Design Considerations

The design variables of bioremediation include the amount of bacteria, oxygen, and nutrients needed for the biodegradability of the contaminant as well as the characteristics of the subsurface environment. Given those variables, environmental engineers can determine an appropriate hydraulic design of the bioremediation system. Computer models such as BIOPLUME II (1986) can assist in the design of bioremediation systems.

The number of bacteria must be sufficient to consume all of the organic contaminants in a timely manner. Most sites have significant populations of indigenous microorganisms that can degrade a variety of organic contaminants. One gram of surface soil can contain from 0.1 to 1 billion cells of bacteria, 10 to 100 million cells of actin-omycetes, and 0.1 to 1 million cells of fungi (Dockins 1980; Whitelaw and Edwards 1980). The microorganism population in soils is generally greatest in the surface horizons where the temperature, moisture, and energy supply is favorable for their growth. As the depth increases, the number of aerobic microorganisms decreases; however, anaerobic microorganisms can exist depending on the availability of nutrients and organic material. The type of microorganisms present on site and their optimal living conditions can be determined in the laboratory. If indigenous microorganisms are not present on site or if their number is not sufficient to consume all organic contaminants, appropriate exogenous microorganisms can be imported, or existing microorganisms can be stimulated with the addition of oxygen and nutrients.

In addition, aerobic bacteria require oxygen for their growth. Because the concentrations of dissolved oxygen in groundwater are generally low, adding oxygen supports the aerobic biodegradation of organic compounds in groundwater. The theoretical quantities of oxygen required to degrade an organic compound can be determined from stoichiometric analysis. For example, degradation of a simple organic acid, such as acetic acid, theoretically requires

In-line i Oxygen Source vMv

In-situ C Aeration

. //AN AMU //AN //AN //AN //AN //AN //AN //AN //AN //AN //AN //A

Soil Flushing

. //AN AMU //AN //AN //AN //AN //AN //AN //AN //AN //AN //AN //A

Soil Flushing

Aeration Well Bank

Injection Well

Aeration Zone Direction of Groundwater Flow / N

Extraction Well

FIG. 9.18.1 Simplified representation of a groundwater bioremediation system. (Reprinted from U.S. Environmental Protection Agency, 1985, Handbook, remedial action at waste disposal sites, EPA/625/6-85/006, Washington, D.C.: U.S. EPA.)

Aeration Well Bank

Injection Well

Groundwater Table Leachate Plume

Aeration Zone Direction of Groundwater Flow / N

Extraction Well

FIG. 9.18.1 Simplified representation of a groundwater bioremediation system. (Reprinted from U.S. Environmental Protection Agency, 1985, Handbook, remedial action at waste disposal sites, EPA/625/6-85/006, Washington, D.C.: U.S. EPA.)

Nutrients

1.1 mg of oxygen. Oxygen can be added in several ways, including aeration, oxygenation, and the use of hydrogen peroxide and other oxygen-containing compounds. Obviously, the use of these compounds requires careful control of the geochemistry and hydrology of the site.

Inorganic nutrients including nitrogen, phosphorous, and potassium are needed for proper bacterial growth and can limit cell growth if they are not present at sufficient levels. The groundwater may already contain levels of phosphorous and nitrogen, but these levels are probably insufficient for bacterial growth (Bouwer 1978; Doetsch and Cook 1973). The addition of nutrients, however, can contaminate the aquifer. Therefore, only the amount needed to sustain biological activity should be added.

Other factors limit the growth rate of bacteria and, therefore, the biodegradation of organic contaminants in groundwater. These factors include the pH, temperature, and toxicity of the contaminant. The appropriate range for these parameters should be determined in a treatabil-ity study.

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