Design Factors

The design objective of primary sedimentation is to produce settled water with the lowest possible turbidity. For effective filtration, the turbidity of settled water should not exceed 10NTU. Since effective sedimentation is closely linked with coagulation and flocculation, the wastewater treatment plant must ensure that the best possible floc is formed.

The flow should be distributed uniformly across the inlet of the basin (see Figure 7.17.6). The solids removal efficiency of a clarifier is reduced by the following conditions:

• Eddy currents induced by the inertia of the incoming fluid

• Surface current produced by wind action (see part D in Figure 7.17.6). The resulting circulating current can short-circuit the influent to the effluent weir and scour settled particles from the bottom.

• Vertical currents induced by the outlet structure

• Vertical convection currents induced by the temperature difference between the influent and the tank contents (see parts B and C in Figure 7.17.6).

• Density currents causing cold or heavy water to underrun a basin, and warm or light water to flow across its surface (see part B in Figure 7.17.6).

• Currents induced by the sludge scraper and sludge removal system

Therefore, factors such as the overflow rate, detention period, weir-loading rate, shape and dimensions of the basin, inlet and outlet structures, and sludge removal system affect the design of a sedimentation basin (Table 7.17.1).

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