Secondary Clarification
Secondary Clarifier Operation
The physical process of removing microorganisms and solids from treated wastewater is known as secondary clarification. The clarifier's job is to produce clear dischargeable effluent, remove excess organisms from the activated sludge system, and replenish the activated sludge process as needed.
The operator's handling of clarifier sludge has an effect on how well the activated sludge process works. The clarified sludge will be disposed of or returned to the aeration tank.
Return Activated Sludge
The sludge settled in the clarifier that is returned to the aeration tank is referred to as Return Activated Sludge (RAS).
RAS management is critical to the efficiency of the activated sludge process because:
- RAS serves as a source of organisms that are recycled into the activated sludge process as needed.
- The operator can control the concentration of organisms in the aeration tank by adjusting the RAS rate in response to the food supply present in the incoming wastewater. This enables him to keep the proper F/M ratio for optimal system performance.
- Increasing the RAS rate reduces the aeration basin's hydraulic detention time. Lowering the RAS rate lengthens the hydraulic detention time.
- As long as the aerobic organisms are in the secondary clarifier, their health deteriorates. If the sludge remains in the clarifier for an extended period of time, the aerobic organisms will perish due to a lack of oxygen.
- When increasing RAS rates, exercise caution because hydraulic overloading of the activated sludge system is possible. Increased RAS rates result in more volume in the aeration tank and a shorter hydraulic detention time.
Waste Activated Sludge
The sludge that is removed from the system to prevent excessive solids buildup in the aeration tank and the activated sludge process is known as WAS. This sludge is removed from the clarifier and disposed of at the plant's sludge disposal facility; it is not reused.
WAS can be used to regulate the concentration of MLSS in an aeration tank. When the WAS rate is reduced, the MLSS concentration rises and vice versa.
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