Pin Floc

1Symptom

The settleometer shows fast settling and a low SVI (often below 80 mL/g), but the supernatant above the blanket is hazy with tiny pinpoint particles. Effluent TSS creeps up even though blankets are low.

2Check first

1. Look at the supernatant in the settleometer against a dark background. Count or describe the pin particles.
2. Check SRT and F/M. Pin floc is classic for long SRT (old sludge) and very low F/M.
3. Look under the microscope for small, round, compact floc with few or no filaments, and for many rotifers, nematodes or testate amoebae.
4. Check for excess turbulence: high aeration in the last zone, hydraulic jumps, drop structures or pumps shearing floc before the clarifier.

3Possible causes

• SRT too long for the current load, often after flows or loads drop seasonally.
• Absence of filaments: a healthy floc needs a few short filaments as a backbone.
• Floc shear from over-aeration, high mixing energy or high velocity in mixed liquor channels.
• Toxic or inhibitory load that breaks floc apart (see toxic upset).
• Anaerobic or very low DO conditions that weaken floc, less common.

4Next investigation

• Calculate SRT from actual wasting records for the last two to three SRTs, not from the target.
• Run a turbidity or effluent TSS profile from the aeration basin effluent through the clarifier to see where fines appear.
• Check airflow and DO in the final aeration zone; DO well above 3 mg/L there often means excess energy.

5Corrective actions

1. Increase wasting gradually to shorten SRT (for example 10% more WAS per day) while protecting nitrification.
2. Reduce air or mixing intensity in the final zone if DO allows.
3. Add a small polymer or coagulant dose ahead of the clarifier only as a short term polishing step.
4. Recheck in two to three SRTs: the floc population turns over slowly.