After spending a substantial amount of time in the biological treatment process in the VLR tanks, the wastewater moves through a series of small waterfalls to a secondary distribution chamber were it can be channeled to one or more, of three final clarifiers depending on the desired pathway and level of flow. In these large 105 wide 16 foot deep tanks water is allowed to calm and settle, and heavier solids separate from the treated water.
The North Olmsted WWTP is equipped with three (3) Final Clarifiers to provide sufficient retention time for suspended solids separation from the waste stream. Sludge and accumulated solids collect at the bottom of the clarifiers, and clarified water leaves via the overflow weirs located around the perimeter of the tank. Each clarifier is equipped with scum baffles located in front of the weirs to contain any floating scum or debris within the clarifiers. The clarifier equipment is manufactured by Walker Process Equipment.
An important function of the final clarifier is to separate biologically activated sludge from the treated water. In order to maintain a constant concentration of mixed liquor suspended solids (MLSS) to mix with the VLR influent, it is necessary to continuously remove settled sludge from the final clarifiers and return a portion of it to the aeration tank.
The final clarifiers provide a zone of calm conditions to allow the mixed liquor floc to separate from the clear water. The settled solids are continuously removed from the bottom of the clarifier through a suction header connected to the RAS pumps. Part of the settled sludge is removed as waste activated sludge (WAS) and pumped to the Sludge Storage Tank. The wasting process helps maintain an effective solids inventory and prevents an over accumulation of solids. The quality and characteristics of the activated sludge generated in the biological process is very important to the operation of the final clarifier.
Return Activated Sludge and Waste Activated Sludge
Return Activated Sludge (RAS) is an essential part of the activated sludge treatment process as it collects settled sludge from the Final Clarifiers and returns it to the RAS conditioning tank (at the Preliminary Treatment Facility) to continue the biological treatment process. RAS helps maintain an effective microorganism inventory for a good treatment system.
The RAS/WAS Pump Station consists of five (5) centrifugal pumps. These pumps are rated for 2,500 gpm. The pump suction for RAS Pumps 1, 2 and 3 come from Final Clarifier #2 and #3. The suction for RAS Pump 4 and 5 come from Final Clarifier #1.
Each set of pumps discharge to a 16-inch force main which conveys flow to the distribution box at the RAS conditioning tank. Flow through each of the RAS lines are monitored separately with dedicated flow meters.
Waste Activated Sludge (WAS) is also important to the biological treatment process as it removes any excess portion of the activated sludge from the biological system to maintain an effective balance of MLSS in the Aeration Basin. The maintenance of this MLSS inventory is balanced between the generation of solids from the raw influent and the removal of excess solids by sludge wasting. Two (2) rotary lobe pumps are used to divert flow to Sludge Holding.