The secondary sedimentation tank is an important component of the activated sludge system. It is used to clarify the mixed liquor and recover and concentrate the activated sludge. Its effect directly affects the quality of the effluent and the concentration of the return sludge. Because the settling and concentration effects are not good, the activated sludge suspended solids in the effluent increase, thereby increasing the BOD mass concentration of the effluent; at the same time, the return sludge concentration also decreases, thereby reducing the mixed liquor concentration in the aeration tank and affecting the purification effect. [1-2]

The activated sludge mixed liquor entering the secondary sedimentation tank also has its own characteristics, namely, the concentration of the activated sludge mixed liquor is high (2,000–4,000 mg/L) and has flocculation performance, therefore it belongs to hindered settling. When it settles, there is a clear interface between sludge and water; the flocs settle as a whole and sink together; the settling velocity of the initial sludge-water interface is fixed and only related to the initial concentration. The concentration of the mixed liquor entering the secondary sedimentation tank is higher than that of the clarified liquid in the secondary sedimentation tank, and secondary flow is easily generated in the secondary sedimentation tank; the relative density of the influent mixed liquor is large and flows in the lower part of the tank. [1]

The mixed liquor entering the secondary sedimentation tank is a three-phase mixture of sludge, water, and gas. The flow velocity in the central inlet pipe should not exceed 0.1–0.3 m/s to facilitate gas-water separation and improve the separation effect of the clarification zone. The flow velocity in the clarification zone of the sedimentation tank should be even smaller (about 0.0004 m/s), because its task of sludge-water separation is more important. [1]

Another characteristic of the activated sludge in the secondary sedimentation tank is that it is light and easily carried away by the effluent, and prone to secondary flow, making the actual flow section far smaller than the designed flow section. Therefore, when designing the secondary sedimentation tank, the maximum allowable horizontal flow velocity should be half that of the primary sedimentation tank; the outlet overflow weir is usually set within a certain distance range at the other end of the tank; the radial-flow secondary sedimentation tank can also use peripheral inlet and peripheral outlet to increase the flow distance and settling effect of the mixed liquor in the tank. [1-2]

The main difference between the secondary sedimentation tank and the primary sedimentation tank lies in the different treatment objects and roles. The treatment object of the secondary sedimentation tank is the activated sludge mixed liquor, which has the characteristics of high concentration (2,000–4,000 mg/L), flocculation, light quality, and slow settling velocity. When settling, there is a clear interface between sludge and water, belonging to hindered settling. In addition to sludge-water separation, the secondary sedimentation tank also plays the role of sludge concentration. Two kinds of settling occur simultaneously in the secondary sedimentation tank: lamellar settling and compression settling. Lamellar settling satisfies the clarification requirement, and compression settling completes the sludge concentration function. Therefore, compared with the primary sedimentation tank, the area required is larger than the tank area required for sludge-water separation. The surface loading rate is used to calculate the area of the secondary sedimentation tank during design, and the solid flux is used for verification. [1-2]

Because the activated sludge in the secondary sedimentation tank is light and prone to decay and deterioration, therefore, for the secondary sedimentation tank using hydrostatic pressure sludge discharge, its hydrostatic head can be reduced to 0.9 m, and the angle between the sludge hopper bottom slope and the horizontal should not be less than 50° to facilitate timely sliding down of the sludge and smooth sludge discharge. For the sedimentation tank using a scraper-suction sludge machine for sludge discharge, the sludge is siphoned into the sludge collection trough by the water level difference between the tank water level and the sludge collection trough water level, and then collected in the sludge discharge well, and the sludge pump in the sludge discharge well discharges the sludge. [1]