BOD Progression in
Soluble Substrates — VI — Cell Recovery
Techniques in the TjjOD Test
LESLIE GRADY. JR., U. S. P. H. S. Research FeUow
A. W. BUSCH, Associate Professor of Environmental Engineering
Department of Chemical Engineering
Rice University
Houston, Texas
ABSTRACT
The total biological oxygen demand (TbOD) test involves tracing the BOD
curve to the plateau and determining the cell oxygen equivalent at that point.
The TbOD is the sum of the two oxygen values. The delineation of a simple,
accurate method for the determination ofthe cell oxygen equivalent is the subject
of this paper.
Three general methods were considered: optical, chemical, and gravimetric.
The use of optical techniques was eliminated as not feasible because of the low
concentrations involved. The gravimetric technique employed the use of MiUi-
pore filtration for which the samples were prepared by centrifugation. The chem -
ical technique involved the use of a modified COD procedure for values below
50 mg/l. Based on the average values from 10 experiments with each of three
substrates, the results of the gravimetric technique were found to be more accurate and precise than those of the COD procedure. A need for further work on
the cell formulation assumption inherent in the gravimetric technique was indicated, however.
INTRODUCTION
A previous paper in this series presented the concept of the total biological
oxygen demand (TbOD) test (1). The technique simply involves tracing the oxygen uptake curve to the plateau representing conversion of all biologically oxidizable substrate to cells (completion of synthesis) and then determining the cell
oxygen equivalent (the amount of oxygen necessary to oxidize the cell material
to carbon dioxide and water). The sum of the oxygen used in assimilating the
substrate and the oxygen equivalent of the cells synthesized is the total biological
oxygen demand. Since the tracing of the BOD curve to the plateau is a relatively
simple task, the most difficult part of the TbOD test is the determination of the
cell oxygen equivalent. The delineation of a simple, accurate method for the
determination of this cell oxygen equivalent is the subject of this paper.
In general, there are three possible methods of determining the oxygen equivalent of the cell material. The first is a direct technique: chemical oxidation in
a chemical oxygen demand test. The other two are indirect methods whose validity is dependent upon a generalized formula for cell material. Porges, et al.
(2) have suggested that the empirical formulation C5H7NO2 is applicable for
young cells. This composition was assumed in the derivation of the TbOD test.
The stoichiometric oxygen requirement (SOR), or the amount of oxygen necessary
for direct oxidation to carbon dioxide and water, for C5H7NO2 is 1.414 grams of
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