Straggler Floc

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

1. Watch the settleometer for the first 5 minutes. Do particles rise or stay suspended while the rest of the sludge settles?
2. Check SRT and F/M. Stragglers are common with young sludge (short SRT, high F/M) that has not formed firm floc.
3. Look for small gas bubbles attached to particles, which points to denitrification instead.
4. 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

1. Reduce wasting to build SRT back to the target range.
2. Raise DO where it is low.
3. Control the load source if one is found; meter septage and hauled waste.
4. If denitrification is involved, follow the rising sludge guide.