32    KINETICS-DYNAMICS OF BIODEGRADATION OF
POTENTIALLY TOXIC ORGANIC CHEMICALS
Walter J. Maier, Professor
Civil and Mineral Engineering Department
University of Minnesota
Minneapolis, Minnesota 55455
INTRODUCTION
It is generally recognized that most of the organic chemicals which manifest toxic properties to
microbial population are nevertheless biodegradable and can serve as carbon and energy sources for
microbial growth under selected environmental conditions. These dual biochemical characteristics
have important practical implication in terms of natural purification processes in surface waters and
soil environments. Quantitative descriptions of toxicity and its effects on rates of metabolism are
needed to provide a rational basis for predicting the fate of such chemicals in the environment as well
as for the design and operation of effective and low cost engineered cleanup strategies for wastewater
treatment and in situ treatment of polluted groundwaters and soils.
Surface and groundwater pollutants usually consist of a mixture of organic chemicals. Individual
toxic chemicals are present at relatively low concentrations along with a large number of naturally
occurring or man-made chemicals that exhibit a broad range of biodegradabilities. This discussion
will therefore focus on the kinetics and dynamics of biodegradation of toxic chemicals in mixtures of
organic substrates.
The first part of the chapter overviews pertinent data on substrate mixtures and discusses microbial
strategies of multiple substrate utilization with a view to defining general behavior patterns. The
second part of the chapter discusses models and rate equations that have been found to be useful for
characterizing biodegradation of mixtures. The last part of the chapter illustrates applications of the
rate equations to predict rates of substrate removal in continuous flow, well mixed reactors.
SUBSTRATE INHIBITION
Growth on toxic substrates that are nevertheless biodegradable have commonly been described in
terms of a modified Monod equation.1-2 Inhibition is ascribed to the inactivation of one or more
necessary enzymes due to formation of multiple substrate enzyme complexes (ESn) as defined by
Haldane's substrate-enzyme inhibition model. The resultant modification of the Monod growth rate
equation for cell mass and substrate removal are given by Equations 1 and 2.
(1)
dx
n      SX
dt
K5 + S + SVKj
ds          1 dx
dt ~ " Y dt
(2)
at r at
where X = cell mass cone.
S = substrate cone.
H = maximum growth rate coefficient
Ks = Monod coefficient
Kj = substrate inhibition coefficient
Y = yield coefficient
The underlying assumptions of this model are that: a) specific growth rate slows down with
increasing substrate concentrations; b) the effects are reversible; and c) there is no shift to other
metabolic pathways.
43rd Purdue Industrial Waste Conference Proceedings, © 1989 Lewis Publishers, Inc., Chelsea, Michigan 48118.
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