Control of Biological Solids Concentration in
The Extended Aeration Process
C.Y. YANG, Research Associate
Department of Agricultural Engineering
Cornell University
Ithaca, New York 14850
A.F. GAUDY, JR., Director
Bioengineering and Water Resources
Oklahoma State University
Stillwater, Oklahoma 74074
INTRODUCTION
The extended aeration (total oxidation) process which has been known for three
decades, has the unique distinction of being one of the most widely used activated sludge
processes, although it has been concluded to be theoretically unsound; i.e., the total
oxidation (autodigestion) of the biological population responsible for purification has
been thought to be an impossibility. Some of the work upon which such conclusions had
been based have been reviewed in previous publications from this laboratory (1,2,3). Our
most recent publication bearing on this process has shown that extracellular bacterial
polysaccharide, previously thought to be biologically inert, served as an excellent source of
carbon for growth of heterogeneous microbial populations. (4).
Also in previous reports (1,2) we have presented results of long-term laboratory pilot
plant studies in which an extended aeration pilot plant was operated for over 1000 days with
positive, total recycly of sludge. There was no inadvertent wastage of solids, since the
clarifier was "backed-up" by centrifuging all effluent, with return of all solids to the aeration
chamber. Throughout this period, excellent substrate removal efficiency was maintained
and the system gave no indication of building up an inert organic fraction in the sludge.
There was no indication of imminent biochemical failure, and we were led to the conclusion
that the total oxidation principle of the extended aeration process was not theoretically
unsound. The periodicity and amplitude of the irregular cycles of solids accumulation and
de-accumulation which were observed could not be predicted. There were times when
biological solids concentration in the mixed liquor was so high as to impair their separation
in the clarifier, and had we not been centrifuging, biological solids would have escaped in
the effluent, it was reasoned that de-accumulation periods (i.e., periods of accelerated
autodigestion) could be initiated by providing an engineering assist to the process by
withdrawing some of the sludge, hydrolyzing it, thus performing chemically the initial
stages of breakdown of macromolecules, and recycling these liquefied cells. We termed this
proposed process the "hydrolytic assist;" a flow diagram embodying the principle is shown
in Figure 1. Preliminary experiments indicated that cell hydrolysate was an excellent
substrate for the growth of microorganisms, and it was decided to run the laboratory pilot
plant in accordance with Figure 1.
The former mode of operation (centrifugation of cells) was continued for some time
beyond the 1000 days of operation previously reported. Finally, after 1202 days, we
terminated this phase herein referred to as Phase A), and initiated operations to examine
the feasibility of the "hydrolytic assist" (herein referred to as Phase B). The purpose of the
present report is to present the results of various experiments designed to gain insight into
the chemical and biochemical nature of the hydrolyzed cells and to present results of pilot
plant operations employing the hydrolytic assist.
MATERIALS AND METHODS
The pilot plant was the same as that employed in Phase A studies. Total operating
volume was 9.4 liters (6.2-liter aeration chamber and 3.2-liter settling chamber). The overall
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