Activated Sludge Process - Wastewater Operators - page 296

systems use pressurized air released through diffusers near the bottom
of the tank. Efficiency is directly related to the size of the air bubbles
produced. Fine bubble systems have a higher efficiency. The diffused air
system has a blower to produce large volumes of low-pressure air (5 to 10
psi), air lines to carry the air to the aeration tank, and headers to distrib-
ute the air to the diffusers, which release the air into the wastewater.
8.2.3 settling Tank
Activated sludge systems are equipped with plain settling tanks
designed to provide 2 to 4 hours of hydraulic detention time.
8.2.4 return sludge
The return sludge system includes pumps, a timer or variable speed
drive to regulate pump delivery, and a flow measurement device to deter-
mine actual flow rates.
8.2.5 Waste activated sludge
In some cases the waste activated sludge withdrawal is accom-
plished by adjusting valves on the return system. When a separate sys-
tem is used it includes pumps, a timer or variable speed drive, and a flow
measurement device.
8.3 overvieW of aCTivaTed sludge ProCess
The activated sludge process is a treatment technique in which
wastewater and reused biological sludge full of living microorganisms
are mixed and aerated. The biological solids are then separated from the
treated wastewater in a clarifier and are returned to the aeration process
or wasted. The microorganisms are mixed thoroughly with the incom-
ing organic material, and they grow and reproduce by using the organic
material as food. As they grow and are mixed with air, the individual
organisms cling together (flocculate). Once flocculated, they more read-
ily settle in the secondary clarifiers.
The wastewater being treated flows continuously into an aeration
tank where air is injected to mix the wastewater with the returned acti-
vated sludge and to supply the oxygen required by the microbes to live
and feed on the organics. Aeration can be supplied by injection through
air diffusers in the bottom of the tank or by mechanical aerators located
at the surface. The mixture of activated sludge and wastewater in the
aeration tank is called the mixed liquor . The mixed liquor flows to a sec-
ondary clarifier where the activated sludge is allowed to settle.
The activated sludge is constantly growing, and more is produced
than can be returned for use in the aeration basin. Some of this sludge
must, therefore, be wasted to a sludge handling system for treatment