One of the important operations in the activated sludge processes is liquid-solid separation. Because of differ ences in material characteristics, models for primary and secondary settlers are different. The secondary settler separates the biomass from the treated wastewater, a key mechanism determining the quality of the effluent since biomass in the effluent affects both the water clarity and oxygen demand. Consequently, more research emphasis is placed on the secondary settlers than primary ones.

Flows in a typical secondary settler are shown in Figure 7. The mixed liquor flow entering the settler splits in two: one flowing upward and over an overflow weir, and

Table 6 Unit operations and devices for pretreatment and primary treatment

Unit operations

Devices

Objectives

Chemicals

Solid classification

Grit separation

Comminution Equalization

Neutralization Gravity sedimentation

(primary settler) Precipitation

Bar racks

Hydro and mechanical fine screens Velocity controlled and aerated grit chambers Comminutors Equalization basins

Neutralization basins Circular tanks and rectangular tanks Sedimentation tanks

Large solid removal Moderate-size particle removal Grit removal to avoid device damage Solid maceration Equalization of flow and/or concentration Regulation of pH level Liquid-solid separation

Agglomeration of tiny particles

H2SO4, HCl, CO2, lime, NaOH

Al2(SO4)3, FeCl3, lime, organic polymers

Table 7 Unit operations and devices for secondary treatment

Unit operations

Objectives

Devices

Hydraulics

Chemical reactions

Fermentation

Biomass growth

Multiphase mass transfer Gravity sedimentation (secondary settler)

Fluid flow through vessels

Organic carbon removal Organic nitrogen removal Organic phosphorous removal

To convert fermentable COD (SF) to VFA (SA) for PAO

microorganism Heterotrophic growth Autotrophic growth PAO biomass growth Oxygen transfer Liquid-solid separation

Concrete tanks with overflow weirs, pumps Aerobic reactor Aerobic and anoxic reactors Aerobic, anoxic, and anaerobic reactors Anaerobic reactor

Aerobic and anoxic reactors Aerobic and anoxic reactors Anaerobic reactor Aerated sludge vessels Circular and rectangular tanks

Anaerobic Anoxic

Figure 6 Nutrient removal mechanisms in activated sludge processes.

Feed

Water flows

Settling solids

Figure 7 Settler flows.

Concentrated underflow

Figure 7 Settler flows.

the other flowing downwards and being withdrawn at the bottom.

The biomass concentration varies with height in the settler as well as with time. This makes the settler a distrib uted parameter system (DPS) described by PDEs, and leads to quite complicated mathematical models. Another key feature of the settler modeling is the flux model for the solid settlement. Two commonly adopted models are the compartment model and multilayer model, which are shown in Figure 8. In the compartment model, a settler is divided into two compartments, namely clear water compartment and sludge compartment. Two sets of differential algebraic equations (DAEs) are developed to describe the dynamics of sludge hold ups in the two compartments, suspended solid concentration in the overflow stream, and sludge blanket concentration in the sludge compartment. The multilayer model is obtained through discretization of PDS models represented by PDEs using a finite difference method. In the multilayer model, the settler is divided into n layers as shown in the bottom diagram in Figure 8. The value of n may vary from 10 to 100. Obviously, the multilayer model provides more accurate prediction of the settler dynamics than the compartment model but at the price of more complicated mathematical representations and much longer computing time.

Both one dimensional (1D) and two dimensional (2D) secondary settler models have been applied to circular tanks. A comparative study between 1D and 2D models has been reported in the literature. The

L |
Clear water compartment |

Sludge compartment |

Compartment model

Overflow

Compartment model

Concentrated underflow

-> |
Layer 1 |

Layer 2 to layer m - 1 | |

Layer m | |

Layer m + 1 to layer n - 1 | |

Layer n |

Multilayer model

Overflow

Multilayer model

Figure 8 Compartment and multilayer models for secondary settlers.

Concentrated underflow

Figure 8 Compartment and multilayer models for secondary settlers.

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