Sludge Production Rates In
Activated Sludge Systems
RAYMOND F. DRNEVICH, Staff Engineer
DANIEL W. GAY, Staff Engineer
Union Carbide Corporation
Linde Division
Tonawanda, New York
INTRODUCTION
The rate of sludge production in activated sludge systems is an important factor
affecting the solids waste handling requirements for most sewage treatment plants. As much
as two thrids of the solid waste produced by primary-secondary sewage treatment schemes
can be a result of the biological activity in the activated sludge unit. Dewatering and
disposal facilities for handling the sludge from a biological treatment plant typically
comprise 25 to 35 percent of the total annual capital, operating and maintenance costs. If
the activated sludge process can be made to produce 30 percent less sludge at comparable
operating conditions and obtain this reduction without an increase in construction,
operating, and maintainance costs a 5 to 8 percent decrease in the annual operating
expenses may be realized.
The factors affecting the rate of sludge production in activated sludge systems have
been the subjects of many investigations (1,2,3,4,5,6,7,8,9,10,11,12). In general, the results
of studies on factors other than  organic loading have been inconsistent.
Work done by Union Carbide Corporation (1,2) at Batavia, NY demonstrated that as
much as a 40 percent decrease in sludge production may be achieved by employing pure
oxygen as the aerating gas in the activated sludge process. The results of the Batavia project
led to a laboratory research program to determine the factor or factors responsible for the
lower sludge production rates. The only independent variables that exhibited significant
variation in the Batavia study were mixed liquor dissolved oxygen concentration (DO),
mixed liquor pH, composition of the aerating gas, and reactor configuration (conventional
air and staged systems for oxygen). In order to determine whether DO, pH, and/or
composition of the aerating gas were responsible for the observed difference sludge
production a set of carefully designed experiments was initiated. The object of the first part
of this paper is a description of these experiments.
A factorial experimental (13) design was chosen so that differences in sludge
production could be evaluated by statistical analysis (analysis of variance). This
experimental technique enables one to distinguish observed differences caused by changes
in the independent variables from those caused by experimental error. Further, the factorial
design gives the investigator the ability to determine the individual effect of the independent
variables (DO, pH, and composition of the aerating gas) as well as the effect of a
combination of these variables.
The Batavia project also prompted the Environmental Protection Agency to conduct
its own investigation on oxygen versus air activated sludge systems. The results of this study
have been reported by Brenner (8) and will be summarized here as well.
Since the Batavia study, Union Carbide Corporation has been involved in an extensive
pilot plant program. Such studies have been completed at 36 locations across the United
States. As a result, several opportunities to compare oxygen and air activated sludge
systems have arisen. However, the operational mode of many of the air activated sludge
pilot plants deviated significantly from the standard operating mode of full scale facilities.
These differences made the comparison between projected full scale air and oxygen systems
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