it. High-molecular-weight organic compounds (i.e., oil sludges and wastes) are spread onto soil and then tilled into the ground with fertilizer, as part of the managed bioremediation process. Good conditions for microbial biodegradation are maintained by controlling soil moisture and soil nutrients. In 1974 R.L. Raymond was awarded a patent for the bioremediation of gasoline. This was one of the first patents granted for a bioremediation process.

Since about 1980, prepared bed systems have been used for bioremediation. In this approach, contaminated soil is excavated and deposited with appropriate fertilizers into a shallow layer over an impermeable base. Conditions are managed to obtain biodegradation of the contaminants of concern.

Composting

Composting has been used as a bioremediation process for many different organic compounds. It is widely employed to recycle nutrients in garden and yard waste. A finished compost can be used as a soil conditioner. Extending composting technology to new bioremediation applications requires experiments. The biodegradation process must be effective within the context of existing environmental conditions, and odors and gases that are generated by the process have to be strictly controlled.

In Situ Bioremediation

In situ processes (degrading the contaminants in place) are often recommended because less material has to be moved. These processes can be designed with or without plants. Plants have been used because they take up large quantities of water. This helps to control contaminated water, such as a groundwater contaminant plume , in the soil. Aerobic (oxygen-using) processes may occur in the unsaturated layer of soil, the vadose zone, which is found above the water table. The vadose zone is defined as the layer of soil having continuously connected passages filled with air, while the saturated zone is the deeper part where the pores are filled with water. Oxygen moves in the unsaturated zone by diffusion through pores in the soil. Some plants also provide pathways to move oxygen into the soil. This can be very important to increase the aerobic degradation of organic compounds.

Fate of Various Organic Contaminants

Petroleum-contaminated soil has been remediated in situ with plants added to enhance the degradation processes. The biodegradation of phenol, oil, gasoline, jet fuel, and other petroleum hydrocarbons occurs in soil. When plants are present, soil erosion is reduced and more microbes are present in the plant root zone. Methyl tertiary butyl ether (MTBE), used in gasoline to enhance the octane rating of the fuel, is difficult to remediate because it is very soluble in water and is hard to break down using microbes normally present in soil. In vegetation-based bioremediation, MTBE is moved from the soil to the atmosphere along with the water that plants take up from soil and release to the air. The MTBE breaks down rapidly in the atmosphere. Benzotriazoles, used as corrosion inhibitors in antifreeze and aircraft deicer fluids, are treated by plant-based bioremediation. The benzotriazole adsorbs or sticks to the plant roots and ends up as part of the plant biomass. Trichloroethylene (TCE) is a common chlorinated solvent that is biotransformed in the soil. It can be taken up by plants along with water. Then the TCE diffuses into the atmosphere where it is destroyed by atmospheric processes.

Bioventing

Bioremediation requires good nutrient and environmental conditions for biodegradation. When oxygen is needed for oxidation of the organic contaminants, bioventing (pumping air into the soil) is often used. Sometimes, fertilizers are added to the soil. In certain places irrigation is necessary so that plants or microbes can grow.

see also Abatement; Biodegradation; Brownfield; Cleanup.

Bibliography

Alexander, Martin. (1994). Biodegradation and Bioremediation. New York: Academic Press.

Davis, Lawrence C.; Castro-Diaz, Sigifredo; Zhange, Qizhi; and Erickson, Larry E. (2002). "Benefits of Vegetation for Soils with Organic Contaminants." Critical Reviews in Plant Sciences. 21 (5):457–491.

Eweis, Juana B.; Ergas, Sarina J.; Chang, Daniel P.Y.; and Schroeder, Edward D. (1998). Bioremediation Principles. New York: McGraw-Hill.

Hannink, Nerissa K.; Rosser, Susan J.; and Bruce, Neil C. (2002). "Phytoremediation of Explosives." Critical Reviews in Plant Sciences. 21(5):511–538.

McCutcheon, Steven C.; Schnoor, Jerald L., eds. (2003). Phytoremediation: Managing Contamination by Organic Compounds. New York: Wiley-Interscience.

Pilon-Smits, Elizabeth, and Pilon, Marinus. (2002). "Phytoremediation of Metals Using Transgenic Plants." Critical Reviews in Plant Sciences. 21(5):439–456.

Rittmann, Bruce E. (1993). In Situ Bioremediation: When Does It Work? Washington, DC: National Academy Press.

Thomas, J.M.; Ward, C.H.; Raymond, R.L.; Wilson, J.T.; and Loehr, R.C. (1992). "Bioremediation." In Encyclopedia of Microbiology, Vol. 1, edited by Joshua Lederberg, pp. 369–385. New York: Academic Press.

Internet Resources

Bioremediation Discussion Group. Available from http://www.bioremediationgroup.org.

Natural and Accelerated Bioremediation Research Web site. Available from http://www.lbl.gov/NABIR.

Larry Eugene Erickson and Lawrence C. Davis