Extended Aeration Process
A Field Study of the
JAMES J. WESTRICK, Research Assistant
JAMES M. MORAND,   Assistant Professor
J. DAVID EYE, Associate Professor
University of Cincinnati
Cincinnati, Ohio
INTRODUCTION
The extended aeration modification of the activated sludge process is being
used extensively for the treatment of low volume wastes. In this process it is theoretically possible to remove a large fraction of the degradable organic matter contained in the influent waste by maintaining a low organic loading to active cell
mass ratio, an ample supply of dissolved oxygen, and a completely mixed system.
The ratio of food to active mass is usually designated as lbs of BOD per day per lb
of volatile suspended solids (lbs BOD/day/lbs VSS). The ratio normally used in
extended aeration units ranges from 0.05 to 0.20 (1). The volumetric loading
generally ranges from 12 to 20 lbs BOD/day/1000 cu ft of aeration tank capacity (2).
The combination of a large mass of organisms in the presence of low available
substrate and a long aeration period leads to the destruction of cell mass by auto-
oxidation. Ideally such a system should provide complete oxidation of the sludge
with no net accumulation of sludge to be disposed of or wasted. It has been reported that about 20 to 25 per cent of the cell mass that is produced is biologically
non-degradable and that there is a slow accumulation of inert biological material
in the system (3). Additionally, non-degradable organic and inorganic solids
brought into the unit by the influent sewage will tend to accumulate unless controlled sludge wasting is practiced.
Without controlled sludge removal or wasting, the solids in an extended aeration plant will increase until the solids handling capacity of the sedimentation unit
is exceeded, with resultant decreased operating efficiency. Under proper conditions of operation the effluent from an extended aeration plant should be well oxidized. Nearly all of the residual carbonaceous BOD in the effluent theoretically
should represent only the oxygen utilized by the active biological mass undergoing
endogenous respiration (4).
Because of the potential advantages of the extended aeration process over
conventional treatment procedures for treating small volume wastes, the State of
Ohio Department of Health by 1962 had authorized the construction of some 2600
individual plants.
The design criteria stipulated by the Ohio Department of Health for extended
aeration plants (5) are: 1) The aeration tank shall provide a detention time of 24
hrs at the design rate of flow not including recirculation; 2) the settling tanks shall
provide a detention period of at least four hrs at the average daily flow rate,
not including recirculation; 3) air will be supplied in an amount of three cfm/ft of
aeration tank length or 2100 cu ft of air/lb of BOD entering the tank each day; and
4) the recirculation ratio shall be at least 1:1 based on the average daily flow.
It appears that most extended aeration plants now in use have been designed in
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