consumed by the microorganisms; others are thought to be converted by enzyme action
from objectionable matter to stable substances. Activated sludge absorbs dissolved
organic material, including ammonia. High quality activated sludge settles rapidly,
leaving a clear, odorless, stable liquid above.
(3)
Principles of the process.
(a) After receiving primary settling, wastewater is mixed with activated
sludge to form a mixed liquor. The mixed liquor then receives prolonged aeration in an
aeration tank and is conveyed to the final settling tank from which a clear, effluent liquid
is usually discharged without further treatment except chlorination. The sludge
collected at the tank's bottom is returned, all or in part, to the influent end of the aeration
tank. It is mixed with incoming settled wastewater to continue the purification process.
Excess sludge is usually pumped to the plant influent and is resettled with the primary
sludge to concentrate it. Figure 2-18 illustrates graphically the flow in an activated
sludge plant.
Figure 2-18. Flow diagram of an activated sludge plant.
(b) Approximately 80 to 85 percent of wastewater treatment occurs
during the first hour of aeration. Aeration not only assists the microorganisms in getting
oxygen, but also causes contact of activated sludge with the wastewater solids to create
floc. Floc becomes heavy and settles. During the rest of the period, sludge is
regenerated, organic matter stabilized, and sludge conditioned for further activity. The
sludge's capacity for taking in organic matter is limited and regeneration is necessary
before an additional load is applied. Activated sludge requires oxygen at a definite,
measurable rate. It absorbs oxygen from the water as rapidly with only a few parts per
million in solution as when the water is almost saturated. The activated sludge process
MD0161
2-28
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