Control of Activated Sludge Processes

A large number of control schemes have been applied to activated sludge processes. The control schemes can be broadly classified into two categories: mea surement based control and model based control. Pure measurement based control schemes - such as proportional (P), proportional integral (PI), propor tional integral derivative (PID), and fuzzy control

Table 8 Problems encountered in sludge-water separation


Possible cause


Deflocculation (dispersed


Turbid effluent


Bacteria failing to aggregate

Slime or nonfilamentous

Development of nonfilamentous microbes leading to

Poor solid settleability


deficiency in either nitrogen or phosphorous

Solid carry-over from clarifier

Filamentous bulking

High numbers of filamentous microbes in the mixed liquor

High SVI

Solid overflow from clarifiers

Pin point floc

Breakage of small and weak flocs

Turbid effluent

Smaller fragments fail to settle


Blanket rising

Gas generation processes lift flocs to the surface of

Surface of clarifiers covered with


buoyant layer of biomass

Exacerbated by long retention of solids in classifiers

Foaming/scum formation

Excessive hydrophobic bacteria

Biomass floatation to surface by foams

Solid carry-over from clarifiers

Table 9 Model classification

Model classes Representations Sub-classes

Empirical (black-box) Various formats for sub-classes Input-output functional models

Neural network models Qualitative models

Distributed parameter models (DPMs) Partial differential equations (PDEs) Mechanistic models

Gray-box models

Lumped parameter models (LPSs) Ordinary differential or differential algebraic Reduced from PDEs equations (ODEs or DAEs) CSTR in series

Linear models Linear difference equations or ODEs Linearization of nonlinear ODEs

Input-output model Reduced order models strategies - do not require mathematical models. In contrast to measurement based control, mathematical models are essential for model based control, includ ing dynamic matrix control (DMC), generic model control (GMC), linear quadratic (LQ) control, generic distributed parameter model control (GDPMC), and Hm control. The main control schemes applied to activated sludge processes are listed in Table 10.

Model types used in various model based control schemes are listed in Table 11 .

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