1Symptom
Light, fluffy floc particles (often 1/8 to 1/4 inch) float in the supernatant of the settleometer and drift over the clarifier weirs. The blanket may be normal. Effluent TSS rises, often worse in the afternoon.
2Check first
- Watch the settleometer for the first 5 minutes. Do particles rise or stay suspended while the rest of the sludge settles?
- Check SRT and F/M. Stragglers are common with young sludge (short SRT, high F/M) that has not formed firm floc.
- Look for small gas bubbles attached to particles, which points to denitrification instead.
- Check the microscope for loose, irregular floc and many free-swimming ciliates or flagellates.
3Possible causes
- Low SRT and high F/M: young sludge with weak floc.
- Sudden increase in organic load, such as an industrial discharge or septage.
- Gas attachment from denitrification in the clarifier (see rising sludge).
- Low DO in part of the basin producing weak floc.
- High shear breaking floc into light pieces.
4Next investigation
- Compare SRT to the last month. A drop of 20% or more in a week is significant.
- Look at influent BOD and COD trends and industrial monitoring for a load spike.
- Do a "stir test": gently stir the settleometer after 30 minutes. If particles sink again, gas is the issue.
5Corrective actions
- Reduce wasting to build SRT back to the target range.
- Raise DO where it is low.
- Control the load source if one is found; meter septage and hauled waste.
- If denitrification is involved, follow the rising sludge guide.
6When to call for help
- Stragglers persist for more than one SRT after correcting wasting.
- Effluent TSS or BOD limits are at risk.
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.