Engineering Aspects of Surface
Aeration Design
W. WESLEY ECKENFELDER, Professor
Environmental Health Engineering
DAVIS L. FORD, Assistant Director
Center for Research in Water Resources
The University of Texas
Austin, Texas
INTRODUCTION
In recent years mechanical surface aerators have become increasingly popular
for application in aerated lagoons and the activated sludge process. The primary
reason for this is the high relative oxygenation efficiency and the relative simplicity of installation and maintenance. Most surface aeration units can be installed on piers mounted on a concrete pad in the bottom of the aeration basin,
off a wall-supported bridge, or on a float or pontoon assembly. In general the
pier or bridge mounted units are most popular in the activated sludge process while
the floating units find principle application in aerated lagoons.
Surface aerators in use today can be roughly classified in three groupings; a)
an impeller which induces flow across the blades resulting in a vortexing action
and a surface liquid spray, b) a surface impeller with a draft tube from which
liquid is sprayed, and c) a submersible pump which pumps liquid against a deflector plate from which it is sprayed over the water surface. The oxygen transfer
rate for units classified under a) and b) is related to impeller diameter and speed
and to the submergence of the rotating element. The performance of these units
is expressed in terms of oxygenation efficiency of lbs 02 transferred/hr/HP. The
oxygenation efficiency is usually reported at 20 C and zero DO in water. The
power drawn may be expressed as shaft or net horsepower or gross horsepower which
includes electrical and mechanical losses. For most units there is a speed and
submergence which will yield an optimum oxygenation efficiency as schematically
shown in Figure 1. Manufacturers' reported oxygenation efficiencies are usually
based on these optimums.
When developing aeration design and process application there are several
factors which must be considered. These are: a) the oxygenation efficiency under
the specified aeration conditions; b) mixing conditions in the aeration basin; and
c) the oxygen transfer in the aeration liquid relative to water ( a) •
It has recently been shown that the oxygenation efficiency of surface aeration
units varies with the volume of liquid under aeration. This is logical when one
considers the mechanism of gas transfer involved. As shown in Figure 2, liquid
from the basin with an oxygen concentration Cj is drawn through the impeller and
sprayed into air. Oxygen is transferred from air to liquid raising the DO concentration to C2. The amount of oxygen transferred will depend upon the interfacial
surface exposed and on the total volume of liquid pumped. When the liquid
strikes the tank surface, turbulent mixing and air entrainment result in additional
oxygen transfer. On one test conducted by the writer on a surface aerator, about
60 per cent of the oxygen transfer resulted from the liquid spray and 40 per cent
from turbulence and entrainment.   If we increase the aeration liquid volume for a
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