Hydraulic and Process Aspects of
Reactor Design — I. Basic Concepts in
Steady-State Analysis
J. C. McLELLAN, FWPCA Fellow
A. W. BUSCH, Professor
Environmental Engineering Department
Rice University
Houston, Texas
INTRODUCTION
There has been a great deal of confusion with respect to the relative roles of
hydraulic and process characteristics of the activated sludge process. The "old"
procedure in activated sludge process design was to choose an aeration tank volume
which afforded some preselected aeration period without regard to the strength of
the incoming waste. Most persons will readily accept the concept that it is not
logical to design a reactor which is intended for use in treating "normal" domestic
sewage on the same basis as a unit which will be used in treating a high-strength
industrial waste. For this reason, the concept arose that there is a limiting
"strength" of waste which can be treated effectively by activated sludge and
therefore "strong" wastes require dilution. The relatively recent utilization of
completely mixed systems has shown that there is no need for dilution of strong
wastes.
The ASCE-WPCF manual on sewage treatment plant design (1) now suggests
that the aeration tank capacity may be computed from the daily weight of BOD
input expected and the BOD-to-solids loading ratio chosen for design. The manual states that neither the sewage hold-up time nor the mixed-liquor aeration period is of prime significance.
The various modifications of the activated sludge process are generally classified in terms of the loading: i.e., the number of lbs of BOD per day per 1, 000 cu
ft of aeration tank or, more logically, lbs of BOD per day per lb of mixed liquor
solids. A generally accepted concept is that the effluent quality, in terms of BOD
remaining in the effluent, is "best" at some loading factor (e.g., 35 lb BOD/day/
100 lb MLSS) and then deteriorates as the loading factor increases (2). Here again,
the difference in industrial and domestic wastes should be considered. For an industrial waste high in soluble organics, it is probably that a loading of, say, 35
lb BOD/day/100 lb MLSS will be too low, whereas with a municipal waste, this
same loading may be too high due to the need for time in which to hydrolize insoluble organic components of the waste. In other words, the rate at which the
process proceeds is not related solely to loading but also involves the characteristics of the waste.
In 1955, Haseltine (3) pointed out that the efficiency of the activated sludge
process in treating domestic waste as measured by BOD removal is related directly
to the weight of total or volatile activated sludge solids carried in the aeration
tanks and inversely to the BOD loading applied to those tanks.
In the design of fermentation units,  the microbiologist has long recognized
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