BOD Progression in Soluble
Substrates
A. W. Busch, Assistant Professor,
Department of Civil Engineering,
Rice Institute, Houston, Texas
Much information has appeared in the literature in recent years
casting doubt on the validity of the first order approximation used in
comparing the "strength" of organic wastes and in predicting the rate
of biological stabilization of putrescible matter in polluted waters. Most
of these papers were concerned with industrial wastes, particularly those
containing predominantly dissolved organics.
Many workers have pointed out the unreliability of "k" values as
parameters of biological activity and some investigators have proposed
theories to justify the apparently erratic characteristics of BOD data.
The fact that the progression of biochemical oxidation of domestic
sewage sometimes conforms to mathematical formulations other than
those expressing the growth of bacteria is an unfortunate coincidence
which has frequently caused valid data to be labeled erratic and hence
unreliable. Phelps (1), in his analysis of the exponential BOD formulation recognized the coincidence and discussed at some length the
possibility of a compound reaction. However, he concluded that the
progression of oxygen utilization in sewage BOD determinations was
best formulated by a first order equation. More recently, Hoover et al.
(2), postulated a two-phase reaction compounded of assimilation and
endogenous respiration. This theory ascribes the oxygen utilization during the second phase to the respiration of the bacteria produced during
the first phase and thus precludes any growth of predator organisms
feeding on the bacterial population.
The intent of this paper is to show that the process of biological
oxidation of soluble organic substrates involves first, conversion of the
material into cell substance with varying degrees of synthesis, and
second, utilization of stored decomposition products (principally glycogen) and of cell substance. The latter phase involves both the oxidative
organisms and predator organisms. The clear delineation of these two
steps depends principally upon the microbial population present. A
single strain of organisms or a predominance of substrate-consuming
organisms over predator organisms yields the most distinct two-stage
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