Basic Wastewater Operator Practice Exam

What is the most likely cause of rising sludge in clarifiers?

1. Young sludge

2. Old sludge

3. Middle age sludge

4. Slime

The correct answer is: Old sludge

The most likely cause of rising sludge in clarifiers is old sludge. As sludge ages, it may develop a low density, often due to the breakdown of floc structures and the presence of inactive microorganisms that do not settle effectively. This phenomenon can lead to buoyancy issues, where the sludge does not settle at the bottom of the clarifier as expected. Instead, the older, lighter sludge may rise to the surface, disrupting the separation process. Young sludge, while it may have settling issues, typically has a higher density and better settling characteristics as it has not yet undergone significant degradation. The term "middle age sludge" does not have a distinct definition in wastewater treatment, and while slime can contribute to settling issues, it is generally more associated with filamentous growth rather than being solely responsible for rising sludge. Therefore, the age of the sludge plays a critical role in its settling behavior, making old sludge the most appropriate answer in this context.