Secondary Sedimentation Tank | Custom Clarifiers for Stable Sludge Separation

A secondary sedimentation tank is a clarification unit installed after biological treatment, especially after an activated sludge aeration tank. Its main function is to separate biological sludge from treated wastewater. In simple terms, microorganisms clean the wastewater in the aeration tank, and the secondary sedimentation tank then settles and separates these microorganisms from the clarified effluent.

The working principle is gravity settling. Mixed liquor from the biological treatment tank enters the secondary clarifier at a controlled velocity. When the flow becomes calm, activated sludge flocs settle to the bottom. Clarified water overflows from the upper outlet or weir, while the settled sludge is collected at the bottom. Part of this sludge is returned to the aeration tank as return activated sludge, commonly called RAS, to maintain the required biomass concentration. Excess sludge, also called WAS or waste activated sludge, is removed for further thickening or dewatering.

A typical secondary sedimentation process is:

A complete secondary clarifier system may include an inlet distribution structure, central feed well or baffle, settling zone, overflow weir, scum baffle, sludge hopper, sludge scraper, return sludge pipeline, excess sludge discharge pipe, pumps, valves, access platform and control system. For compact projects, different tank forms or clarifier configurations can be selected according to site space and hydraulic conditions.

Unlike a primary sedimentation tank, which mainly removes settleable raw solids before biological treatment, a secondary sedimentation tank focuses on separating biological flocs after biochemical reaction. Its performance directly affects effluent suspended solids, sludge concentration in the aeration tank and the stability of the whole biological process.

For B2B buyers and engineers, tank selection should not be based only on water flow. Key parameters include influent flow rate, peak flow, MLSS concentration, sludge volume index, sludge settling velocity, surface overflow rate, solids loading rate, hydraulic retention time, sludge blanket depth, RAS ratio, WAS quantity, effluent suspended solids target and available installation area. If the tank is undersized or the hydraulic distribution is poor, activated sludge may wash out with the effluent, causing high suspended solids and unstable biological treatment.

Common problems in secondary sedimentation include sludge bulking, floating sludge, uneven flow distribution, excessive sludge blanket, scraper failure and solids carryover. Some problems are caused by biological process conditions, while others are related to tank design, sludge return control or maintenance. Therefore, the secondary clarifier should be considered together with the aeration tank, sludge return system and sludge treatment line.

A secondary sedimentation tank is suitable for conventional activated sludge systems, oxidation ditch systems, A/O and A2/O processes, SBR-related clarification systems where applicable, and industrial biological wastewater treatment lines. However, it cannot remove dissolved pollutants by itself. If the biological treatment is insufficient, clarification alone cannot guarantee COD, ammonia nitrogen or total nitrogen removal.

Qingdao Jinwantong provides customized secondary sedimentation tanks and related wastewater treatment equipment. Buyers can send flow rate, MLSS, SVI, sludge return requirements, effluent target, site layout and project location for technical selection and quotation.