Overview

Soil that is accidentally contaminated by releases of hazardous chemicals may be classified as hazardous waste under state or federal environmental statutes. When the amounts of contaminated soil are sufficiently large, the most often used methods of disposal, incineration or land disposal, can become prohibitively expensive. In addition, soil pollutants act as a secondary source for groundwater contamination. The U.S. Environmental Protection Agency has recently implemented a strategy to address the complex problems associated with soil pollution through the application of innovative technologies [1], One of the most frequently studied alternatives of late for the treatment of several classes of organics, notably petroleum hydrocarbons, is bioremediation.

Long maligned as a technology that simply uses pollutants to attack other pollutants, the method was revived by Lee Thomas, former EPA administrator, and its use has spread to 130 contaminated sites throughout the country. Out of these 130 sites, 85 are targeted to treat soils, 55 groundwater, and to a much lesser extent sediments, sludge, and surface water (Figure 1). Of the two classical methods for bioremediation, ex situ and in situ, fully 75% are using the ex situ method via some form of bioreactor, landfarming, static pile, or aerated lagoon. The vast majority of sites undergoing bioremediation use aerobic as opposed to anaerobic methods [1].

Although considered a new "innovative" technology by the EPA, bioremediation is anything but new. The mechanisms from which bioremediation research and application were derived are deeply rooted in biological wastewater treatment. Groundwater is routinely extracted and treated in on-site bioreactors that behave much like activated sludge systems, with three unique differences: groundwater influent is more dilute than wastewater, groundwater influent can contain a single or select few substrate types, and if bioremediation is properly performed, substrate levels continuously decline. A similar analogy can be drawn between soil treatment and fixed-film wastewater reactors. Kinetic equations for activated sludge and trickling filter reactors have been applied since at least the 1930s. R. L. Raymond was the first researcher to

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SOIL GROUND SEDIMENTS SLUDGE SURFACE WATER WATER

Figure 1 Types of media treated by bioremediator. (After U.S. EPA [1].)

SOIL GROUND SEDIMENTS SLUDGE SURFACE WATER WATER

Figure 1 Types of media treated by bioremediator. (After U.S. EPA [1].)

patent the application of subsurface microorganisms to degrade a variety of groundwater contaminants as early as the 1970s [2-4].

The process of degrading hazardous organics in shallow soils to be discussed here usually involves stimulating the indigenous subsurface microflora to enhance the decomposition rate. Although engineered microorganisms with specialized metabolic capabilities have been developed and applied, indigenous organism stimulation is being used at three out of every four sites [1]. Irrespective of the microorganism selection, the goal of biodégradation is to convert organic wastes into biomass and harmless by-products of microbial metabolism such as H20, C02, CH4, and inorganic salts. To investigate the abilities of this remedial alternative, one must understand the waste characteristics and microbial ecology unique to each site, the limitations of the method, and the mechanisms by which its effectiveness can be optimized.

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