STUDIES ON THE RELATIONSHIP BETWEEN FEED COD AND
EFFLUENT COD DURING TREATMENT BY
ACTIVATED SLUDGE PROCESSES
T. S. Manickam, Research Assistant
A. F. Gaudy, Jr., Professor
Bioenvironmental Engineering Laboratories
School of Civil Engineering
Oklahoma State University
Stillwater, Oklahoma 74074
INTRODUCTION
Increasingly stringent effluent requirements emphasize the need for more "finely
tuned" predictive equations for describing effluent quality in the design stage and
throughout the life of a treatment facdity. The greatest advances in modeling predictive
behavior have been those dealing with fluidized culture processes, such as activated
sludge. These advances have been made possible because of the research of various
investigators in the areas of both wastewater treatment and industrial fermentation.
However, the greatest steps forward have been the adoption and the adaptation of
the general "first principles" embodied in the theory of continuous culture of microorganisms which was first described (independently) in  1950 by Monod [1]   and by
Novick and Szdard [2]  and elaborated upon by Herbert et al.  [3]  in  1956 and by
Herbert in  1961   [4].
One prediction of the theory is that in the steady state the concentration of soluble
carbon source in the reactor effluent Se is not dependent on the concentration in the
feed Sj.   This comes about in a once-through system because the specific growth rate
M is related to Se by the Monod relationship, and also to ddution rate D. Since D
can be held constant by controlling flowrate F (i.e., D = F/V), Se wdl be constant.
Furthermore, since the cell yield Yt and the cell decay factor kj are biological "constants," an increase in Sj is registered only as an increase in the biomass concentration
X in the system. Se in a once-through system is given in Equation 1 of Table I. If
Sj is increased it does not affect p. and therefore does not affect Se. The same line
of reasoning holds true for a system in which cell recycle is employed according to
the model of Herbert, since the recycle cell concentration Xr is made to be dependent
upon the biomass concentration X because of the selection of the concentration factor
c (Xr/X = c) as a design and/or operational constant. Equation 2 is the predictive
relationship for Se and it is independent of Sj. This feature of the kinetic theory has
caused some concern among microbial kineticists and pollution control researchers
because in continuous culture experiments some have noted an increase in Se for increased values of Sj. This matter is a source of controversy in the field [5-14]. The
divergence from theory generally is not so great as to interfere with general use of the
theory for predicting the useful yield of a bioculture process. However, in wastewater
treatment, small divergences from the predicted Se for an increase in organic loading
Sj can be important in view of the very low Se values demanded by enforcement of
PL 92-500, and a number of investigators have proposed adjustments or modifications
to accommodate or account for increases in Se for increased values of S;. The situation
is complicated in water pollution control by the fact that the carbon source can be
measured by a variety of codective or gross parameters of substrate, e.g., BOD, COD,
ACOD, ATOC, and the fact that Se refers to soluble substrate in the effluent which is
as yet not routinely assessed and reported in field operations.
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