What is a common cause of rising sludge and gas bubbles in the final clarifier?

Denitrification is a biological process where nitrate is reduced to nitrogen gas, typically occurring in the anoxic or anaerobic zones of a treatment system. In the context of a final clarifier, if denitrification occurs excessively, it can produce gas bubbles, which rise to the surface. These gas bubbles can cause rising sludge, a condition where solids that should remain settled in the clarifier are lifted and carried into the effluent. This phenomenon can lead to poor settling characteristics and may result in an increased concentration of solids in the treated effluent, thereby violating discharge standards.

Additionally, while other factors like infilling of sludge, insufficient aeration, and influent overloading can affect the performance of a clarifier, they do not specifically lead to the unique combination of rising sludge and gas bubbles that is characteristic of excess denitrification in the final clarifier. These conditions tend to indicate a balance issue in the biological processes occurring upstream of the clarifier rather than a direct issue with the clarifier itself.