Concepts and definitions of the activated sludge process
For the plant shown in Fig 4.1:
Mx=Vz · Xz
and Xz = X3 (based on ideal mix)
(4.15)
where ex
is sludge age,
e
is hydraulic retention time.
In the simple plant in Fig 4.1 the retention times for biomass and water are identical.
This is not the case for an activated sludge plant with recycle. The whole idea of
recycling is actually to increase the sludge age over and above the hydraulic
retention time.
For the plant in Fig 4.4:
Mx=Vz · Xz
(the sludge mass in the settling tank being set at 0 which is not quite correct)
Substitution into Expression (4.14) gives:
(4.16)
Expression (4.16) is general and also applies for an activated sludge plant without
recycle where Xz = Yobs · (C1 - C3) (Expression (4.4)), which, if substituted into
Expression (4.16), gives ex= e (=Expression (4.15)).
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Example4.5
Calc,:ulate the hydraulic retention time in the aeration tank (ideal mix) and the sludge
age in the following activated sludge plant:
OaXa
The following is known:
V2 = 15,000 m 3
X2 = 4.0 kg COD/m 3
a, = 1 ,500 m 3 /h