McKay, Donald. "Front matter" Multimedia Environmental Models ...

halogentated hydrocarbons), they experience considerable difficulty, and they may
or may not be able to perform useful chemical conversions. In such cases, if environmental
degradation does take place, it is often the result of abiotic processes such
as photolysis or reaction with free radicals.
Our aim is to be able to define a half-life or rate constant for microbial conversion
of the chemical, usually in water but often also in soil and in sediments. These rate
constants may be measured by introducing the chemical into the medium of interest
and following its decay in concentration. If first-order behavior is observed, a rate
constant and half-life may be established. Care must be taken to ensure that the
decay is truly attributable to biodegradation and not to other processes such as
volatilization.
In many cases, non-first-order behavior occurs. For example, it is suspected that,
in some situations, the concentration of chemical is so low that the enzymes necessary
for conversion do not become adequately activated, and the chemical is essentially
ignored. At high concentrations, the presence of the chemical may result in
toxicity to the microorganisms, and therefore the conversion process ceases. The
number of active enzymatic sites may also be limited, thus the rate of conversion
of the chemical species becomes controlled not by the concentration of the species
but by the number of active sites and the rate at which chemicals can be transferred
into and out of these sites. Under these conditions of saturation, a Michaelis–Menten
type equation can be applied as described earlier.
Much to the chagrin of microbiologists, we will adopt a simple expedient assuming
that a first-order rate constant (or half-life) applies and that the rate constant can
be estimated by experiment or from experience. This is necessarily an approximation
to the truth and often involves merely a judgement that, in a particular type of water
or soil, this compound is subject to biodegradation with a half-life of approximately
x hours. The rate constant is therefore 0.693/x hours. Valiant efforts have been made
to devise experimental protocols in which chemicals are subjected to microbial
degradation conditions in the field or in the laboratory using, for example, innoculated
sewage sludge. Such estimates are of particular importance in the prediction
of chemical fate in sewage treatment plants. Even more valiant attempts are being
made to predict the rate of biodegradation of chemicals purely from a knowledge
of their molecular structure. Others have been content to categorise organic chemicals
into various groups that have similar biodegradation rates or characteristics.
Several standard and near-standard tests exist for determining biodegradation
rates under aerobic and anaerobic conditions in water and in soils. Simplest is the
biochemical oxygen demand (BOD) test as described in various standard methods
compilations by agencies such as ASTM and APHA. More complex systems involve
the use of chemostats and continuous flow systems, which are analogous to benchtop
sewage treatment plants.
An important characterization of biodegradation relates to whether the organism
requires an oxygenated environment to thrive. All organisms require energy, which
is obtained by performing chemical reactions. The most common reaction is oxidation,
which is performed by aerobic organisms when oxygen is present. Oxidation
of ethanol to acetic acid is an example. When oxygen is absent and anaerobic
conditions prevail, the organism can obtain energy by processes such as reducing
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