Mass balances, anaerobic plants
Sludge age,
80
days
~
70
~
~
60
~
~
50
~
~
40
~
~
30
~
~
~
20
~
~
10
~
0
0
Temperature
20
30
40
50
60
70
c
Fig 9.3
Necessary sludge age in an anaerobic wastewater treatment plant as a function of the
temperature.
In Fig 9.3, the necessary sludge age, 9x,M is shown as a function of the temperature.
The volume of the tank, V'b can be found from Expression (9.4):
288
SxM · Fsr
v 2 = _:..:•::.:.:...___:c:..
XB,M,2
(9.4)
Example 9.1
An anaerobic contact plant (suspended sludge) is going to treat wastewater from a sugar factory. The net growth rate of the methane producing bacteria at 30°C has been
measured at 0.05 d- 1 . How high is the sludge age going to be?
From Expression (9.3) we find:
9x,M = 1/IJobs.M.net = 1/0.05 d- 1 = 20 d
From Fig 9.3 we can estimate the necessary sludge age at 20-28 d. The necessary
sludge age is therefore in the lower end of the normal experience range.
What would the necessary sludge age be estimated at if a thermophilic process at
55°C was to be used?
We see from Fig 9.3 that ex.M- 6-12 d. If it is assumed that the net growth rate is high
at 55°C, just like at 30°C, the necessary sludge age can be estimated at 6 d.