Increasing Clarifier Blanket

1Symptom

Secondary clarifier blanket depth increases from its normal range (at White River WRRF, 1 to 3 ft) to 4 ft or more, either gradually over days or quickly during peak flows. At worst, solids reach the weirs.

2Check first

1. Measure the blanket in every clarifier with a core sampler or sludge judge at the same time of day.
2. Check SVI trend and settleometer appearance.
3. Check RAS flow per clarifier and RAS concentration.
4. Check flow split: are all clarifiers getting equal flow and solids?
5. Check MLSS and influent flow trends.

3Possible causes

• Thickening overload: too much mass for the clarifier area and the current settleability.
• RAS rate too low, or a RAS pump or line problem.
• Rising SVI.
• High MLSS from under-wasting.
• Unequal flow split.
• Hydraulic overload during wet weather.

4Next investigation

• Run the state point tool with current SVI, flow and RAS.
• Calculate solids loading rate on the clarifiers.
• Compare the measured RAS concentration with the predicted value from a mass balance.

5Corrective actions

1. Increase RAS to move the underflow line below the flux curve. At White River WRRF, RAS was raised from 4.0 to 5.2 MGD during the September storm.
2. Increase wasting gradually to bring MLSS down if inventory is high.
3. Put any standby clarifier in service.
4. Fix the flow split.
5. Address the SVI cause.