Rising Sludge (Denitrification in the Clarifier)

1Symptom

Clumps or rafts of brown sludge rise to the clarifier surface, often in patches, with small gas bubbles visible. In the settleometer, sludge settles well at first and then rises after 1 to 2 hours. It is worse in warm weather and when the plant is nitrifying.

2Check first

1. Run the settleometer for 2 hours. Note the time the sludge starts to rise.
2. Check effluent nitrate. Rising sludge needs nitrate, usually above about 5 to 6 mg/L nitrate-N.
3. Measure blanket depth. Deep blankets hold sludge longer in an anoxic zone.
4. Check RAS rate and whether any RAS pumps or collector arms are out of service.
5. Stir a floating clump: gas bubbles released confirm nitrogen gas.

3Possible causes

• Nitrate in the mixed liquor plus long sludge holding time in the clarifier.
• RAS flow too low for the current solids loading, so the blanket gets deep.
• Collector problem or a plugged suction header leaving dead zones.
• Warm water speeding up denitrification.
• High MLSS with modest clarifier capacity.

4Next investigation

• Calculate how long sludge sits in the blanket (blanket volume divided by RAS flow).
• Check the anoxic zone performance: is nitrate getting denitrified upstream?
• Walk the clarifier and look for uneven rise (dead spots) versus uniform rise.

5Corrective actions

1. Increase RAS to lower the blanket and shorten sludge holding time.
2. Improve upstream denitrification: increase internal recycle, check anoxic zone mixing and DO, add carbon only if justified.
3. Do not starve nitrification to stop rising sludge if you have an ammonia limit. Fix the clarifier holding time and upstream denitrification instead.
4. Repair collectors or suction headers; clear plugged lines.
5. Use surface sprays or skimmers to break up and remove floating sludge.