A Laboratory Study of Mixing
Conditions in Small Aeration Vessels
W. F. MILBURY, Graduate Student
W. O. PIPES, Associate Professor of Engineering
The Technological Institute
Northwestern University
Evanston, Illinois
R. B. GRIEVES, Assistant Professor of Civil Engineering
Illinois Institute of Technology
Chicago, Illinois
INTRODUCTION
Since its inception, the activated sludge process has drawn the attention of
investigators who were interested in understanding the mechanisms by which the
process operated. In recent years there have been several attempts to describe
and evaluate the mixing regime which exists in the aeration portion of the system.
The quantitation of the time distribution of oxidizable substrate and "active" suspended solids in the system will contribute significantly to the definition and
understanding of the efficiency of this waste treatment process.
Last year we presented a paper introducing a mathmetical model describing
the effect of the hydraulic regime upon the activated sludge process (1). This
model combined equations describing the mixing pattern and biological growth
kinetics to produce overall system performance equations. In that paper eight
different mixing configurations were considered involving combinations such as
complete mixing, plug flow, short-circuiting, stagnant zones, and recycle.
In this paper some laboratory studies of mixing conditions in small aeration
vessels conducted for experimental verification of this mixing model are reported.
Because our biological studies of this model are still in progress, only verification
of the hydraulic portion of the mixing model is presented. First a brief review of
the hydraulic model is presented, then the experimental methods and equipment
are discussed, and finally the experimental results verifying the model are reported and commented upon.
HYDRAULIC MODEL
The mixing regime is described in this hydraulic model by a volume apportionment method which theoretically divides the aeration tank into several volumes with different mixing patterns. Cholette and Cloutier (2) have presented
relationships which consider an "effective" volume of complete mixing, stagnant
zone, and short-circuit and plug flow. The actual quantitation of a continuous
flow system by this method of hydraulic description is performed by creating a
step-change in concentration of some inert component of the feed. Then the
change in concentration of this substance in the effluent is observed with time.
The nature of the effluent response curve can then be used to characterize the degree  of mixing in the system according to this method of hydraulic analysis.
The theoretical results which would be obtained from a tracer analysis performed by this method are presented in Figure 1. The vessel of volume, v, initially has a uniform tracer concentration,  c0,   the same as that of the feed,
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