Operational Performance Of An Activated Sludge
Process With Constant Sludge Feedback
R. SRNIVASARAGHAVAN, Graduate Student
A.F. GAUDY, Jr., Professor
Bioengineering Laboratories
School of Civil Engineering
Oklahoma State University
Stillwater, Oklahoma 74074
INTRODUCTION
There is an ever increasing need for reliable delivery of a high degree of removal of
biochemical oxygen demanding organic matter. This has fostered a trend in design of such
biological treatment processes as the activated sludge process toward the use of descriptive
kinetic models purporting to relate the variables controlling metabolism. Such a trend away
from strictly empirical "rules of thumb" is a forward step and has led to many varying
approaches to design. All too often these approaches to design neglect to consider operational aspects, and in some cases one might argue that the design models have little
communion with theories of continuous culture which have been developed to depict the
growth of microorganisms.
Thus, the approach in our laboratories to development of kinetic models has been to
seek relationships useful both in design and in operation. Also, much investigative effort
has been directed toward determining whether the theory of continuous culture of single
species developed by Monod (1) and Novick and Szilard (2), and elaborated upon by
Herbert, Elsworth and Telling (3) and by Herbert (4) was applicable or could be made
applicable to heterogeneous microbial populations such as exist in activated sludge
processes. If it were, we would then have the basis for a model which could claim the desired communion with the basic concepts of microbial growth. Various research papers on
that portion of the research have been published and need not be reviewed in detail here.
Briefly, it was found that the kinetic relationships for describing the biomass in pure cultures were in general valid for heterogeneous populations if one would accept reasonable
variations in the biological "constants" (5, 6, 7, 8, 9). Also, the continuous growth equations of Herbert for once-through reactors were useful for description of effluent substrate
and biomass concentrations (5,6). However, theequationsfor"steady state"concentration
of effluent substrate and cell or biomass concentration, S and X, for cell recycle systems
were not entirely useful (6). One of the operational or design constants was defined by
Herbert as the recycle concentration factor, c, which is the ratio between the concentration
of recycle cells (or sludge), Xr, and the aeration tank suspended solids concentration, X.
Attempts to operate a system using this parameter caused rather severe fluctuation in the
"steady state" values of cells, X, and substrate S, when heterogeneous populations were
employed (6). Thus, it was found necessary to change the equations, and the aim was to do
so in such a way as not to lose the basic tie to proven concepts of continuous culture. It was
decided to_discard the use of c as a system constant and to derive mass balance equations
in X and S, assuming Xr itself to be the system constant. The model equations for design
and operation were presented in 1969 (6). The mathematical (or "theoretical" ramifications of maintaining Xr as a system constant have also been discussed and the relationships between the operational parameters have been presented (10).
In Table I the basic equations as given by Herbert (4) ( with c as a selectable system
constant) are compared with those of Ramanathan and Gaudy (with Xr as a selectable system constant). In both sets of equations, the effluent substrate concentration, S, and the
biological solids concentration, X, are determined by the descriptive biological "constants,"
753