Rising sludge in secondary clarifiers is usually a result of what process?

• A. Denitrification
• B. Aeration
• C. Filtration
• D. Sludge digestion

Answer: (A)

Rising sludge in secondary clarifiers usually results from gas bubbles forming inside the sludge due to denitrification. In denitrification, bacteria use nitrate as an electron acceptor under anoxic conditions and convert it to nitrogen gas. Those gas bubbles can become trapped in the flocs, reducing their density and making the solids buoyant so they rise toward the surface. This disrupts settling and can carry solids into the effluent if not controlled.

Aeration, filtration, and sludge digestion operate differently and don’t create the gas-filled buoyancy that denitrification does in the clarifier, so they’re not the primary cause of rising sludge.