What is the most likely cause of sludge floating to the surface of a secondary clarifier?

The most likely cause of sludge floating to the surface of a secondary clarifier is that settled sludge is not being removed quickly enough. In a secondary clarifier, the process relies on proper settling of solids to separate them from the treated water. If the removal of settled sludge is delayed, it can lead to resuspension of the sludge, causing it to float rather than settle at the bottom.

When the rate at which the sludge is removed from the clarifier is insufficient, it can disrupt the equilibrium of the system. The weight of the sludge, combined with factors like gas production or changes in water flow, can contribute to this phenomenon, leading to poor clarification and potential operational issues. Proper maintenance of the sludge removal system is crucial to ensure effective settling is achieved, preventing any accumulation from rising to the surface.

The other choices do not directly contribute to the phenomenon of floating sludge in the same way. For instance, ineffective screening might lead to larger solids entering the system but doesn't directly relate to sludge behavior in the clarifier. Similarly, the composition of the sludge and scraper speed have their own impacts but do not lead primarily to floating sludge in the manner described.