39    ESTIMATION OF PROCESS KINETICS FOR AN EXTERNAL
RECYCLE EXTENDED AERATION PILOT PLANT
W.L. Lowe, Technical Manager
Roy F. Weston,
West Chester, Pennsylvania 19380
A.F. Gaudy, Jr., Professor Emeritus
University of Delaware
Newark, Delaware 19716
A. F. Rozich, Program Manager
ERM Group
Exton, Pennsylvania 19341
INTRODUCTION
Previously reported work has demonstrated excellent performance and stability in organic substrate
removal achievable by a modified extended aeration process in which an aerobic digester was used as a
sludge inventory holding/dosing tank.' The potential advantages of this system, as compared to
conventional extended aeration, include the high degree of operating flexibility, with the possibility of
managing short- and long-term fluctuations in operating conditions by control of the biomass recycle
stream.
Design of this, or any other, extended aeration system should be based upon the kinetics exhibited
by heterogeneous populations under such conditions. Pertinent parameters include not only the
growth rate, yield and substrate removal terms but also the endogenous/autodigestion decay kinetic
parameters. In fact, these endogenous/autodigestion decay "coefficients" are of great significance in
determining the final configuration of the extended aeration system. They are, at the same time,
somewhat difficult to evaluate, due in part to the wide variety of metabolic events which may
contribute to the destruction of microbial biomass, and in part to the fact that these events are not, as
currently understood, under direct experimental or engineering control. However, the flow scheme
represented by the external recycle extended aeration process may allow the separation of the substrate removal and biomass decay phases of an extended aeration system so that an evaluation of the
magnitude of these factors under various operating conditions might be considered.
MATERIALS AND METHODS
The modified extended aeration flow scheme and the laboratory scale pilot plant used in this study
have been previously presented.1 The system is illustrated with mass balance notations in Figure 1.
Soluble organic synthetic wastewater consisting of 1,000 mg/L glucose with nutrients and buffer was
fed to the reactor. The reactor was operated according to the constant recycle concentration model of
Ramanathan and Gaudy,2 while the digester was operated as a semi-batch, variable volume reactor to
retain all biomass (except experimental and sampling losses) within the system. A flow through
centrifuge was used to capture effluent biomass for return to the digester. Separate models were
adopted for estimating biomass decay rates in the reactor and the aerobic digester. Operating data
from six experimental runs, using reactor dilution rate, D, as the independent variable, were used to
estimate the kinetic parameters. Exhaustive mass, flow, and constituent balances were used to examine the quality of the data from which biokinetic coefficients were estimated.
REACTOR MODEL
While a variety of ecological events may contribute to the loss of cellular mass under continuous
flow conditions, conventional models incorporate a single coefficient into either the biomass or
substrate balance to account for the observed effects.5"5 This parameter is often conceptualized as
representing the endogenous or maintenance energy requirement expressed by microbial populations
47th Purdue Industrial Waste Conference Proceedings, 1992 Lewis Publishers, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.
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