Oxygen Supply Limitations in Full Scale Biological
Treatment Systems
J. V. MATSON, Graduate Student
W. G. CHARACKLIS, Assistant Professor
Rice University
Houston, Texas
A. W. BUSCH, Regional Director
Environmental Protection Agency
Dallas, Texas
INTRODUCTION
There is a diversity of opinion over the mechanisms that can control the rate of reaction
in biological treatment systems. A strong case has been made for kinetic models based on
the concentration of substrate as the rate controlling factor. Accordingly, many of the waste
treatment designs contain variations of Michaelis Menten or Monod formulations which
.relate reaction rate as a function of substrate concentration.
Under certain conditions the mass transfer of substrate can control the reaction rate-
so can the mass transfer of oxygen. With no mass transfer limitations, the reaction rate is
dependent on cellular biochemical mechanisms.
The object of this paper is to define under what conditions the mass transfer of
substrate and of oxygen are controlling and when neither is controlling. Also, it will be
demonstrated that many waste treatment plants are oxygen limited, and are performing
below their potentials.
THE BIOLOGICAL REACTOR
On the micro-scale a biological waste treatment basin can be visualized as composed of
biological floe particles, each surrounded by a layer or film of water (Figure 1). The
thickness of the water layer is a function of the turbulence level. The floe size is a function of
turbulence level, solids concentration, and solids retention time (SRT).
Figure I - Microscale representation
of an aerobic biological reactor.
Both oxygen and substrate must diffuse through identical resistances - the attached
water layer and the floe - to reach the reaction sites in the floe. The rate processes are defined
as follows:
Ri - diffusion of substrate through attached water layer
R2 - diffusion and reaction of substrate in floe
R3 - diffusion of oxygen through eddy
R4 - diffusion and reaction of oxygen in floe
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