Biogeochemical Cycles - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 169

summary of pseudo-first-order and second-order rate constants for the disap-
pearance of representative toxic organic compounds from natural waters via
biotransformation. 131
For modeling, first-order biodegradation rate constants or half-lives for the water
column and the benthos may be specified. If these rate constants have been measured
under similar conditions, this first-order approach is likely to be as accurate as more
complicated approaches. If first-order rate constants are unavailable, or if they must
be extrapolated to different bacterial conditions, then the second-order approach may
be preferred. It is assumed that bacterial populations are unaffected by the presence
of the compound at low concentrations.
Certain organic pollutants such as PCBs, dioxin, and some pesticides (e.g., DDT)
are not biodegradable, and thus persist in the environment despite their use being
banned. Therefore, current environmental policy in most countries dictates that only
readily biodegradable compounds be used in commerce. Additionally, a large part
of wastewater disposal regulations is based on the ability of various organic wastes
to be biodegraded in the ecosystem.
Various procedures have been developed to measure the biodegradability of
chemical compounds. One procedure is the inoculated die-away test where water,
salts, and biomass are incubated for 5 to 40 days with the test compound. Another
is the activated sludge test method where large amounts of biomass and several test
organic substrates are continuously fed with air or oxygen to provide for better
adaptation to degrade the multiple substrates.
Many factors can affect the rate of biodegradation: pH, sorption, populations
and types of microorganisms, moisture, the presence of other chemicals, and the
concentration of chemicals present. 166 Biodegradation is also affected by numerous
factors that influence biological growth. 131

• Temperature effect on the biodegradation of toxic compounds is similar
to that on biochemical oxygen demand (BOD), and is quantified using an
Arrhenius-type relationship,
• Nutrients are necessary for growth and often limit growth rate. Other
organic compounds can serve as the primary substrate so that the chemical
of interest is degraded via co-metabolism or as secondary substrate (the
utilization of organic chemicals at concentrations lower than are required
for growth in the presence of one or more primary substrates that are used
as the carbon and energy sources),
• Acclimation is the process of adaptation necessary to express repressed
enzymes or foster those organisms that can degrade the toxicant through
gradual exposure to the toxicant over time. A shock load of toxicant can
kill a culture that would otherwise adapt if gradually exposed. On the
other hand, the chronic presence of some toxic organic chemical some-
times spurs on the induction or expression of an enzyme that helps to
degrade other organic chemicals.

• Population density or biomass concentration dictates that organisms must

be present in numbers large enough to significantly degrade the toxicant
(a time lag often occurs if the organisms are too few).