Figure 2-21. Mechanical aeration tank.
(5) Variations. A number of possibilities exist for varying the activated
sludge process. Three will be mentioned here.
(a) Step aeration. This process is designed to offset the effects of
fluctuations in influent and sludge quality. Instead of introducing the influent at one
point, as in the conventional unit, it is introduced uniformly at a number of points along
the aeration tank as shown in Figure 2-22. Thus, the large aeration basin is actually
broken down into smaller basins. The use of this modification allows for absorption of
organics at more than one point in the tank. Therefore, there is accelerated growth and
oxidation spread out over the basin. Also, by distributing the organic load over the
entire tank, it is possible to decrease the detention time in the tank and to maintain a
lower solids concentration in the mixed liquor. BOD reduction lies between 85 percent
and 95 percent. To implement this type of upgrading, it is necessary to modify influent
piping, renovate the air system, and expand the sludge recycle capacity. Primary and
secondary clarifier capacity must be checked and increased to handle the higher loads.