possible, resulting in a significant reduction in unit size compared to the conventional process.
Either a long rectangular aeration tank or a rectangular tank subdivided into four
or more parallel channels can be used. In either case, the first portion of the aeration
tank can be used for reaeration of the return activated sludge alone. This adds to the
flexibility of operation since recondition of return sludge by sludge aeration sometimes is essential for maintaining top treatment performance. The arrangement of
subdividing the aeration tank into multiple parallel channels (Fig. 3.10b) further adds
flexibility to the process operation. In small cities and communities where fluctuations of wastewater flow and strength are expected, a portion of the aeration tank can
be shut off to accommodate lower flow. This operation is very important if uniform
treatment performance is to be maintained. Flexibility in operation provided is
therefore an important feature of the step aeration process.
3.3 Complete Mix Process
The complete mix process is most useful for the treatment of wastewater with
fluctuating organic strength because it is least susceptible to shock loads among all
activated sludge processes. For this reason, the process has found increasing popularity among those particularly concerned with the treatment of industrial wastewater
with moderate to high strength. A mechanically stirred reactor test simulates a
complete mix condition (Fig. 3.10c). When diffused aeration in a rectangular tank
is used, the mixture of wastewater influent and the return sludge should enter the
aeration tank at several points in a central location with the effluent going into
channels on the sides of the aeration tank.
The process can be better described mathematically than other activated sludge
processes (see Sect. 2). In general, both volumetric loading and F/M ratio are higher
than that used in conventional and step aeration processes. A high MLVSS concentration is maintained to accommodate the strong organic wastewater that leads to the
higher sludge recycle ratio required, even though the sludge retention time is
comparable to those of the step aeration and conventional processes.
It should be emphasized here that although comparable treatment efficiencies are
obtained for the complete mix process, the conventional process, and the step
aeration process, the higher F/M ratio employed in the operation of a complete
mix process tends to leave more soluble in the effluent. Consequently an effluent
with a lower quality is expected.
3.4 Extended Aeration Process
The extended aeration process is operated hydraulically, rather more like a complete
mix than any other activated sludge process (Fig. 3.10d). This is made possible by
3 Biological Processes
101
Either a long rectangular aeration tank or a rectangular tank subdivided into four
or more parallel channels can be used. In either case, the first portion of the aeration
tank can be used for reaeration of the return activated sludge alone. This adds to the
flexibility of operation since recondition of return sludge by sludge aeration sometimes is essential for maintaining top treatment performance. The arrangement of
subdividing the aeration tank into multiple parallel channels (Fig. 3.10b) further adds
flexibility to the process operation. In small cities and communities where fluctuations of wastewater flow and strength are expected, a portion of the aeration tank can
be shut off to accommodate lower flow. This operation is very important if uniform
treatment performance is to be maintained. Flexibility in operation provided is
therefore an important feature of the step aeration process.
3.3 Complete Mix Process
The complete mix process is most useful for the treatment of wastewater with
fluctuating organic strength because it is least susceptible to shock loads among all
activated sludge processes. For this reason, the process has found increasing popularity among those particularly concerned with the treatment of industrial wastewater
with moderate to high strength. A mechanically stirred reactor test simulates a
complete mix condition (Fig. 3.10c). When diffused aeration in a rectangular tank
is used, the mixture of wastewater influent and the return sludge should enter the
aeration tank at several points in a central location with the effluent going into
channels on the sides of the aeration tank.
The process can be better described mathematically than other activated sludge
processes (see Sect. 2). In general, both volumetric loading and F/M ratio are higher
than that used in conventional and step aeration processes. A high MLVSS concentration is maintained to accommodate the strong organic wastewater that leads to the
higher sludge recycle ratio required, even though the sludge retention time is
comparable to those of the step aeration and conventional processes.
It should be emphasized here that although comparable treatment efficiencies are
obtained for the complete mix process, the conventional process, and the step
aeration process, the higher F/M ratio employed in the operation of a complete
mix process tends to leave more soluble in the effluent. Consequently an effluent
with a lower quality is expected.
3.4 Extended Aeration Process
The extended aeration process is operated hydraulically, rather more like a complete
mix than any other activated sludge process (Fig. 3.10d). This is made possible by
3 Biological Processes
101