Settleability-Based RAS Rate (from SVI)
The RAS number straight from the settleometer, no RAS_SS lab result needed: the achievable return concentration is Xr = 1,000,000 / SVI, so the ratio for a target MLSS is R = MLSS / (Xr - MLSS) and Q_RAS = R x Q. Poor settling forces a much higher return - holding 2,500 mg/L on a good SVI of 100 needs a 33% return, but a bulking sludge at SVI 150 wanting 3,000 mg/L needs 82%, and if the pumps cannot deliver it the MLSS target is unreachable. The settleometer path to the same mass balance ras-flow-rate does from a measured RAS_SS. An operating aid, not a process design.
Formula and source
Xr = 1,000,000 / SVI; ras_ratio = MLSS / (Xr - MLSS); Q_RAS = ras_ratio x Q.
Settleability-based RAS from the sludge volume index (WEF MOP / Sacramento activated-sludge manuals), by name.
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