(b) Contact aerator. The contact aerator (see Figure 2-20) consists of
a contact bed made up of cement-asbestos plates on which a biological film develops.
An aerobic condition is maintained by blowing air through the contact section from
perforated pipes mounted below the contact plates.
Figure 2-20. Contact aerator.
(c) Mechanical type. Mechanical aerators use impellers, revolving
disks, or brushes for spraying wastewater into the air or pulling air down into the
wastewater. Figure 2-21 illustrates equipment for pulling wastewater up through a
cone-shaped draft tube from the bottom of the tank and spraying it into the air by means
of an impeller. Air mixed with the spray is diverted downward into the tank by the
diffuser cone. This method of aeration is used in small plants. If circular tanks are
used, internal wall baffles will be provided to prevent potential vortexing (liquid vacuum
in the center) problems.