Environmental Engineering Reference
In-Depth Information
8
ChaPTer
aCTivaTed sludge
ProCess
The biological treatment systems discussed to this point—ponds,
trickling filters, and rotating biological contactors (RBCs)—have been
around for years. The trickling filter, for example, has been around and
successfully used since the late 1800s. The problem with ponds, trick-
ling filters, and RBCs is that they are temperature sensitive, remove less
BOD, and cost more to build (particularly trickling filters) than the acti-
vated sludge systems that were later developed.
The activated sludge process follows
primary settling. The basic components
of an activated sludge sewage treatment
system include an aeration tank and a
secondary basin, settling basin, or clari-
fier (see Figure 8.1). Primary effluent is
mixed with settled solids recycled from the secondary clarifier and is
then introduced into the aeration tank. Compressed air is injected con-
tinuously into the mixture through porous diffusers located at the bot-
tom of the tank, usually along one side. Wastewater is fed continuously
into an aerated tank, where the microorganisms metabolize and bio-
logically flocculate the organics. Microorganisms (activated sludge) are
settled from the aerated mixed liquor under quiescent conditions in the
final clarifier and are returned to the aeration tank. Left uncontrolled,
the number of organisms would eventually become too great; therefore,
some must periodically be removed (wasted). The concentrated solids at
the bottom of the settling tank that must be removed from the process
are known as waste activated sludge (WAS). Clear supernatant from the
final settling tank is the plant effluent.
Key Point:
Although trickling filters and other
systems cost more to build than activated sludge
systems, it is important to point out that activated
sludge systems cost more to operate because of
the need for energy to run pumps and blowers.
