If solids are rising in the secondary clarifier, which process is likely occurring?

When solids are observed to be rising in the secondary clarifier, it is indicative of denitrification. This process occurs in environments with low dissolved oxygen, where bacteria reduce nitrates to nitrogen gas, leading to the formation of gas bubbles. These bubbles can adhere to the solids, causing them to become buoyant and rise to the surface of the clarifier rather than settling to the bottom as intended.

In contrast, flocculation involves the agglomeration of small particles into larger flocs, which typically enhances settling rather than causing solids to rise. Filtration is a process used to remove suspended solids from liquids but is not directly related to the behavior of solids in a clarifier. Decanting is the act of carefully pouring off the liquid from the top while leaving the solid sediment at the bottom, which also does not explain the rising solids phenomenon.

Thus, the behavior of rising solids aligns directly with the gas-producing conditions found in denitrification, making this the most accurate choice in the context of the question.