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.