Environmental Engineering FOURTH EDITION

Wastewater Treatment 187

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Diffused Aeration

Figure 9-15. Activated systems with diffused aeration. (Photo courtesy of Envirex.)

shown in Fig. 9-17. Some causes of poor settling are improper or varying FIM ratios,
fluctuations in temperature, high concentrations of heavy metals, or deficiencies in
nutrients. Cures include chlorination, changes in air supply, or dosing with hydrogen
peroxide to kill the filamentous microorganisms. When sludge does not settle, the return
activated sludge is “thin” because SS concentration is low, and the microorganism
concentration in the aeration tank drops. A higher FIM ratio results, since there are
fewer microorganisms to handle the same food input, and the BOD removal efficiency
is reduced.

Biological Process Dynamics Applied to the Activated Sludge System

The organic materials in the influent meet two fates in an activated sludge system. Most
are oxidized to C02 and water, but some high-energy compounds are used to build new
microorganisms. The latter are known as substrate in biological process dynamics.