Environmental Engineering Reference
In-Depth Information
solubility in soil water, (3) nutrient availability in soil water, and (4) heavy metal
solubilities. Most natural environments (i.e., soils) have pH values between 5.0
and 9.0. Consequently, this range is optimal for microbial-enhanced biodegra-
dation of waste contamination. This pH range is maintained by a natural buff-
ering capacity that exists in most fertile native soils due to the presence of
carbonates and other minerals. However, this buffering capacity can be
depleted over time as a result of acidic by-products of degradation. Although
microbes can adapt to a broader range of pH values, there typically is an
accompanying decrease in growth/metabolic rates. Likewise, there is a reduc-
tion in the variety of microbial strains, with the microbes themselves becoming
more specialized for living under certain environmental conditions.
3.2.5 Toxic compounds
Just as contaminant concentrations that are too low can complicate bioreme-
diation, high aqueous phase concentrations of some contaminants can create
problems. At high concentrations, some chemicals are toxic to microbes, even
if the same chemical is readily degraded at lower concentrations. Toxicity
prevents or slows metabolic reactions and often prevents the growth of new
biomass needed to stimulate rapid contaminant removal. The degree and
mechanisms of toxicity vary with specific toxicants, their concentration, and
the exposed microorganisms. Microbial cells cease to function when at least
one of the essential steps in their numerous physiological processes is
blocked. The blockage may result from gross physical disruption of the cell
structure or competitive binding of a single enzyme essential for metaboliz-
ing the toxicant (National Research Council, 1993).
By design, some organic compounds are toxic to targeted life forms such
as insects and plants and may also be toxic to microbes. These compounds
include herbicides, pesticides, rodenticides, fungicides, and insecticides. In
addition, some classes of inorganic compounds such as cyanides and azides
are toxic to many microbes; however, these compounds may be degraded
following a period of microbial adaption.
3.2.6 Effects of soil type
The soil type affects the rate of mass transport of nutrients, contaminants,
water, air (i.e., oxygen), and pH adjusters. This effect on mass transport in
return affects the operation of the degradation process and the potential for
migration of the wastes and amendments. In addition, highly organic soils
can be sorptive and act as a barrier to organic migration.
3.2.7 Nontechnical issues
As already established, the technical parameters for biotreatment can be
complex and numerous; however, nontechnical issues are also of vital impor-
tance for an effective design. Some considerations may include the following:
