Studies on Sludge Yield in Aerobic Systems
M. RAMANATHAN, Project Engineer
Roy F. Weston, Inc.
West Chester, Pennsylvania
A. F. GAUDY, Jr., Professor
Bioenvironmental Engineering Laboratories
Oklahoma State University
Stillwater, Oklahoma
INTRODUCTION
The need to make a reliable estimate of the amount of biological solids
produced when a given amount of substrate is utilized is of obvious importance in
the field of biological waste treatment. A numerical value for sludge yield (or cell
yield) is needed to size sludge disposal facilities. Also, the sludge yield (Y) is an
important "constant" in design and operational models for biological treatment
systems (e.g., activated sludge processes). It is also of obvious importance in various
commercial fermentations employing specific "pure" cultures, and the cell yield has
received considerable investigational attention in the basic microbiological field.
In the pollution control field, Burkehead and McKinney have reviewed various
thermodynamic parameters given by McKinney (1), Servizi and Bogan (2), and
McCarty (3), relating cell yield to changes in heat content or free energy of the
carbon source. These concepts have been criticized by various workers, for example,
Hetling, et al (4) and Schroeder and Busch (5). Regardless of the question of the
utility or of the validity of such concepts, past work reported by Rao and Gaudy has
provided evidence that, in heterogeneous populations, changes in predominance of
species bring about changes in Y which are sufficiently large to make highly
questionable the utility of such thermodynamic properties of the substrate for
predicting cell yield (6). For example, in a study which included 44 determinations
of Y, on a single carbon source (i.e., constant thermodynamic parameters), the
average yield value was 65 percent with a coefficient of variation of 17 percent (i.e.,
a range of 48 to 82 percent). Large differences in Y were correlated to gross changes
in species predominance in the system. Gross prediction of Y for heterogeneous
populations, such as found in activated sludge, purely on thermodynamic properties
of the waste (provided these can be readily determined, in any event), would seem to
be an overly simplistic approach. Gaudy and Gaudy have discussed problems
encountered in evaluation of cell yield and biological bases for variations in this
important parameter for both pure cultures and heterogeneous microbial populations
(7). Other recent reviews on cell yield are those by Forest (8) and by Payne (9).
MATERIALS AND METHODS
One of the problems that workers have when compiling and analyzing
published cell yield values is the different methods of measurement used by various
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