California Wastewater Treatment Operator Practice Exam

What might cause sludge to rise in a clarifier?

1. Increased settling rate

2. High dissolved oxygen levels

3. Gasification from aging sludge

4. Low organic content

The correct answer is: Gasification from aging sludge

Gasification from aging sludge is a valid reason for sludge to rise in a clarifier. As sludge ages, it may undergo anaerobic decomposition, leading to the production of gases, such as methane or carbon dioxide. These gases can become trapped in the sludge flocs, causing a buoyancy effect that leads to rising sludge within the clarifier. When the solid particles in the sludge are uplifted by these gas bubbles, they tend to cluster and float instead of settling, which disrupts the clarification process. The other potential causes do not align with this phenomenon. For example, an increased settling rate would typically lead to better sedimentation and would not contribute to rising sludge. High dissolved oxygen levels are generally associated with aerobic processes that promote healthy sludge and do not contribute to the formation of gas bubbles. Lastly, low organic content would lead to less gas production during decomposition, potentially resulting in more stable sludge that settles effectively rather than rising.