Bulking in sewage treatment refers to which condition?

Bulking in sewage treatment specifically refers to a condition where sludge predominately floats rather than settles at the bottom of treatment tanks. This phenomenon often occurs due to the presence of filaments, which can be caused by various factors, including nutrient imbalances or the growth of certain microorganisms that don't settle well. When bulking occurs, it disrupts the proper functioning of the treatment process by preventing the effective separation of solids from liquids during sedimentation. Effective treatment relies on sludge settling properly to reduce the organic load and improve the quality of the effluent.

The other options describe issues that do not accurately depict the bulking phenomenon. For instance, excessive sludge settling pertains to the opposite problem, while over-aeration may lead to different quality concerns but is not directly related to the floating of sludge. Sludge that solidifies in treatment tanks indicates an entirely different malfunction within the system, again not aligned with the definition of bulking. Understanding the specific characteristics of bulking helps operators manage sewage treatment processes more effectively.