Components of the
Sludge Loading Ratio and Their Effect
on the Bulking of Activated Sludge
E. J. GENETELLI, Research Associate,
H. HEUKELEKIAN, Professor and Chairman,
Department of Environmental Science,
Rutgers, the State University
New Brunswick, New Jersey
Considerable attention has been devoted in recent years to the theoretical
aspects of the activated sludge process. We are all familiar with the conventional
flow diagram of an activated sludge plant and with the phenomena known as
"bulking." One of the most important factors affecting this bulking phonomena
is the type of organic material present in the substrate. The relationship existing
between the quantity of material in the substrate and the BOD removal efficiency
of the process has led to the development of many concepts proposed for use as a
loading parameter. Loadings have been expressed in terms of lbs of BOD/1000 cu
ft of aeration tank volume. In the literature, however, constant BOD removal
efficiencies (87-90) per cent have been reported (1, 2) at loadings of from 30-120
lbs/1000 cu ft, thus indicating the ineffectiveness of this ratio as a general loading parameter. Parameters based on tank volumes do not take into account two
major factors, namely, the amount of solids under aeration and the length of the
aeration period.
Gould (1939) suggested the term "sludge age" as a measure of the quantity
of organic material entering the activated sludge process (3). This term may be
defined as the average length of time a particle of suspended solids remains under
aeration. It was expressed as the ratio of the lbs of mixed liquor solids under
aeration to the lbs of suspended solids entering the system per day and thus had a
unit of days.
When dealing with sewage, sludge age is adequate since the suspended solids
concentration in mg/1 usually is about equal to the BOD concentration in mg/1.
The substitution of the lbs of BOD entering the system per day in place of the lbs
of suspended solids is necessary when dealing with relatively soluble organic
wastes.
Actually Harris, Cockburn and Anderson (4), back in 1926, were the first to
propose the combination of the three major factors into a loading parameter applicable to the activated sludge process. The factors evaluated were the BOD of
the applied waste, the quantity of sludge present in the aeration tank, and the
period of aeration. Since then ample evidence has been presented to demonstrate
that the effectiveness of the activated sludge process and the amount of air required for its operation are primarily dependent upon the daily BOD input/mixed
liquor solids ratio (MLSS).
The National Research Council Sub-Committee on Military Sewage Treatment (5) established a loading parameter expressed as lbs of BOD applied daily
per 1000 lbs of suspended solids in the aeration tank.
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