130 Environmental Biotechnology
Figure 6.4 Schematic activated sludge system
clarifier for secondary sedimentation. The first element of the set-up allows heavy
particles to settle at the bottom for removal, while internal baffles or a specifically
designed dip pipe off-take excludes floating materials, oil, grease and surfactants.
After this physical pretreatment phase, the wastewater flows into, and then
slowly through, the activated sludge tanks, where air is introduced, providing
the enhanced dissolved oxygen levels necessary to support the elevated micro-
bial biomass present. These micro-organisms represent a complex and integrated
community, with bacteria feeding on the organic content in the effluent, which are
themselves consumed by various forms of attached, crawling and free-swimming
protozoa, with rotifers also aiding proper floc formation by removing dispersed
biomass and the smaller particles which form. The action of aeration also creates
a circulation current within the liquid which helps to mix the contents of the tank
and homogenise the effluent while also keeping the whole sludge in active suspen-
sion. Sludge tanks are often arranged in batteries, so that the part-treated effluent
travels though a number of aeration zones, becoming progressively cleaned as
it goes.
At the end of the central activated phase, the wastewater, which contains
a sizeable sludge component by this stage, leaves these tanks and enters the
clarifiers. These are often designed so that the effluent enters at their centre and
flows out over a series of weirs along the edge of the clarifier. As the wastewater
travels outward, the heavier biological mass sinks to the bottom of the clarifier.
Typically, collector arms rotate around the bottom of the tank to collect and
remove the settled biomass solids which, since they contain growing bacteria that
have developed in the aeration tanks, represent a potentially valuable reservoir
of process-acclimatised organisms.
Accordingly, some of this collected biomass, termed the return activated sludge
(RAS), is returned to the beginning of the aeration phase to inoculate the new