After undergoing biological treatment, wastewater enters the secondary sedimentation tank (SST) for sludge-water separation. Only the clarified effluent that meets discharge standards can be released. At the same time, a certain concentration of return sludge must be provided to the biological treatment facility. Therefore, the performance of the SST directly affects the operation of the activated sludge system.
1. Concept and Function of Secondary Sedimentation Tanks
1.1 Definition of Secondary Sedimentation Tank
Sedimentation tanks in wastewater treatment are classified based on their position in the treatment process into primary sedimentation tanks and secondary sedimentation tanks. Primary tanks are usually located after grit chambers and before biological reactors, whereas secondary sedimentation tanks are located after biological treatment and before advanced treatment or discharge.
After biological treatment, wastewater enters the SST for sludge-water separation. Clarified water meeting standards is discharged, and part of the sludge is returned to the biological treatment tank to maintain microbial activity. Therefore, SST performance is crucial for the efficiency of the activated sludge system.
Common types of SSTs include horizontal flow sedimentation tanks, radial flow sedimentation tanks, and inclined plate sedimentation tanks.
1.2 Function of Secondary Sedimentation Tanks
The main functions of an SST are to: Clarify the biologically treated mixed liquor by separating solids from water. Concentrate the sludge for recirculation.
As a critical stage in suspended solids treatment, if the SST is poorly designed, it will compromise sedimentation and separation. Ineffective sludge concentration will reduce the biomass returned to the biological tank, leading to lower mixed liquor concentrations and poor treatment performance, ultimately impacting effluent quality.
2. Classification and Design Requirements
Common types of secondary sedimentation tanks include horizontal flow sedimentation tanks, radial flow sedimentation tanks, and inclined plate sedimentation tanks.
Design Requirements for SSTs：
• Surface Hydraulic Loading Rate: Typically 0.6–1.5 m³/(m²·h). For industrial wastewater, this value should be adjusted according to water quality.
• Solids Loading Rate: To ensure effective sludge concentration for return, a design value of 150 kg/(m²·d) is common. Radial flow tanks may allow up to 200 kg/(m²·d), but no more.
• Side Water Depth: Should be 2.5–4 m depending on treatment capacity and retention time. For circular tanks:Diameter 10–20 m: depth 3.0 m； Diameter 20–30 m: depth 3.5 m；Diameter >30 m:depth 4.0 m. If depth requirements cannot be met, surface loading should be reduced to maintain retention time.
• Weir Loading: Should be below 1.7 L/(m·s). For radial flow tanks, up to 3.4 L/(m·s) may be acceptable.
• Sludge Zone Volume:For activated sludge systems: calculated based on 2–4 hours of sludge volume and must have continuous sludge removal.For biofilm processes: based on 4 hours of sludge volume.
• Sludge Return Equipment: To reduce energy consumption, use low-lift, high-flow devices such as screw pumps or axial flow pumps. For air-lift systems, air-lift pumps may be used to simplify maintenance.
3. Routine Monitoring Parameters
SST performance should be monitored using the following parameters:
• pH: Slightly lower than influent values, typically between 6 and 9.
• Suspended Solids (SS): Should be <30 mg/L under normal operation; must not exceed 50 mg/L.
• Dissolved Oxygen (DO): Lower than that in the biological tank due to microbial oxygen consumption.
• Ammonia Nitrogen & Phosphates: Should meet national effluent standards. Class 1A: Ammonia nitrogen <5 (or 8) mg/L,Phosphates <0.5 mg/L.
• Toxic Substances: Must meet national limits for toxic discharge.
• Sludge Level: Online sludge level meters can automate excess sludge control.
• Clarity (Transparency): Visual indicator of effluent quality.
4. Operation and Management of SSTs
• Regularly check and adjust influent distribution to ensure even flow to each SST.
• Inspect and clean scum hoppers; ensure the scum scraper and baffle cooperate properly.
• Check and level weir plates to avoid uneven flow and short-circuiting. Remove scum and biofilm from the weirs and outlet troughs.
• During inspections, observe:Sludge interface height;Suspended sludge quantity;Sludge rising/floating phenomena;Take corrective actions immediately if anomalies are detected.
• Listen for abnormal noises from sludge scraping/removal equipment. Check for loose parts and repair as needed.
• Perform annual emptying and maintenance of SSTs to inspect underwater equipment, piping, tank bottom, and their interfaces.
• Before emptying tanks with high groundwater levels, confirm groundwater conditions to prevent tank uplift. De-watering may be required in advance.
• Perform timely chemical analysis and testing as per monitoring standards.