Use of the TbOD Test with Colloidal
Wastewaters
MICHAEL K. MULLIS, Graduate Student
EDWARD D. SCHROEDER, Assistant Professor
Department of Civil Engineering
University of California
Davis, California
INTRODUCTION
Operational control of biological treatment systems, such as the activated
sludge process, cannot be accomplished without a rapid technique for determining
the biochemical oxygen demand of the wastewaters. Many wastewaters contain high
concentrations of non-soluble organics and a suitable measurement method is
particularly needed if process control is to be possible. Currently used methods, the
five day BOD test, the COD test and instrumental methods of COD or total carbon
determination, are unacceptable for several reasons. The five day BOD test is based
upon an arbitrary time period which is unrelated to the biochemical events taking
place within the bottle or within a treatment process (1,2). Because the five day
period is unrelated to the conversion of organics to cells and carbon dioxide, there is
no way to develop consistent results. For example, any material in the wastewater
that inhibits the biological activity will depress the BOD curve without being
detected. Advantages of the BOD5 test include low cost and the fact that recorded
oxygen uptake is due to biological activity. Because there is no stoichoimetric
evaluation (Eq 1)
n. [Organics] +n2[02]    -^-»   n3[New Cells] + n4[H20] + n5(C02]    (1)
of the metabolic conversion of the organics to C02 and water, the obtained values
are of questionable validity and meaning. Hence, it can be concluded that the five
day BOD test should not be reproducible. This conclusion is well supported in the
literature (1,2).
The COD test like the BOD test is reasonably inexpensive and simple. Because
chemical oxidation rather than biochemical oxidation is measured, an accurate
estimate of the BOD is not provided. An important advantage of the COD test is the
relatively short determination time. If oxygen demand is to be used for process
control a rapid technique is necessary. COD determinations can be made in about
three hr, a time span that is acceptable for operational control of activated sludge
processes.
Instruments now available for measuring the COD and organic carbon content
of wastewater provide a more rapid determination technique than the COD test.
Values can be obtained in a matter of minutes rather than hours. However, as in the
COD test, there is no differentiation between biodegradable and non-biodegradable
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