RAS Calculator (Return Activated Sludge Flow)
Return activated sludge (RAS) carries settled solids from the secondary clarifier back to the aeration basin. This simplified steady-state mass balance estimates the RAS flow needed to hold your MLSS.
Result
RAS flow
0.6 MGD
RAS rate
60% of influent
Same flow in GPM
417 GPM
To hold MLSS at 3,000 mg/L with RAS at 8,000 mg/L, this mass balance estimates a RAS flow of about 0.6 MGD (60% of influent). Check it against blanket depth and settling.
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The formula
Q_RAS = Q × MLSS ÷ (RAS TSS − MLSS) RAS (%) = Q_RAS ÷ Q × 100- Q
- Influent flow rate (MGD or GPM). Keep the same unit throughout; Q_RAS comes out in that same unit
- MLSS
- Aeration basin MLSS (mg/L)
- RAS TSS
- Return sludge suspended solids (mg/L)
- Q_RAS
- Estimated RAS flow, in the same units as Q
Good to know
- The formula works in MGD or GPM, but the units must stay the same: if influent flow is in GPM, RAS flow comes out in GPM. Use the MGD / GPM switch next to influent flow to pick your unit.
- Use this estimate with clarifier blanket depth, settling, solids loading and actual pump output.
- Operating decisions stay with qualified personnel following your plant's approved procedures.
Read the full guide: RAS Rate Calculation Wastewater: A Plain-Language Guide
Frequently asked questions
Why must RAS TSS be higher than MLSS?
The formula divides by RAS TSS minus MLSS. If return sludge is not thicker than the mixed liquor, the mass balance can't hold MLSS and gives no valid answer.
Is this RAS flow the setting I should run?
It is an estimate from a simplified steady-state mass balance. Use it with clarifier blanket depth, settling, solids loading and actual pump output, and follow your plant's approved procedures.
How do I get RAS as a percent?
Divide the RAS flow by the influent flow and multiply by 100. The calculator shows both.
Stop running this math by hand
ComplyFlo for wastewater plants can calculate and trend SRT, F/M, HRT, RAS, SVI and removal efficiency from your daily readings, next to your plant dashboard and DMR-ready reports.
See ComplyFlo for wastewater plantsFree checklist
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