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RAS Flow Calculator

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)

  1. 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)

  1. Ratio = 330 / (1,000 - 330) = 0.4925
  2. 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)

  1. RAS conc = 1,000,000 / 118 = 8,475 mg/L
  2. 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.

This calculator also appears on ActivatedSludge.com, our consulting site. Results are for training and operator use; always follow your plant’s procedures.