CURRENT PRACTICE OF BIOREMEDIATON
The key players in bioremediation are bacteria-microscopic organisms that live virtually everywhere. Microorganisms are ideally suited to the task of contaminant destruction because they possess enzymes that allow them to use environmental contaminants as food and because they are so small that they are able to contact contaminants easily. In situ bioremediation can be regarded as an extension of the purpose that microorganisms have served in nature for billions of years: the breakdown of complex human, animal, and plant wastes so that life can continue from one generation to the next. Without the activity of microorganisms, the earth would literally be buried in wastes, and the nutrients necessary for the continuation of life would be locked up in detritus.

The goal in bioremediation is to stimulate microorganisms with nutrients and other chemicals that will enable them to destroy the contaminants. The bioremediation systems in operation today reply on microorganisms native to the contaminated sites, encouraging them to work by supplying them with the optimum levels of nutrients and other chemicals essential for their metabolism. Researchers are currently investigating ways to augment contained sites with nonnative microbes including genetically engineered microorganisms specially suited to degrading the contaminants of concern at particular sites. It is possible that this process, know as bioaugmentation, could expand the range of possibilities for future bioremediation systems.

Regardless of whether the microbes are native or newly introduced to the site, an understanding of how they destroy contaminants is critical to understanding bioremediation. The types of microbial processes that will be employed in the cleanup dictate what nutritional supplements the bioremediation system must supply. Furthermore, the byproducts of microbial processes can provide indicators that the bioremediation is successful.

Whether microorganisms will be successful in destroying manmade contaminants in the subsurface depends on three factors: the type of organisms, the type of contaminant, and the geological and chemical conditions at the contaminated site.

Biological and nonbiological measures to remedy environmental pollution are used the same way. All remediation techniques seek first to prevent contaminants from spreading. In the subsurface, contaminants spread primarily as a result of partitioning into ground water. As the ground water advances, soluble components from a concentrated contaminant pool dissolve, moving forward with the ground water to form a contaminant plume. Because the plume is mobile, it could be a financial, health, or legal liability if allowed to migrate off site. The concentrated source of contamination, on the other hand, often has settled into a fixed position and in this regard is stable. However, until the source can be removed (by whatever cleanup technology), the plume will always threaten to advance off site.

Depending on the nature of the site, the types of contaminants, and the needs of the parties responsible for the contaminated site, the treatment technologies administered may vary. The source area and the ground water plume may be treated by aumented bioremediation, intrinsic bioremediation, a combination of the two, or a mixture of bioremediation with nonbiological treatment strategies. Contaminant concentrations in ground water plumes are typically much lower than in the source area. Because of this concentration difference, management procedures for the source area and the plume may be quite different. When the source area is highly contaminated, aggressive containment and treatment are often required to bring the site under control.

Selection and application of a bioremediation process for the source or the plume require the consideration of several factors. The first factor is the goals for managing the site, which may vary from simple containment to meeting specific regulatory standards for contaminant concentrations in the ground water and soil. The second factor is the extent of contamination. Understanding the types of contaminants, their concentrations, and their locations is critical in designing in situ bioremediation procedures. The third factor is the types of biological processes that are effective for transforming the contaminant. By matching established metabolic capabilities with the contaminants found, a strategy for encouraging growth of the proper organisms can be developed. The final consideration is the site's transport dynamics, which control contaminant spreading and influence the selection of appropriate methods for stimulating microbial growth.

Once site characteristics have been discerned, strategies for gaining hydrologic control and for supplying the requisite nutrients and electron acceptors for the microorganisms can be developed. if there is sufficient natural supply of these substances, intrinsic bioremediation may be effective. On the other hand, if these biochemical or environmental requirements must be artificially supplied to maintain a desire level of activity, bioremediation is the desired course. The ultimate consideration is if and when the targeted cleanup goal can be achieved.

Augmented bioremediation may be chosen over intrinsic bioremediation because of time and liability. Because augmented bioremediation accelerates biodegradation reaction rates, this technology is appropriate for situations where time constraints for contaminant elimination are short or where transport processes are causing the contaminant plume to advance rapidly.

When subsurface contamination exists substantially or entirely above the water table (in what is known as the unsaturated, or vadose, zone), the treatment system relies on transport of materials through the gas phase. Thus, bioremediation is effected primarily through the use of an aeration system, oxygen being the electron acceptor of choice for the systems used so far to treat contamination. If the contamination is shallow, simple tilling of the soil may accelerate oxygen delivery sufficiently to promote bioremediation. For deeper contamination, aeration is most commonly provided by applying a vacuum, but it may also be supplied by injecting air. In either case the three primary control parameters are, in order of importance, oxygen supply, moisture maintenance, and the supply of nutrients and other reactants.

The design and implementation of an effective vacuum or injection system for oxygen delivery require knowledge of the vertical and horizontal location of the contaminants and the geological characteristics of the contaminated zones. Because air flow is proportional to the permeability characteristics of each geological stratum, aeration points must be separately installed at depths that correspond to every contaminated geological unit. For effective oxygen delivery, the spacing of the aeration points within a geological unit is a function of the soil permeability and the applied vacuum (or pressure). Determination of spacing should be based on field data and/or computer models. In some clay-rich soils the circulation of sufficient oxygen to promote bioremediation is extremely difficult because such soils are relatively impermeable. In these soils hydraulic fracturing or another engineered approach may be required to facilitate air flow.

The passage or air through the subsurface will remove moisture. This can cause drying that, if severe enough, may impede biological processes. Therefore, maintaining a proper moisture balance is critical to the system's success. Moisture is sometimes added to the treatment area by spraying or flooding the surface (if the surface is relatively permeable) or by injecting water through infiltration galleries, trenches, or wells. Care must be taken that excess water is not added, because it can leach contaminants into the ground water or decrease the amount of air in the subsurface pores.

If inorganic nutrients or other stimulants are required to maintain the effectiveness of the bioremediation system, they may be added in soluble form through the system used for moisture maintenance. In some cases, nutrients and stimulants could be added as gases. At some sites, nitrogen has been added in the form of gaseous ammonia.

Bioremediation systems for treating ground water below the water table fit two categories: water circulation systems and air injections systems. Most aquifer bioremediation systems have used the former approach, but in the last few years air injection systems have become increasingly common.

Water circulation systems work by circulation water amended with nutrients and other substances required to stimulate microbial growth between injection and recovery wells. The method has typically incorporated an optional above-ground water treatment facility into the ground water circulation system, with oxygen supplied by hydrogen peroxide (H2O2) and the recovered water treated with an air stripper to remove any remaining volatile contaminants.

All of the ground water is recovered, and all or a portion of the treated ground water is reinjected after being amended with nutrients and a final electron acceptor. Recovery systems most frequently use wells, although trenches can be used in some situations. Injection is commonly achieved with wells, but several systems have used injection galleries. In some systems all of the recovered water is discharged to an alternate reservoir, and either drinking water or uncontaminated ground water is used for injection. The injected ground water moves through the saturated sediments toward the ground water capture system. As the amended water moves through the contaminated portions of the site, it increased microbial activity by providing the elements that limit intrinsic biodegradation.

Microbial transformation of organic contaminants normally occurs because the organisms can use the contaminants for their own growth and reproduction. Organic contaminants serve two purposes for the organisms: they provide a source of carbon, which is one of the basic building blocks of new cell constituents, and they provide electrons, which the organisms can extract to obtain energy.

Microorganisms gain energy by catalyzing energy-producing chemical reactions that involve breaking chemical bonds and transferring electrons away from the contaminant. The type of chemical reaction is call an oxidation-reduction reaction: the organic contaminant is oxidized, the technical term for losing electrons; correspondingly, the chemical that gains the electrons is reduced. The contaminant is called the electron donor, while the electron recipient is called the electron acceptor. The energy gained from these electron transfers is then "invested," along with some electrons and carbon from the contaminant, to product more cells.

The process of destroying organic compounds with the aid of O2 is called aerobic respiration. In aerobic respiration, microbes use O2 to oxidize part of the carbon in the contaminant to carbon dioxide (CO2), with the rest of the carbon used to produce new cell mass. In the process the O2 gets reduced, producing water. Thus the major byproducts of aerobic respiration are carbon dioxide, water, and an increased population of microorganisms.