Activated Sludge Treatment Plants
(it is assumed that Mx = Vz · Xz, and this is usually a good assumption if the aerobic
sludge age is used as the design basis and hence is included in the expression).
Example 4.8
Using the sludge loading, design an activated sludge plant to treat the wastewater in
Table 4.6 to an effluent concentration of 0.020 kg BOD/m 3 .
The sludge concentration in the activated sludge plant is in dry weather 4.5 kg SS/m 3
and in wet weather 4.0 kg SS/m 3 .
From Example 4.7:
Esoo = 0.92 dry weather
Esoo = 0.88 wet weather
From Expression (4.21) is found:
V2.dry weather= (1 ,200 kg BOD/d)/(4.5 kg SS/m 3 ) · Bx,BOD
Table 4.5 shows that a sludge loading of 0.05-0.1 kg 800/(kg SS · d) gives a treatment efficiency for BOD of 90-95 per cent. As a treatment efficiency of 92 per cent is needed, the
necessary Bx.soo - 0.07 (dry weather) is estimated. For wet weather Bx,soo - 0,2 is assumed.
v2.dry weather= 1 ,200/(4.5 . 0.07) = 3,810 m 3
v2.wet weather= 1 ,440/(4.0 . 0.20) = 1,800 m 3
The aeration tank must be approx. 3,800 m 3 , as the dry-weather loading here proves
to be the design basis. The volume found is here greater than the 3,000 m 3 which was
found in Example 4.7. The reason is that the sludge concentration in the aeration tank
is somewhat low. As design by means of volumetric loading does not take this into consideration, the necessary volume is underestimated. (The result may at worst be that
the plant does not meet the effluent requirements, and that those who designed the
plant may be held liable in damages).
4.3.8 Computer-aided plant design
Computer programs of varying complexity are available which may support the
design of activated sludge plants. The programs can calculate the influence of the
diurnal variations on the treatment result. Some of the programs are based on the
process matrix shown in Table 4.7. For the design of a conventional, uncomplicated
activated sludge plant, such programs may not be needed whereas they are vital for
the design of more complicated plant constructions.
An essential problem in connection with the use of the computer models is the
characterization of the wastewater and the determination of stoichiometric and
kinetic constants. If there are no detailed analyses of the discharged wastewater, an
estimate must be made based on Tables 1.7-1.10. For ordinary municipal waste137
(it is assumed that Mx = Vz · Xz, and this is usually a good assumption if the aerobic
sludge age is used as the design basis and hence is included in the expression).
Example 4.8
Using the sludge loading, design an activated sludge plant to treat the wastewater in
Table 4.6 to an effluent concentration of 0.020 kg BOD/m 3 .
The sludge concentration in the activated sludge plant is in dry weather 4.5 kg SS/m 3
and in wet weather 4.0 kg SS/m 3 .
From Example 4.7:
Esoo = 0.92 dry weather
Esoo = 0.88 wet weather
From Expression (4.21) is found:
V2.dry weather= (1 ,200 kg BOD/d)/(4.5 kg SS/m 3 ) · Bx,BOD
Table 4.5 shows that a sludge loading of 0.05-0.1 kg 800/(kg SS · d) gives a treatment efficiency for BOD of 90-95 per cent. As a treatment efficiency of 92 per cent is needed, the
necessary Bx.soo - 0.07 (dry weather) is estimated. For wet weather Bx,soo - 0,2 is assumed.
v2.dry weather= 1 ,200/(4.5 . 0.07) = 3,810 m 3
v2.wet weather= 1 ,440/(4.0 . 0.20) = 1,800 m 3
The aeration tank must be approx. 3,800 m 3 , as the dry-weather loading here proves
to be the design basis. The volume found is here greater than the 3,000 m 3 which was
found in Example 4.7. The reason is that the sludge concentration in the aeration tank
is somewhat low. As design by means of volumetric loading does not take this into consideration, the necessary volume is underestimated. (The result may at worst be that
the plant does not meet the effluent requirements, and that those who designed the
plant may be held liable in damages).
4.3.8 Computer-aided plant design
Computer programs of varying complexity are available which may support the
design of activated sludge plants. The programs can calculate the influence of the
diurnal variations on the treatment result. Some of the programs are based on the
process matrix shown in Table 4.7. For the design of a conventional, uncomplicated
activated sludge plant, such programs may not be needed whereas they are vital for
the design of more complicated plant constructions.
An essential problem in connection with the use of the computer models is the
characterization of the wastewater and the determination of stoichiometric and
kinetic constants. If there are no detailed analyses of the discharged wastewater, an
estimate must be made based on Tables 1.7-1.10. For ordinary municipal waste137