The performance of large scale aeration tanks was evaluated from the mutual relationship between the flow characteristics snd the substrate removal patterns. The results of the tracer studies indicated that the flow conditions in the aeration tanks were too complicated to describe satisfactorily with the mathematical models. In the light of the transient nature of physical, chemical, and biological factors controlling the mechanism of the activated sludge process, it would seem most unlikely to provide a unified kinetics model. As regards the load distribution, the empirical equation C/C0 = u/(u + Kx) well described the BOD distribution in the aeration tanks whatever the flow characteristics and tank geometry. The equation of C/C0 = u/(u + Kx) or C/C0 = 1/(1 + Kt) is definitely valid in field installations operating under conventional conditions.

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