RAS Flow Rate Calculator
Return activated sludge keeps the clarifier sludge blanket stable and the aeration basin seeded. Enter MLSS, RAS solids, and influent flow to get the RAS rate from a clarifier mass balance.
RAS Rate
| Item | Value |
|---|
How the calculation works
Formulas (clarifier mass balance, effluent solids neglected)
(Q + Qr) x MLSS = Qr x RAS TSS
Qr = Q x MLSS / (RAS TSS – MLSS)
RAS % = 100 x MLSS / (RAS TSS – MLSS)
RAS (gpm) = RAS (MGD) x 1,000,000 / 1,440
(Q + Qr) x MLSS = Qr x RAS TSS
Qr = Q x MLSS / (RAS TSS – MLSS)
RAS % = 100 x MLSS / (RAS TSS – MLSS)
RAS (gpm) = RAS (MGD) x 1,000,000 / 1,440
| Typical values | Range |
|---|---|
| RAS rate (% of influent flow) | 25–150% |
| RAS underflow TSS | 8,000–12,000 mg/L |
| MLSS (conventional activated sludge) | 1,500–3,000 mg/L |
Notes & limitations
The mass balance assumes effluent TSS is negligible next to the underflow solids. It gives the RAS rate that holds the blanket steady for the MLSS and thickening you measured — not a universal setpoint.
- Use a fresh RAS TSS sample: underflow concentration changes with blanket depth, SVI, and pumping rate. Recompute when sludge settleability shifts.
- This balance ignores effluent solids. If effluent TSS is high, the clarifier is already losing solids and RAS alone will not fix it — check hydraulic loading and sludge age.
- Higher RAS thins the blanket but returns more solids to the aeration basin; coordinate RAS changes with wasting (WAS) to hold your target MLSS.
- For plants with step-feed or contact stabilization, apply the balance per pass or use the combined mixed liquor entering the clarifier.
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