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 | ||

Problem |
Possible cause |
Consequence |

Deflocculation (dispersed |
Overaeration |
Turbid effluent |

growth) |
Bacteria failing to aggregate | |

Slime or nonfilamentous |
Development of nonfilamentous microbes leading to |
Poor solid settleability |

bulking |
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 |
Low SVI | |

Blanket rising |
Gas generation processes lift flocs to the surface of |
Surface of clarifiers covered with |

equipment |
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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