Return Activated Sludge (RAS) Flow Rate
The return-pump rate from the solids mass balance.
Example
You enter
- Plant influent flow Q (MGD) 5
- Mixed-liquor solids MLSS (mg/L, target) 2500
- Return solids RAS_SS (mg/L, > MLSS) 8000
You get
- RAS flow rate 2.27 MGD
- Ras ratio (%) 45
Details, formula, and sources
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.
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.
The solids-balance return-rate relation is a public wastewater-operations result; the settleability and clarifier performance govern the actual set point.
Estimate. AHJ and licensed professional govern.
Field names used by the API: plant_flow_mgd, mlss_mg_l, ras_ss_mg_l, q_ras_mgd, ras_ratio_pct
- Mass balance Q_RAS = Q x MLSS / (RAS_SS - MLSS); the return solids must exceed the mixed-liquor solidsactivated-sludge solids balance
- Thickening cap the clarifier thickens sludge only ~3-4x; the settleability (SVI) and clarifier performance cap RAS_SSWEF / Sacramento manuals