Floating sludge has been increasing on a final clarifier with normal DO levels. What is the most likely cause?

The presence of floating sludge in a final clarifier, even with normal dissolved oxygen (DO) levels, often indicates issues related to the settling characteristics of the sludge. When there is a clogged return pump, it can lead to a condition known as denitrification, where anaerobic processes dominate in the clarifier. This condition can produce gases such as nitrogen, which can create buoyancy in the sludge particles, causing them to float instead of settle effectively.

In this context, a clogged return pump disrupts the normal flow of sludge back to the aeration basin, which is crucial for maintaining a healthy biomass concentration necessary for effective treatment. When denitrification occurs due to the lack of proper return flow, the generated gases can cause the sludge to become less dense, resulting in the observed floating sludge phenomenon.

While the other options hint at potential issues, they do not directly relate to the symptoms described. Low DO levels would have different implications, and excessive chemical dosing might lead to different types of sludge issues rather than specifically causing flotation. A high concentration of influent solids can affect clarifier performance but is less likely to be the immediate cause of floating sludge under normal DO conditions.