At a conventional activated sludge plant, the operator observes clumps of sludge floating on the surface of the secondary clarifier. The final effluent is still meeting typical performance expectations with good BOD/TSS removal, and the clarifier looks clear except for these floating sludge patches. The sludge blanket is deeper than normal. When a sludge blanket sample is gently placed in a clear container, small gas bubbles form and lift bits of floc (a “boiling” appearance), indicating denitrification and dissolved nitrogen gas in the blanket.

Which operational response is the best immediate corrective action to stop the clumps of sludge from floating in the secondary clarifier?

Question

At a conventional activated sludge plant, the operator observes clumps of sludge floating on the surface of the secondary clarifier. The final effluent is still meeting typical performance expectations with good BOD/TSS removal, and the clarifier looks clear except for these floating sludge patches. The sludge blanket is deeper than normal. When a sludge blanket sample is gently placed in a clear container, small gas bubbles form and lift bits of floc (a “boiling” appearance), indicating denitrification and dissolved nitrogen gas in the blanket.

Which operational response is the best immediate corrective action to stop the clumps of sludge from floating in the secondary clarifier?

Accepted Answer

A) Increase the RAS rate (or otherwise reduce secondary clarifier blanket depth) to shorten solids detention time in the clarifier and prevent denitrification in the blanket. Correct

Correct. Rising sludge from denitrification happens when nitrate-bearing mixed liquor sits in an anoxic clarifier blanket long enough for bacteria to convert nitrate to nitrogen gas, which attaches to floc and makes it float. Increasing RAS lowers the blanket depth and reduces clarifier solids detention time, removing solids before significant denitrification gas can form. This matches the “boiling/bubbly” blanket observation and explains why BOD/TSS removal can still look good even while sludge is popping up.

Answer

B) Decrease the RAS rate to keep solids in the clarifier longer so they compact better and stop floating.

Wrong. Decreasing RAS increases blanket depth and keeps sludge in the clarifier longer, which increases the chance of anoxic conditions and denitrification gas production. That typically makes rising sludge worse, not better.

Answer

C) Increase aeration basin DO as high as possible to strip nitrogen gas and eliminate denitrification in the secondary clarifier.

Wrong. Higher aeration basin DO can help reduce the tendency for anoxic carryover, but it does not directly fix an anoxic, nitrate-rich clarifier blanket in the short term. The immediate issue is excessive blanket detention time in the secondary clarifier, so blanket control (RAS/blanket depth) is the primary operational knob.

Answer

D) Add polymer to the secondary clarifier influent to make the sludge heavier so gas bubbles cannot lift the floc.

Wrong. Polymer may improve settling/compaction in some situations, but it does not remove the root cause of gas formation in the blanket. Gas bubbles attached to floc can still lift “heavier” floc, and polymer use can create other problems (cost, overflocculation, higher sludge handling issues) if the real problem is denitrification.

Rising Sludge in Secondary Clarifier (Denitrification)

Answer Choices

Practice More Activated Sludge Questions