Aerobic Cell Yield and Theoretical
Oxygen Demand
W. O. PIPES,   Associate Professor, Civil Engineering
Northwestern University
Evanston, Illinois
E. M. MIHOLITS, Graduate Student
University of Texas
Austin, Texas
and
O.   W.  BOYLE
Galway, Ireland
INTRODUCTION
The concept of growth yield has been widely employed in the various attempts
to develop a theory of biological waste treatment processes (1). The basic idea is
that if an organic compound can serve as a carbon and energy source for the growth
of microorganisms, there is a quantitative relationship between the amount of the
compound which is assimilated and the quantity of microorganisms produced. In
describing organic wastes it is customary to measure the concentration of organic
matter which may serve as a carbon and energy source for microbial growth as BOD
and thus growth yield is often related to BOD. For instance, Sawyer (1) has stated,
"... it is quite definite that the amount of growth can be predicted from the
five-day BOD."
The concept of growth yield has been used to relate sludge growth in the activated sludge process to the BOD of the influent waste. The question is now
raised as to whether or not it is possible to measure BOD by measuring growth.
Since it is possible under the proper conditions to determine growth yield more
rapidly and at least as easily and as precisely as it is to determine five-day BOD,
the answer to this question might provide the basis for developing a better method
for estimating the strength of an organic waste.
Monod (2) reviewed the literature describing investigations of growth yield in
pure bacterial cultures. He stated that in a pure culture the growth yield on a
given substrate should be constant; that is, the quantity of bacteria produced should
be proportional to the amount of substrate consumed. However, he also pointed
out that there are several conditions which must be met before the growth yield
on a substrate can be accurately measured. The substrate must be the sole carbon
and energy source for the bacterial growth, the substrate must be the growth limiting factor, and the growth yield must be measured during the exponential growth
phase or the stationary phase.
Monod (2) further stated that if the growth yield is accurately determined it
should be a measure of the energy available from the substrate for growth. The
energy available for growth per unit weight of substrate varies considerably from
substrate to substrate thus the growth yield per unit weight of substrate also varies
greatly. However, the energy available for growth per unit weight of oxygen consumed varied  only slightly from substrate to substrate.    The amount of oxygen
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