Carbon - Freshwater Ecology: Concepts and Environmental Applications - page 235

METHOD 12.1.
What Is Biochemical Oxygen Demand and How Is It Measured?
Biochemical oxygen demand (BOD) is a simple method for measuring the
total organic content that is available to organisms plus any chemicals that
spontaneously react with O 2 . The procedure is simple: Water is incubated
in sealed bottles, and the decrease in O 2 over time is monitored. If all the
O 2 is used up over the time of the incubation, the original water sample
must be diluted and analyzed again. During the incubation, naturally pres-
ent heterotrophic organisms respire the organic carbon that is biologically
available and any chemicals that spontaneously react with O 2 (e.g., sulfide)
will also do so, allowing analysts to assess total BOD in wastewaters
(Eaton et al., 1995).
When sewage is released into natural waters, it provides heterotrophs
with additional substrate and creates a demand for dissolved O 2 . High
sewage influx leads to anoxic conditions in the waters, particularly during
summertime low flows. During summer low-flow, dilution is at a minimum
and with high temperatures dissolved O 2 concentrations are low. Thus,
regulations for the degree of sewage treatment are based on the amount of
BOD released into the receiving waters. BOD measurements are often
made daily to assess the efficiency of sewage treatment facilities.
Oxidation of Organic Carbon with Inorganic Electron Acceptors Other
Than O 2
Organic carbon can release the most energy to organisms if it is oxi-
dized with O 2 . In the absence of O 2 , the next best thing is to use other
electron acceptors (such as nitrate, sulfate, and oxidized iron) to oxidize
organic C. The degree of efficiency of these oxidations depends on the ox-
idation state of the compound. In other words, O 2 is the most oxidized
compound abundant in the natural environment that reacts with the re-
duced organic carbon, so the greatest amount of energy is released. Other
compounds are used in order of redox (see Fig. 11.6), but an oxidation of
carbon that is not the most efficient can occur (e.g., a bacterium that can
only use nitrate to oxidize organic carbon may continue to do so even in
the presence of O 2 ). If the transformation is less efficient, the organism that
relies on the less efficient mode of oxidizing carbon will ultimately be out-
competed, unless it is able to switch to the more efficient mode. In variable
environments, conditions may change so the metabolic strategy of the less
efficient organism becomes more efficient.
In anoxic habitats, this series of organic carbon oxidations can be dis-
tributed across a redox gradient (across millimeters or centimeters in sed-
iments, micrometers or millimeters in decaying organic particles, or meters
at the interface of an anoxic hypolimnion). The O 2 is used first where con-
tact with the atmosphere occurs. After O 2 is used up, then each successive
type of oxidant is used up according to the maximum potential energy