By: Kate
Email:kate@aquasust.com
Date: 4th December 2024
The role of the secondary sedimentation tank in sewage treatment
The role of the secondary sedimentation tank is to separate mud and water to clarify the biologically treated mixed liquor, and at the same time to concentrate the sludge in the mixed liquor. The secondary sedimentation tank is the last link in the biological treatment of sewage, and plays a decisive role in ensuring that the suspended solids content of the effluent water quality is qualified.
If the secondary sedimentation tank is not set up reasonably, even if the biological treatment effect is very good, the content of dissolved organic matter in the mixed liquor is already very small, and the effect of mud and water separation of the mixed liquor in the secondary sedimentation tank is not ideal, the effluent water quality may still be unqualified.
If the sludge concentration effect is not good, the amount of microorganisms returned to the aeration tank will be difficult to guarantee, and the reduction in the concentration of the aerated mixed liquor will lead to a decrease in the sewage treatment effect, which will in turn affect the effluent water quality.
Basic requirements for setting up secondary sedimentation tanks
1. The hydraulic load is generally 0.5-1.8m3/(m2·h). When treating industrial wastewater, the proportion of organic matter in the activated sludge is relatively large, and the SVI of the mixed liquor in the aeration tank is relatively high. The secondary sedimentation tank matched with it should adopt a lower surface hydraulic load.
2. In order to ensure that the sludge can be sufficiently concentrated in the secondary sedimentation tank so as to supply the return sludge of the required concentration of the aeration tank, the solid surface load of the secondary sedimentation tank is 150kg/(㎡·d), and the solid surface load of the inclined tube (plate) secondary sedimentation tank can be expanded to 192kg(㎡·d).
3. The water depth at the edge of the secondary sedimentation tank should be 2.5-4m. The specific value is related to the size of the tank body. The larger the diameter of the secondary sedimentation tank, the water depth at the edge should be appropriately increased, otherwise the hydraulic efficiency of the secondary sedimentation tank will be reduced and the effective volume will be reduced. For secondary sedimentation tanks with diameters of 10-20m, 20-30m, 30-40m and >40m, the water depths at the pool edge are 3.0m, 3.5m, 4.0m and 4.0m respectively. When the above pool edge water depths cannot be achieved due to various reasons, a lower surface load value must be used to maintain the sedimentation time unchanged.
4. The overflow rate (or load) of the secondary sedimentation tank outlet weir is 1.5-2.9L/(m·s).
5. When mechanical sludge discharge is used, the volume of the sludge zone of the secondary sedimentation tank shall be calculated according to the residence time for the sludge to be concentrated to the required concentration. The volume of the sludge zone of the activated sludge method secondary sedimentation tank is generally 2-4h sludge volume, and continuous sludge discharge measures shall be taken. The volume of the sludge zone of the biofilm method secondary sedimentation tank is generally 4h sludge volume.
6. In order to reduce energy consumption, it is best to use low-head, high-flow equipment such as screw pumps or axial flow pumps for sludge return. If forced aeration is used, an air lift pump can also be used to simplify equipment management and maintenance.
Precautions for the operation and management of secondary sedimentation tanks
1. Check and adjust the water distribution equipment of the secondary sedimentation tank regularly to ensure that the mixed liquid flow entering each secondary sedimentation tank is uniform.
2. Check the slag accumulation in the slag bucket and discharge it in time, and rinse the slag bucket with water frequently. At the same time, pay attention to whether the slag scraper and the slag bucket baffle are properly matched, and adjust or repair them in time.
3. Check and adjust the flatness of the water outlet weir plate regularly to prevent uneven water outlet and short-flow phenomena, and promptly remove the slag hanging on the weir plate and the biofilm hanging on the water outlet tank.
4. During the inspection, carefully observe the sensory indicators of the effluent, such as the height changes of the sludge interface, the amount of suspended sludge, whether there is sludge floating, etc., and take targeted measures to solve the problem in time after abnormalities are found, so as not to affect the water quality.
5. During the inspection, pay attention to listen to whether there are abnormal sounds in the sludge scraping, slag scraping, and sludge discharge equipment, and check whether there are loose parts, etc., and adjust or repair them in time.
6. Regularly (usually once a year) empty the secondary sedimentation tank for inspection and maintenance, focusing on checking whether there are any abnormalities in the underwater equipment, pipelines, pool bottom and equipment coordination, and repair according to the specific situation.
7. Since the secondary sedimentation tank is generally buried deep, when the groundwater level is high and the secondary sedimentation tank needs to be emptied, in order to prevent the phenomenon of drifting, it is necessary to confirm the specific situation of the groundwater level in advance. If necessary, the water level can be lowered before emptying.
8. Carry out timely analysis and testing of the routine monitoring items of the secondary sedimentation tank according to regulations.