Return Activated Sludge

Purpose: The purpose of the RAS pumping system is to return the settled sludge from the clarifiers back to the aeration basins to maintain the logical balance of the process.

Description: The RAS pumping system includes the RAS pumps, flow meters and control valves. These components are located in the lower level Pump Room of the Operations Building. The RAS pumps are horizontal centrifugal non-clog type pumps. Three pump units are provided, two operating and the third as a standby. Piping is arranged to allow any of the three pumps to draw sludge from either of the two clarifiers and discharge it to either of the aeration basins. Normally, two of the three pumps would operate continuously. Each pump would draw sludge from one clarifier and return it to the aeration basin feeding that clarifier. RAS is pumped to the aeration basin splitter box where it mixes with the influent wastewater as it enters the basin. This arrangement effectively creates two treatment process trains that operate independently. The rate of RAS return is controlled by motor operated plug valves in the discharge lines. A magnetic flow meter measures the rate of return flow in each line and the motor operated plug valve adjusts automatically to maintain the desired rate. The operator enters the RAS rates through a programmable logic controller (PLC) using the keypad on the front of the plant Main Control Panel.

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