Monod Kinetics — Explanation of Historical Pitfalls
In BOD Interpretation
WILLIE P. ISAACS, Professor
Department of Civil Engineering
New Mexico State University
Las Cruces, New Mexico 88001
INTRODUCTION
Organic wastes from our industrialized society are polluting and destroying the purity
of our natural streams. The Water Quality Act of 1965, requiring standards and clean-up
plans for all interstate and coastal waters, establishes the basis of a comprehensive plan for
the future. The standards of each state will allow us to carry out river basin pollution control
plans which will encompass coordination between all Federal, State, local and private
investments to achieve our goals for water quality. This type of composite national view of
water problems provides the water management program required for our Nation's
economy and health for the future. Will engineering "know-how" delay the orderly
development of this comprehensive plan?
The interrelationships between the many oxygen transfer processes which occur in
natural streams have not been determined, and basic understanding of the self-purification
process which occurs in the natural stream has been hindered due to inabilities to study
separately each of the oxygen transfer processes. There is general agreement that the
process of surface reoxygenation is a true first order reaction. It is now quite possible to
predict with accuracy the reaeration rate to be expected, reach by reach, in an open river
under unstratified conditions from knowledge of mean stream velocity and mean stream
depth of flow. The kinetics of deoxygenation has been less clearly defined and general
agreement has not been realized. The lack of concensus is primarily due to a lack of
differentiation between bacterial growth and death in conjunction with the lack of adequate
testing techniques to describe complex wastewaters in terms of parameters which serve the
required utility for rigorous mathematical treatment.
Monod (1) presented an expression which is highly adaptable to stream analysis. It
describes the growth of organisms as they utilize a substrate as an energy source. Monod's
investigation showed that his kinetic model could be used to describe the growth of pure
cultures on several well defined simple substrates. Marlar (2) has shown that Monod
kinetics describe the interaction of a heterogeneous population of bacteria with a simple
well defined substrate (glucose). It appears that the next logical step would be to study more
complex substrates. Furthermore, in order to reap practical results "real natural problem"
substrates should be studied and characterized.
The use of trickling filters and activated sludge processes constitute essentially 100
percent of our now required secondary treatment of waste outfalls. Most all of the
municipalities in this country use one or both of these processes. Essentially all industries
with organic type waste use these types of treatment processes. The ultimate disposal of
waste treatment plant effluents generally are placed into our natural streams. The impact
and magnitide of the effects of these waste effluents uponthe kinetics of natural stream self-
purification is virtually unknown.
Presently, economic, governmental, and social pressures are such that predictions of
future downstream conditions must be based upon the best predictor systems available. The
classical analysis (Streeter-Phelp's Oxygen Sag Equation) of the assimilative capacity of
streams is being scrutinized both as to its engineering adequacy and theoretical basis. It
appears now that streams receiving domestic waste, or a variety of soluble industrial waste
can be analyzed using Monod kinetics.
Monod kinetics has been used successfully to describe simultaneously; the growth of
organisms, oxygen utilization, and substrate utilization (2). The processes of bacterial
growth, substrate removal, bacterial deoxygenation, and atmospheric reoxygenation, as it
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