Activated Sludge Treatment Plants
If the tank has a plug flow system, the concentration varies throughout the tank
from XB,2.1 in the influent to XB,2.2 in the effluent.
For activated sludge plants with recycle it often applies that X2.1- X2.2· This does not
apply to plants without recycle.
Example 4.3
Calculate the effluent concentration, X6,2.2, of activated sludge from the aeration tank
of an activated sludge plant. The following is known (for symbols, see Fig 4.2):
Xa,2.1 = 4.5 kg SS/m 3
C, = 0.4 kg BOD/m 3
C3 = 0.015 kg BOD/m 3 (15 mg BOD/I)
Y obs = 0.9 kg SS/kg BOD
a, = 2,500 m 3 /d
a4 = 2,000 m 3 /d
X6.2.2 can be calculated from Expression (4.6)
Xs.2.2 = Xa.2.1 + (C2.1 - C2.2l · Y obs
C2.1 and C2.2 are not known.
(4.6)
C2.1 and C2.2 are not the total BOD-concentration, but should be assumed to represent the BOD content of the raw wastewater (in the plant it is mixed into the activated
sludge with its very high BOD concentration).
It is normally assumed that no reactions take place in the settling tank, that is, C2_ 2 =
Cs.
C2.1 can be found by means of a BOD-balance at point a (Fig 4.2):
a,· c, + a4 · c4 =(a,+ a4) · c2.1
Assuming that removals only take place in the aeration tank, C4 = C3 = C2.2; substitution of these values gives:
C2.1 =(a,· c, + a4 · Cs)/(a, + a4)
Substitution of these values by known values gives:
C2; = (2,500 · 0.4 + 2,000 · 0.015)/(2,500 + 2,000) = 0.229 kg BOD/m 3
Substitution of these values by known values into Expression (4.6) gives:
Xa.22 = 4.5 + (0.229- 0.015) · 0.9 = 4.69 kg SS/m 3 .
The example shows that the variation from the influent entering the aeration tank to
the effluent leaving the tank is, in this case, only an increase of the sludge concentration of 4-5 per cent. Considering the uncertainties in the determination of the incoming
quantities, the variation may often be ignored and the sludge concentration may be assumed to be constant throughout the tank.
NOTE! The variation cannot be ignored during the process of calculating the sludge production which takes place in the plant (see next chapter).
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If the tank has a plug flow system, the concentration varies throughout the tank
from XB,2.1 in the influent to XB,2.2 in the effluent.
For activated sludge plants with recycle it often applies that X2.1- X2.2· This does not
apply to plants without recycle.
Example 4.3
Calculate the effluent concentration, X6,2.2, of activated sludge from the aeration tank
of an activated sludge plant. The following is known (for symbols, see Fig 4.2):
Xa,2.1 = 4.5 kg SS/m 3
C, = 0.4 kg BOD/m 3
C3 = 0.015 kg BOD/m 3 (15 mg BOD/I)
Y obs = 0.9 kg SS/kg BOD
a, = 2,500 m 3 /d
a4 = 2,000 m 3 /d
X6.2.2 can be calculated from Expression (4.6)
Xs.2.2 = Xa.2.1 + (C2.1 - C2.2l · Y obs
C2.1 and C2.2 are not known.
(4.6)
C2.1 and C2.2 are not the total BOD-concentration, but should be assumed to represent the BOD content of the raw wastewater (in the plant it is mixed into the activated
sludge with its very high BOD concentration).
It is normally assumed that no reactions take place in the settling tank, that is, C2_ 2 =
Cs.
C2.1 can be found by means of a BOD-balance at point a (Fig 4.2):
a,· c, + a4 · c4 =(a,+ a4) · c2.1
Assuming that removals only take place in the aeration tank, C4 = C3 = C2.2; substitution of these values gives:
C2.1 =(a,· c, + a4 · Cs)/(a, + a4)
Substitution of these values by known values gives:
C2; = (2,500 · 0.4 + 2,000 · 0.015)/(2,500 + 2,000) = 0.229 kg BOD/m 3
Substitution of these values by known values into Expression (4.6) gives:
Xa.22 = 4.5 + (0.229- 0.015) · 0.9 = 4.69 kg SS/m 3 .
The example shows that the variation from the influent entering the aeration tank to
the effluent leaving the tank is, in this case, only an increase of the sludge concentration of 4-5 per cent. Considering the uncertainties in the determination of the incoming
quantities, the variation may often be ignored and the sludge concentration may be assumed to be constant throughout the tank.
NOTE! The variation cannot be ignored during the process of calculating the sludge production which takes place in the plant (see next chapter).
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