Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>
4.2.1 Activated sludge treatment
The term 'activated sludge' refers to sludge in the aeration
tank of an activated sludge treatment process. It consists of flocs of bacteria,
which consume the biodegradable organic substances in the wastewater. Because of
its usefulness in removing organic substances from wastewater, the sludge is
kept in the process by separating it from the treated wastewater and
re-circulating it. A typical arrangement of an activated sludge process is
schematically shown in Figure 2.24.
Wastewater entering an activated sludge treatment plant is
usually passed through a bar screen to remove gross materials such as napkins,
rags and other materials which may damage mechanical equipment further down the
treatment plant. The bar screen consists of vertical bars separated by a
distance of about 1 cm. Screened solids are continually scraped off the bars.
The screenings can be landfilled or incinerated.
Figure 2.24: Schematic diagram of an activated sludge wastewater treatment
Sand and similar heavy particles are removed next in a grit
chamber. This chamber can be aerated to separate these particles from other
suspended solids. The wastewater spends a relatively short period in the grit
chamber (in the order of minutes). The sedimented sand and grit is usually
The finer solids are removed in a settling or sedimentation
tank, where the wastewater spends of the order of an hour to allow the solids to
settle or float. The mechanical removal of solids as described above is usually
called 'primary treatment', the sedimentation tank as primary sedimentation
tank, the overflow from the sedimentation tank as primary-treated wastewater
(primary effluent) and the sludge produced as primary sludge.
The primary-treated wastewater is then passed to an aeration
chamber. Aeration provides oxygen to the activated sludge and at the same time
thoroughly mixes the sludge and the wastewater. Aeration is by either bubbling
air through diffusers at the bottom of the aeration tank, or by mechanically
agitating the surface of the water.
In the aeration tank the bacteria in the activated sludge
consume the organic substances in the wastewater as described in Section 2
(2.3). The organic substances are utilised by the bacteria for energy, growth
and reproduction. The wastewater spends in the order of a few hours in the
aeration chamber before entering a second sedimentation tank to separate the
activated sludge from the treated wastewater. The activated sludge is returned
to the aeration tank. There is an increase in the amount of activated sludge
because of growth and reproduction of the bacteria. The excess sludge is wasted
to maintain a desired amount of sludge in the system. This part of the treatment
process is called 'secondary treatment', the sedimentation tank as secondary
sedimentation tank, the overflow from the sedimentation tank as
secondary-treated wastewater (secondary effluent) and the excess activated
sludge as secondary sludge.
Depending on the flow rate of wastewater, several parallel
trains of primary and secondary stages can be employed. There are several ways
to operate an activated sludge process. In a 'high rate' process a relatively
high volume of wastewater is treated per unit volume of activated sludge. The
high amount of organic waste consumed by the activated sludge produces a high
amount of excess sludge. In an 'extended aeration' mode of operation the
opposite condition takes place. A relatively low amount of organic waste is
treated per unit volume of sludge with little excess sludge to be removed.
Removal of BOD is higher in the extended aeration mode compared to the high rate
mode, but more wastewater can be treated with the latter mode.
An activated sludge treatment plant is a highly mechanised
plant, and is suited to automated operation. The capital cost for building such
a plant is relatively high. The energy requirement, particularly for providing
air to the aeration tank, is also relatively high. There is a need for regular
maintenance of the mechanical equipment, which requires skilled technical
personnel and suitable spare parts. The operation and maintenance costs of an
activated sludge treatment plant are therefore relatively high.
An activated sludge treatment process can be operated in batches
rather than continuously. One tank is allowed to fill with wastewater. It is
then aerated to satisfy the oxygen demand of the wastewater, following which the
activated sludge is allowed to settle. The treated wastewater is then decanted,
and the tank is filled with a new batch of wastewater. At least two tanks are
needed for the batch mode of operation, constituting what is called a
‘sequential batch reactor (SBR)'. SBRs are suited to smaller flows, because the size of
each tank is determined by the volume of wastewater produced during the
treatment period in the other tank.