Three ways to estimate the RAS flow you need. When they agree, you can be confident. When they do not, the difference tells you something.
RAS flow: mass balance method
Formula
RAS flow = Q x MLSS / (RAS conc - MLSS)
Inputs used
- Influent flow (MGD)
- MLSS (mg/L)
- RAS concentration (mg/L)
Worked example (White River WRRF, fictional)
- RAS = 8.0 x 3,000 / (9,000 - 3,000) = 4.00 MGD (50% of Q)
RAS flow: settleometer method
Formula
RAS ratio = SSV30 / (1,000 - SSV30) RAS flow = Q x RAS ratio
Inputs used
- Influent flow (MGD)
- SSV30 (mL/L)
Worked example (White River WRRF, fictional)
- Ratio = 330 / (1,000 - 330) = 0.4925
- RAS = 8.0 x 0.4925 = 3.94 MGD
RAS flow: SVI method
Formula
Estimated RAS conc, mg/L = 1,000,000 / SVI RAS flow = Q x MLSS / (RAS conc - MLSS)
Inputs used
- Influent flow (MGD)
- MLSS (mg/L)
- SVI (mL/g)
Worked example (White River WRRF, fictional)
- RAS conc = 1,000,000 / 118 = 8,475 mg/L
- RAS = 8.0 x 3,000 / (8,475 - 3,000) = 4.38 MGD
Tips
- Mass balance uses measured RAS TSS and is best for current conditions.
- The settleometer method assumes the clarifier thickens like the cylinder.
- Confirm any change with blanket depth and the state point tool.
Results are estimates for operator use. Check units and inputs, and confirm decisions with plant data and procedures.