Kinetics of the
Steady-State Bacterial Culture
II.    Variation in Synthesis
L. J. HETLING, Public Health Service Fellow
D. R. WASHINGTON, Associate Professor
Department of Environmental Engineering
Rensselaer Polytechnic Institute
Troy, New York
S. S. RAO, Research Microbiologist
Canadian National Research Council
Toronto, Canada
INTRODUCTION
One of the more important parameters used in the process design of activated
sludge plants is the quantity ofsludge (cell material) produced or synthesized. The
relationship between the quantity of cell material synthesized and the amount of
substrate utilized is usually expressed as an effective yield coefficient (Y) having
the units of weight of cell produced per weight of substrate removed. Experimentally, engineers and biologists have measured the effective yield coefficient
by means of a material balance relating the accumulation of cell material to the
removal of substrate under batch growth conditions. A tabulation of values found
in this manner and reported in the biological and engineering literature are shown
for pure culture in Table I and for mixed culture in Table II.
These tables show a significant spread in the reported values of the effective
yield coefficient, even for the same substrate, when measured by different investigations.
It is the purpose of this paper to postulate reasons for these differences.
VARIATIONS OF THE EFFECTIVE YIELD COEFFICIENT WITH
SUBSTRATE AND ORGANISM
The relationship between the effective yield coefficient and the nature of the
organic substrate metabolized is an area in which there has been considerable
speculation and confusion both in the biological and engineering literature.
In the biological literature, most of the published work on this topic indicates that a stoichiometric relationship exists between the quantity of substrate
oxidized and the quantity used in cell synthesis for a given organism and a given
compound under similar conditions. Many attempts to relate the synthesis to the
change in the free energy (AF) of the substrate were made but no experimental
evidence could be found to support this relationship. In reviewing the subject,
Lamanna and Mallette (13) conclude that the chemical nature of a substrate rather
than its change in free energy controls synthesis during growth.
A different approach to the problem was recently developed by Bauchop and
Eldsen (14) who related observed cell growth to the calculated theoretical production of ATP from the energy source during metabolism.   Experimental results
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