Return Activated Sludge (RAS) Flow Rate

The return-pump rate from the solids mass balance: Q_RAS = Q x MLSS / (RAS_SS - MLSS), plus the return ratio. The clarifier only thickens sludge 3-4x, so RAS_SS is capped -- cranking the pump to chase a higher MLSS floods the clarifier and washes solids over the weir. The mass balance, not the pump maximum, sets the rate.

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Formula and source

Q_RAS = Q x MLSS / (RAS_SS - MLSS); ras_ratio = Q_RAS / Q x 100.

RAS flow from the activated-sludge solids mass balance (WEF MOP / Sacramento activated-sludge manuals), by name.

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