The process of removing, or at least significantly degrading, the presence of contaminants in soils and groundwater using biology at the microbial level alone is known as bioremediation (Anderson et al., 1993). Bacteria and fungi play the dominant role in these systems. They metabolically degrade a variety of industrial, and in some instances even hazardous, wastes into less toxic, or perhaps nontoxic, products. There are two basic options to the circumstances under which a bioremediating activity might develop: the intrinsic and the engineered options. The underlying differences between them are related to the source of the responsible microbial agents and whether their presence stems from
naturally occurring organisms or a manipulated, and possibly even externally seeded, colonization.
At those sites where intrinsic bioremediation has been found to occur, the environmental conditions and contaminant characteristics of the site (i.e., types of contaminants, relative concentrations, etc.) are such that microbes already present in the soil and/or groundwater will naturally begin the remediating process of chemical degradation. In fact, it is likely that many contaminated sites could experience some degree of intrinsic bioremediation, given the biochemical diversity and metabolic flexibility of the microbial world. However, the unfortunate reality of intrinsic bioremediation is that it often fails to develop at a rate or degree that is sufficiently robust to fully remediate a given site within a time frame that is acceptably short. There are many reasons for these shortcomings. The chemistry of the soils might not have been conducive to microbial growth, as would be the case with pH extremes or nutrient limitations. There may also be physical limitations or constraints with these soils, created perhaps by extreme (whether high or low) moisture contents or unduly low soil permeabilities that hinder the movement of the bacteria or the necessary substrates and nutrients. Yet another set of problems could develop due to the contaminants themselves, which might either be recalcitrant in form or present at levels sufficiently high to inhibit, or possibly even kill, the existing microbes.
The strategy of invoking engineering methods was developed to enhance the attainable rate and efficacy of bioremediation. A wide variety of engineered bioremediation procedures have now been developed, including plans that attempt to optimize the environmental conditions under which these microbial reactions might occur and also to instigate and promote the growth of suitable microorganisms. These engineered bioremediation systems can largely be subdivided into two primary options (see Figure 16.65) In situ treatment means, literally, "in place,'' as opposed to ex situ treatment, in which soil is excavated for treatment aboveground.
The synopsis given in le 16.17 provides an overview of the basic contaminant categories with which bioremediation has already proven to be a potentially suitable procedure, as well as information regarding the relative characteristics of the various chemical forms and the appropriate mode (aerobic or anaerobic) of their biochemical transformation. Several factors play an important role in this regard, including that of the
TABLE 16.17 Contaminant Forms Relative to Potential Bioremediation Treatment
Biodegradability In Situ
Contaminant Compounds (/ KH) (/ KOW) Aerobic Anaerobic Potential
Domestic fuel (C9-C24)
Aromatic hydrocarbons (BTEX)
Polycyclic aromati chydrocarbons
Light (2-3 rings)
Heavy (4-5 rings)
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