Concepts and definitions of the activated sludge process
Fig 4.4
Activated sludge plant with sludge recycle and excess sludge withdrawal,
Qs · Xs.
The sludge in the influent (Q1 · X1) is included in the sludge production and should
therefore not be deducted in Expression (4.12).
The sludge production can be measured in many ways and hence be given in
different units, for example kg SS/d, kg VSS/d and kg COD/d.
Table 4.4 shows which components are included in the different measuring methods
for sludge and sludge production.
Sludge production Inorganic susOrganic suspended Biological growth, C hemical
component ~
pended solids, raw solids, raw
a era lion tank
precipitants
wastewater
wastewater
Unit for sludge
production!
kg SS/ d
+
+
+
+
kgVSS / d
+
+
(+)
kgCOD/ d
+
+
Table 4.4
Measuring methods lor sludge and sludge production and the sludge components included in the
methods.
Different units for the sludge production are needed, depending on the purpose of
application. For sludge dewatering, the unit kg of SS/ dis relevant; in the case of an
anaerobic or an aerobic sludge stabilization, the unit kg of VSS/ d or kg of
COD(B)/d is relevant.
The volume of the sludge production thus depends both on the influent of wastewater to the plant, and on the process in the plant.
The sludge production can be measured in an existing plant and is calculated from
Expression (4.12).
The sludge production can be estimated in a design situation by means of Expression (4.13):
(4.13)
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