1Symptom
RAS concentration is lower or higher than expected, blankets differ between clarifiers, or RAS flow readings do not match pump performance.
2Check first
- Grab RAS samples from each clarifier and test TSS.
- Verify RAS flow meters against pump curves or a drawdown test.
- Check for plugged suction headers, rags, or air binding.
- Look for a solids mass balance mismatch: Q x MLSS should roughly equal RAS x (RAS TSS minus MLSS) at steady state.
3Possible causes
- RAS rate too high: thin RAS and high clarifier underflow loading.
- RAS rate too low: deep blanket, thick RAS, septic sludge.
- Plugged suction or collector arm.
- Meter calibration drift.
- One pump much weaker than others.
4Next investigation
- Calculate the RAS ratio needed from settleometer, SVI and mass balance methods.
- Do a clarifier mass balance.
5Corrective actions
- Set RAS from the mass balance or state point, then fine tune using blanket depth.
- Clear plugs and repair pumps.
- Calibrate meters.
- Balance RAS per clarifier.
6When to call for help
- A collector or suction header needs a drain-down.
- Mass balance cannot be closed.
Make one change at a time, record it, and follow your plant procedures and permit. If you suspect a toxic discharge, a safety hazard or a permit violation, notify your supervisor right away.