Aerobes and Effluents - Environmental Biotechnology: Theory and Application - page 131

Activated Sludge Systems
This approach was first developed in Manchester, just prior to the outbreak of the
First World War, to deal with the stronger effluents which were being produced in
increasingly large amounts by the newly emerging chemicals industry and were
proving too toxic for the currently available methods of biological processing.
Treatment is again achieved by the action of aerobic microbes, but in this method,
they form a functional community held in suspension within the effluent itself and
are provided with an enhanced supply of oxygen by an integral aeration system.
This is a highly biomass-intensive approach and consequently requires less space
than filter to achieve same treatment. The main features are shown in Figure 6.4.
The activated sludge process has a higher efficiency than the previously
described filter system and is better able to adapt to deal with variability in the
wastewater input, both in terms of quantity and concentration. However, very
great changes in effluent character will challenge it, since the resident microbial
community is generally less heterogeneous than commonly found in filters.
Additionally, as a more complex system, initial installation costs are higher
and it requires greater maintenance and more energy than a trickling filter of
comparable throughput.
In use, the sludge tanks form the central part of a three-part system, comprising
a settlement tank, the actively aerated sludge vessels themselves and a final
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 pre-treatment 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 microbial
biomass present. These micro-organisms represent a complex and integrated
Figure6.4 Schematicactivatedsludgesystem