Activated Sludge Treahnent Plants
In order to be able to estimate the sludge production by means of Expression (4.13)
the yield constant, Yobs must be estimated. This constant varies, for example
according to the loading of the activated sludge plant, see Fig 3.5.
Example 4.4
Calculate sludge loading and sludge production for an activated sludge plant for the treatment of municipal wastewaters. The following is known:
0 1 = 11 ,000 m 3 /d
C1 = 0.4 kg BOD/m 3
C3 = 0.04 kg BOD/m 3
The concentration of activated sludge in the aeration tank is set at 5.2 kg SS/m 3 . The
volume, V2, of the aeration tank is calculated at 5,600 m 3 .
The sludge loading is calculated from Expression (4.11 ):
Bx = 01 · C1/(V2 · X2)
Substitution of these values gives:
Bx = 11,000 · 0.4/5,600 · 5.2 = 0.15 kg BOD/(kg SS · d).
The sludge production is estimated from Expression (4.13):
Fsp = Yabs (C1- C3) · 01
(4.13)
Y obs is estimated (for example from Fig 3.5) at 0.8 kg SS/kg BOD. Substitution of these
values gives:
Fsp = 0.8 (0.4- 0.04) · 11 ,000 = 3,170 kg SS/d.
(Note that in the denominator the yield constant, Y obs· must have the same unit as the
concentration, C, in this case BOD. The numerator may have any unit (SS, VSS, COD,
TN, TP, etc.)
It is not all of the sludge production which is further treated in the sludge treahnent
section of the activated sludge plant. Some of the sludge production leaves the
treahnent plant together with the treated water. It corresponds to the quantity Q3 ·
X3 in Fig 4.4.
Excess sludge production, FEsr, is that part of the sludge production which is
actively withdrawn and further treated at the treahnent plant. Using the symbols
from Fig 4.4, the quantity is Qs · Xs. The excess sludge production can be withdrawn
from the recycle flow as shown in Fig 4.4, together with the primary sludge by
leading it to the influent of the treahnent plant if there is mechanical treahnent.
If sludge age control is required in the plant, direct excess sludge withdrawal from
the aeration tank is the best solution, see Example 4.5.
Sludge age is the mean cell residence time of sludge (biomass) in the plant. It can be
estimated on the basis of measurements on an existing plant by means of Expression
(4.14):
8x= Mx/Fsr
(4.14)
123
In order to be able to estimate the sludge production by means of Expression (4.13)
the yield constant, Yobs must be estimated. This constant varies, for example
according to the loading of the activated sludge plant, see Fig 3.5.
Example 4.4
Calculate sludge loading and sludge production for an activated sludge plant for the treatment of municipal wastewaters. The following is known:
0 1 = 11 ,000 m 3 /d
C1 = 0.4 kg BOD/m 3
C3 = 0.04 kg BOD/m 3
The concentration of activated sludge in the aeration tank is set at 5.2 kg SS/m 3 . The
volume, V2, of the aeration tank is calculated at 5,600 m 3 .
The sludge loading is calculated from Expression (4.11 ):
Bx = 01 · C1/(V2 · X2)
Substitution of these values gives:
Bx = 11,000 · 0.4/5,600 · 5.2 = 0.15 kg BOD/(kg SS · d).
The sludge production is estimated from Expression (4.13):
Fsp = Yabs (C1- C3) · 01
(4.13)
Y obs is estimated (for example from Fig 3.5) at 0.8 kg SS/kg BOD. Substitution of these
values gives:
Fsp = 0.8 (0.4- 0.04) · 11 ,000 = 3,170 kg SS/d.
(Note that in the denominator the yield constant, Y obs· must have the same unit as the
concentration, C, in this case BOD. The numerator may have any unit (SS, VSS, COD,
TN, TP, etc.)
It is not all of the sludge production which is further treated in the sludge treahnent
section of the activated sludge plant. Some of the sludge production leaves the
treahnent plant together with the treated water. It corresponds to the quantity Q3 ·
X3 in Fig 4.4.
Excess sludge production, FEsr, is that part of the sludge production which is
actively withdrawn and further treated at the treahnent plant. Using the symbols
from Fig 4.4, the quantity is Qs · Xs. The excess sludge production can be withdrawn
from the recycle flow as shown in Fig 4.4, together with the primary sludge by
leading it to the influent of the treahnent plant if there is mechanical treahnent.
If sludge age control is required in the plant, direct excess sludge withdrawal from
the aeration tank is the best solution, see Example 4.5.
Sludge age is the mean cell residence time of sludge (biomass) in the plant. It can be
estimated on the basis of measurements on an existing plant by means of Expression
(4.14):
8x= Mx/Fsr
(4.14)
123