3 System Modifications and Design Criteria
In the past, the conventional activated sludge process was used exclusively because
engineers were handling relatively small-volume and low-strength wastewaters.
Later in dealing with wastewater of greater volume and strength and with industrial
wastewater in particular, which has a complex chemical nature, engineers were
forced to operate the activated sludge process differently as well as to modify the
process design in order to suit special needs. This led to the development of many
process modifications that have become standardized today [28].
In this section, both the conventional activated sludge process and its modifications are discussed. The characteristics, removal efficiency, and application and
design parameters are listed for comparison [29].
3.1 Conventional Activated Sludge Process
The process employs a long rectangular aeration tank. Biological sludge is collected
from the final clarifier. Part of the sludge is wasted from the sludge return line, and
the recycled portion, together with the wastewater influent (overflow from primary
clarifier), enters the aeration tank at the head end. The mixed liquor is aerated for
approximately 4–8 h during which time synthesis and respiration take place with the
organic matter being oxidized. The mixed liquor has a MLSS (mixed liquor
suspended solids) of 1500–3000 mg/L. The sludge settles in the final clarifier until
there are 8000–10,000 mg/L, when it is withdrawn off for partial return. The return
rate is approximately 15–50% of the influent flow rate (Fig. 3.10a).
Hydraulically, the conventional process is operated in a quasi-plug flow mode
using a long and deep rectangular tank. Some longitudinal dispersion and some short
circuiting are expected, and a true plug flow regime is not obtained. A true plug flow
system is a more efficient process, as was discussed earlier. It is, however, more
susceptible to shock loads if the aeration tank is divided into a series of complete mix
reactors; Jenkins [30] has shown that improvement in treatment performance can be
obtained without a major loss in the ability of the system to handle shock loads.
Toerber [31] reports that in response to a severe shock load, the plug flow operation
in long rectangular tanks has a removal efficiency only 10% smaller than does the
complete mix operation on a BOD basis. A conventional process in reality comes
close to a series of complete mix reactors, and consequently its treatment performance is one of the best among all activated sludge processes in use. Nevertheless,
the conventional process is designed for and applicable to low-strength wastewater
only, for example, domestic wastewater. The requirement of a larger aeration tank,
and consequently a larger land area, further inhibits its use.
3 Biological Processes
99
In the past, the conventional activated sludge process was used exclusively because
engineers were handling relatively small-volume and low-strength wastewaters.
Later in dealing with wastewater of greater volume and strength and with industrial
wastewater in particular, which has a complex chemical nature, engineers were
forced to operate the activated sludge process differently as well as to modify the
process design in order to suit special needs. This led to the development of many
process modifications that have become standardized today [28].
In this section, both the conventional activated sludge process and its modifications are discussed. The characteristics, removal efficiency, and application and
design parameters are listed for comparison [29].
3.1 Conventional Activated Sludge Process
The process employs a long rectangular aeration tank. Biological sludge is collected
from the final clarifier. Part of the sludge is wasted from the sludge return line, and
the recycled portion, together with the wastewater influent (overflow from primary
clarifier), enters the aeration tank at the head end. The mixed liquor is aerated for
approximately 4–8 h during which time synthesis and respiration take place with the
organic matter being oxidized. The mixed liquor has a MLSS (mixed liquor
suspended solids) of 1500–3000 mg/L. The sludge settles in the final clarifier until
there are 8000–10,000 mg/L, when it is withdrawn off for partial return. The return
rate is approximately 15–50% of the influent flow rate (Fig. 3.10a).
Hydraulically, the conventional process is operated in a quasi-plug flow mode
using a long and deep rectangular tank. Some longitudinal dispersion and some short
circuiting are expected, and a true plug flow regime is not obtained. A true plug flow
system is a more efficient process, as was discussed earlier. It is, however, more
susceptible to shock loads if the aeration tank is divided into a series of complete mix
reactors; Jenkins [30] has shown that improvement in treatment performance can be
obtained without a major loss in the ability of the system to handle shock loads.
Toerber [31] reports that in response to a severe shock load, the plug flow operation
in long rectangular tanks has a removal efficiency only 10% smaller than does the
complete mix operation on a BOD basis. A conventional process in reality comes
close to a series of complete mix reactors, and consequently its treatment performance is one of the best among all activated sludge processes in use. Nevertheless,
the conventional process is designed for and applicable to low-strength wastewater
only, for example, domestic wastewater. The requirement of a larger aeration tank,
and consequently a larger land area, further inhibits its use.
3 Biological Processes
99