Mechanism and Kinetics of
Substrate Utilization at High Biological
Solids Concentrations
P. KRISHNAN, Graduate Research Assistant
A. F. GAUDY, JR., Director Bioengineering Laboratories
School of Civil Engineering - Oklahoma State University
Stillwater, Oklahoma
INTRODUCTION
In a previous paper Rao and Gaudy (1) reported on the effect of sludge concentration on various aspects of biological activity in activated sludge, such as
the relationship between rate of substrate removal and initial biological solids
concentration, cell yield, and the percentage of the theoretical oxygen demand
exerted at the time of substrate and COD removal. The relationship between substrate removal and initial solids concentration was found to be linear, but the
value of the slope of the relationship depended not solely upon the type of substrate but upon predominating microbial species. The sludge yield varied between
48 and 82 per cent for a single substrate, glucose. Concerning the per cent theoretical parameters, the results were in general accord with those of McWhorter
and Heukelekian (2) with respect to COD, but not with respect to substrate removal as measured by a test specific for carbohydrate. A theory and experimental evidence of its validity were presented to explain the results. Since the
highest biological solids concentration employed in the previous study was approximately 2000 mg/1, it was felt that further studies at higher solids concentrations
were warranted. In addition, there is still much controversy over the mechanism
of purification during rapid substrate removal, i.e., adsorption of soluble substrate on the floe as opposed to metabolism as the primary mechanism of rapid
substrate removal. While many workers have described purification using an adsorption isotherm (3,4,5,6), it is emphasized that kinetic formulation is not proof
of mechanism. On the other hand, the work of other investigators supports the action of biochemical processes as the primary mechanism of rapid substrate removal (7,8,9,10,11,12). Smallwood's (13) studies indicated that soluble substrates are removed primarily through metabolic assimilation while colloidal
substances are possibly removed by adsorption. Since very rapid purification is
accomplished only at high solids concentration, the present study offered an opportunity to gain further insight into this mechanistic problem, and also to investigate me degree of immediate uptake of substrate by the sludge. All of these
aspects are of basic importance to more complete understanding of all types of
activated sludge processes, and are particularly pertinent to the biosorption process and the applicability of a process modification recently studied by Kromolrit,
Goel, and Gaudy (14).
MATERIALS AND METHODS
Two batch activated sludge units were started using seed from the municipal
treatment plant at Stillwater, Oklahoma. Sludges were developed on glucose and
sorbitol (5000 mg/1) using the 24-hour batch feeding cycle previously described
(15). After a four-week period during which flocculated cells were developed and
the units came into solids balance, sludge was periodically harvested for experimentation.    Sludge was centrifuged, washed twice in 0.05M potassium phosphate
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