Kinetics of Removal of Mixed
Carbohydrates in Activated Sludge Processes
SAMBHUNATH GHOSH, Postdoctoral Fellow
FREDERICK G. POHLAND, Associate Professor
School of Civil Engineering
Georgia Institute of Technology
Atlanta, Georgia
WILLIAM E. GATES, Associate Manager
Engineering-Science Inc.
Oakland, California
INTRODUCTION
In the last decade there has been an upsurge of interest in the development of
kinetic models describing the courses of microbial growth and removal of organics in
biological waste stabilization processes. These models are generally based on
microbial growth kinetics and deal with the production of biomass at the expense of
a single growth controlling substrate. The general applicability of these single
substrate models to complex multi-substrate media such as wastewaters is questionable considering that uptake rates of the individual substrates in a mixture may be
markedly modified due to enzymatic interactions stimulated by the presence of
other substrates. A well known example of such interaction is the occurrence of
diauxic growth and sequential substrate utilization in batch cultures (1,2,3,4,5). How
the kinetics of biological assimilation of various organics in mixed substrate systems
differ from the kinetics of uptake in systems where these same organics serve as sole
substrates has not been well established.
The objectives of the research reported herein were to: 1) formulate
generalized equations expressing the course of uptake of individual sugar substrates
from a mixture; 2) identify the substrate which controls growth and substrate
utilization rates in mixed substrate systems and to delineate its biochemical
attributes; and 3) compare the performance of single and mixed substrate processes
in terms of effluent quality and substrate requirements for synthesis and energy
production. To accomplish these objectives, the research was performed in a
laboratory continuous flow reactor charged with synthetic carbohydrate wastes with
the assumption that this approach, analyses and the conclusions drawn could be
extended to other multi-substrate processes.
THEORETICAL CONSIDERATIONS
Derivation of a Basic Growth Kinetic Relationship
Since substrate assimilation is a consequence of growth due to enzymatic
synthesis of protoplasm and cell division, equations for substrate utilization should
be based on the kinetics of enzymatic reactions and also on the rate of cell division.
The basic differential equation for increase of cell concentration due to binary fission
is:
dN0
"dg-    =N0rln2 (1)
where:  Na = cell concentration at time 6
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