Environmental Engineering Reference
In-Depth Information
biological growth. The availability or toxicity of these metals to the microbes
is usually dependent on pH, with the metals becoming more mobile/avail-
able at lower values of pH.
Metals can be actively accumulated by certain microorganisms and plant
species. Living cells can adsorb metals and concentrate inorganics within
the cell. Although heavy metals may not be metabolically essential, they are
taken up by the biomass as a side effect of the normal metabolic activity of
the cell. Activated biomass removes metals from solution by a variety of
mechanisms, which include ion exchange at the cell walls, complexation
reactions at the cell walls, and intra- and extracellular complexation reactions
(Benemann, 1991). Inactivated biomass removes metals primarily by adsorb-
ing metals to the ionic groups either on the cell surface or in the polysac-
charide coating found on most forms of bacteria. The metals are bound by
exchange of functional groups or by sorption on polymers.
3.2.2 Temperature
Most microbes prefer to grow at temperatures in a range of about 10 to 38˚C.
The rate of biochemical reactions in cells increases with temperature up to
a maximum, above which the rate of activity declines as enzyme denatur-
ation occurs and organisms either die or become less active. Low tempera-
tures seldom kill the microbes, and with warming, the microbes typically
recover. Temperature also affects gas solubilities and must be taken into
account when designing a remediation system. However, it is extremely
difficult to control the temperature of
in situ
processes, and the temperature
of
ex situ
processes can only be moderated, sometimes with great expense.
Consequently, the effects of the expected temperatures should be factored
into the design-basis expected degradation rate.
3.2.3 Water
Microorganisms do not grow without adequate water, which is the universal
solvent for their cellular biochemicals, growth substrates, oxygen, and nutri-
ents. However, too much water may saturate the soil and result in anaerobic
activity, which may or may not be beneficial, depending on whether aerobic
or anaerobic degradation processes are preferred. If aerobic conditions are
desired, a 30 to 35% saturation of the pore spaces is usually sufficient to still
enable the passage of air through the subsurface. On the other hand, if
anaerobic conditions are desired, a 100% saturation of the pore spaces will
more than suffice.
3.2.4 pH
Extreme values of pH (i.e., pH values of <3 and >9 or 10), as well as sudden
changes in the pH of the waste-treatment system matrix, can inhibit microbial
growth by interfering with the following: (1) microbial metabolism, (2) gas
