40    STUDIES ON BIODEGRADATION KINETICS FOR
MIXED SUBSTRATE SYSTEMS
Chao Gao, Ph.D. Student
Rakesh Govind, Professor
Department of Chemical Engineering
University of Cincinnati
Cincinnati, Ohio 45221
Henry H. Tabak, Research Biochemist
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
INTRODUCTION
Wastewater typically contains a mixture of noninhibitory and inhibitory chemicals or substrates. Biodegradation of mixed substrates is encountered in a majority of biological waste treatment processes. Most previous biokinetic studies were limited to single chemicals'-2--1 and few
studies on mixtures have been conducted. Bitzi et al? emphasize that more information concerning the biodegradation kinetics of mixtures of common industrial chemicals needs to be gathered
before wastewater biotreatment process performance can become a matter of design.
The study of mixed substrate utilization began from Monod's analysis of bacterial growth on
two sugar mixtures.5-6 When a microorganism is placed in an environment consisting of several
limiting substrates, it may use all the substrates simultaneously or alternately it may consume the
substrates preferentially. Glucose is preferred over lactose by E. Coli. Monod called this diauxic
growth. The manner of utilization of these mixed substrates is controlled by cultures, the properties of these substrates, and the operation conditions (concentration, etc.).
The modeling of microbial kinetics in the fermentation industry was reviewed by Nielsen and
Villadsen.7 The most fundamental observations concerning microbial growth processes are as
follows : (1) that the rate of cell mass production is proportional to biomass concentration: (2)
that there is an upper (saturation) limit for growth rate on each substrate: and (3) that the cells
need substrate and may synthesize products even when they do not grow. The models can be divided into unstructured models in which the population's internal "structure" is not included and
structured models which account for changes in the population's internal state.8 A summary of
the studies of mixed substrate systems is listed in Table I.4- 9~24
There are very few mathematical models that can describe the biodegradation of mixed substrate in wastewater treatment systems in which inhibitions and pronounced lag time exist. A
model frame called cybernetic was developed by Ramkrishna et al.14,25"28 for mixed substrate
systems in the fermentation industry. The basic assumption of this model frame is that over many
years of evolutionary processes, in environments with varying menus of substrates, microbes
have acquired the capability to control their regulatory processes to optimize their growth pattern. The basic merit of the cybernetic approach is that it adopts a simple mathematical description of a complex organism but compensates for the over-simplification by assigning an optimal
control motive to its response.
The objective of this chapter is to conduct a study of biodegradation kinetics of toxic organic
chemicals in mixed substrate systems and to test the validity of cybernetic model applied to
those systems.
51st Purdue Industrial Waste Conference Proceedings, 1996, Ann Arbor Press, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.
381