Study of an
Extended Aeration Plant and Effluent
Effect on the Receiving Watercourse
GROVER L. MORRIS, Senior Sanitary Engineer
LOWELL A. VAN DEN BERG, Assistant Sanitary Engineer
GORDON L. CULP, Assistant Sanitary Engineer
JACK R. GECKLER, Biologist
RALPH PORGES, Sanitary Engineer Director
Technical Advisory and Investigations Section
Division of Water Supply and Pollution Control
Robert A. Taft Sanitary Engineering Center
Cincinnati, Ohio
INTRODUCTION
Extended-aeration sewage treatment is a modification of the activated sludge
process. In its basic form it consists of an aeration tank with 18 to 24 hrs' detention, a final settling tank with three to six hrs detention, and 100 per cent return
of settleable solids to the aeration tank. Many modifications of this basic concept
have been developed in an effort to provide better control of the process.
A preliminary evaluation of extended aeration as used in the United States
has been presented by Porges and Morris (1). There has been an impressive increase in the number of plants from three in 1950 to 1,224 in 1960, and to over
2,600 in 1962.
The statement has often been made that the extended-aeration process is
capable of producing a high-quality effluent if plants are properly designed and
operated (2,3,4). When separate sludge wasting is not practiced, a clear effluent with a low BOD can be produced until the mixed liquor suspended solids build
up to equilibrium, after which solids are discharged in the effluent and lower the
effluent quality (5).
Early studies reported lack of data on the fate of the aerobically digested
sludge discharged in the plant effluent and the effect of those solids on the receiving stream (1). Recent reports indicate that it is impossible to operate an extended aeration system without sludge accumulation (5,6,7,8).
It has been noted that incomplete nitrification in an extended-aeration plant
can cause a substantial increase in the effluent BOD over that exerted by the solids
and the small demand exerted by unassimulated organics. Generally, control of
the discharge of solids in the effluent has been considered the primary factor affecting the efficiency of extended-aeration plants.
Because many small plants have been built without provisions for separate
sludge wasting, state agencies have been concerned about the effects of discharged
solids from these plants on the receiving watercourses.
To determine the effect of a basic (no control of solids) extended-aeration
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