SELF-PROPELLED AERATORS
Boris M. Khudenko, Associate Professor
Department of Civil Engineering
Wayne State University
Detroit, Michigan 48202
For small communities and some industrial plants, extensive (low rate) treatment process,
such as aerated lagoons, oxidation ditches and their modifications, offer several advantages
over other types of treatment processes. These processes demonstrate better stability in
treatment efficiency and simpler operation than activated sludge processes. Extensive treatment processes with forced aeration are also superior to nonaerated lagoons because mixing
and aeration permit a two- to sixfold increase in pond or tank depth, and produce overall
treatment rates at least two to four times greater than in nonaerated lagoons.
The major disadvantage of extensive treatment methods with artificial aeration is their
relatively high energy consumption, which is about 3-10 times greater per kg of BOD
removed, than the conventional activated sludge process. Only a small fraction of this
excessive energy consumption is due to the nitrification process. The high energy requirement is primarily due to the stationary nature of presently employed aeration equipment,
including both anchored mechanical and diffused air aerators. Since the effectiveness of the
aeration process depends on the amount of mixing in the reservoirs, thereby suspending the
activated sludge, either four to ten times the number of stationary aeration units needed to
satisfy the oxygen requirement are installed, or additional mixing devices are employed.
Either of these approaches causes additional, and perhaps unnecessary, energy consumption.
An alternative approach would be the use of mobile, and particularly self-propelled,
aerators. Such devices can provide a more balanced energy usage between aeration and
mixing, thus reducing energy consumption and operation costs for wastewater treatment in
lagoons. The major parameters of self-propelled aerators are the linear velocity of their
movement upon the water surface, their period of rotation, the service area, and oxygenation capacity. The determination of these parameters for mechanical and jet aerators is
discussed in the following sections.
SELF-PROPELLED MECHANICAL AERATORS
A self-propelled mechanical aerator is illustrated in Figure 1. This aerator consists of an
open-bladed-rotor and motor-gear mounted on a platform supported by floats. A hinged arm
links the floating structure to the pile. The aerator is free to rotate around the pile. The
hinges compensate for water level fluctuations in the basin.
Linear Velocity and Period of Revolution
The linear velocity and period of revolution of the aerator can be determined by equating
the tractive, F, and drag, D, forces:
I=D (1)
The tractive force depends on the torque MQ and the arm length L, or,
F = M„/L (2)
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