also requires nutrients in the form of carbon, nitrogen, and phosphorous. These
nutrients help the microorganisms grow.
(c) Bulking of activated sludge caused by excessive growths of
hairlike bacteria can usually be corrected and controlled by applying chlorine to the
return sludge. Overchlorination upsets the process and must be avoided. When
bulking has been continuous, constant application of small amounts of chlorine to the
return sludge may maintain a low sludge index; partial prechlorination of the raw
wastewater may have the same effect.
(d) Volume of activated sludge returned from final settling tanks to
aeration tanks normally ranges from 20 to 40 percent of the raw wastewater flow. A
high rate of return reduces aerator detention time but keeps sludge fresh and may
return needed dissolved oxygen to the aerator inlet. A low rate of return increases
aerator detention time; it is feasible when the sludge has a low rate of oxygen utilization
and does not readily become septic. A high return sludge concentration is obtained with
low return rate.
(4) Types of aerators. Three types of aerators in common use in activated
sludge treatment plants are different air, contact aeration, and mechanical.
(a) Diffused air type. These tanks are rectangular in shape and allow
compressed air to be admitted through diffuser plates or tubes. About 95 to 98 percent
of the air keeps the contents in motion, the rest is for oxidation. Figure 2-19 pictures an
empty tank equipped with diffuser tubes.
Figure 2-19. Aeration tank with diffuser tubes.