4.3. Activated sludge
The essence of the continuous flow activated sludge
process is the integration of the aeration tank
(aerobic biological reactor), secondary sedimentation tank and sludge recirculation line. These
three components are maintained in the alternative of activated sludge systems acting as posttreatment of effluents from anaerobic reactors.
The intermittent flow activated sludge system
(sequencing batch reactors) can also be adopted
as post-treatment, requiring, in this case, only the
tanks that alternate in the functions of reaction
and sedimentation. Recent developments regarding the application of such system for the posttreatment of anaerobic effluents are discussed in
the last item of this paper.
When the activated sludge system acts as
post-treatment of anaerobic effluents, the anaerobic reactor is used instead of the primary sedimentation tank (which is an integral part of the
conventional activated sludge system). The aerobic sludge is recirculated in the usual manner,
that is, from the bottom of the secondary sedimentation tank to the entrance of the aerobic
reactor (aeration tank).
The excess aerobic sludge generated in the
activated sludge stage, not yet stabilised, is sent
to the UASB reactor, where it undergoes thickening and digestion, together with the anaerobic
sludge. As the return flow of the excess aerobic
sludge is very low compared with the influent
flow, there are no operational disturbances in the
UASB reactor. The sludge treatment is largely