Temperature Prediction in Activated Sludge
Basins Using Mechanical Aerators
DAVIS L. FORD, Manager
Engineering-Science of Texas
Austin, Texas
C. S. SHIH, Associate Professor
Industrial Engineering Department
Texas A & M University
College Station, Texas
ED C. SEBESTA, Project Engineer
Engineering-Science of Texas
Houston, Texas
INTRODUCTION
The expanding use of mechanical aerators for oxygenating activated sludge basins
coupled with increasingly stringent temperature and organic effluent criteria underscores
the need for accurately predicting temperature balances throughout the wastewater
treatment system. It should first be recognized that a mechanically aerated activated sludge
basin is both a cooling pond and a biological reactor. As the degree of heat dissipation
dictates the equilibrium basin temperature which in turn influences the efficiency of organic
removal via biochemical oxidation, the importance of temperature prediction is apparent.
Paradoxically, many biological treatment systems are designed with little or no reference to
thermal effects. The purpose of this paper, therefore, is to present a design approach for
predicting a temperature profile across a mechanically aerated basin, and estimating the
resultant biological removal capacity and effluent quality of the system.
GENERAL REVIEW
A review of pertinent historical information is necessary in order to provide a basis for
developing a rational temperature-prediction approach. As heat loss from mechanical
aerators, temperature effects on biological systems, and regulatory constraints with respect
to effluent temperature and organic residuals are all interrelated, each of these aspects is
included in this review:
Aeration Basin Heat Loss
Mechanical aerators in activated sludge basins serve not only to oxygenate the mixed
liquor, but also to increase heat transfer from the basins. The intimate contact of air and the
water of the aerator spray achieve this heat loss although the heat dissipation per kw-hr is
much higher in cooling towers. Mechanically aerated basins used as cooling ponds per se
are difficult to justify on a strict economical basis, although their cooling capacity may be an
added benefit when utilized as an oxygenating device in an activated sludge system receiving
high-temperature wastewaters.
The total heat dissipated in a mechanically aerated basin is the sum of the losses in the
aerator spray cloud and the losses due to exposed water surface. Assuming the usual "water
warmer than air" case, the heat loss through a spray cloud can be estimated by multiplying
the enthalpy (heat content) change of the air flowing through the cloud by the air flow rate
(1). The net heat loss from the unaerated water surface includes the effects of evaporation,
convection, conduction, radiation, solar radiation, aerator mechanical energy, and heats of
reactions. The general heat loss equation can be written as follows:
Heat Loss = H£V * Hcy + HCD + HRA - H§R - HM - H£x - HBX (I)
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