Tennessee Wastewater Operator Practice Exam

When should air off cycles be terminated in an aerobic digester?

1. When sludge is agitated

2. When solids have settled to the bottom

3. When pH levels stabilize

4. When temperature exceeds 30°C

The correct answer is: When solids have settled to the bottom

In an aerobic digester, the primary goal is to promote the breakdown of organic material by microorganisms in the presence of oxygen. Terminating air-off cycles when solids have settled to the bottom is crucial as it indicates that the digestion process has progressed sufficiently. When solids settle, it typically means that the biological activity has reduced the volume of sludge, allowing for easier management and removal of digested materials. This settling indicates that the microorganisms have effectively digested the organic material, creating a more stable biosolid. It also suggests that the volatile solids have been sufficiently reduced, which is a key indicator of the effectiveness of the digestion process. Continuing to aerate the system beyond this point may lead to inefficient energy use and the potential for operational issues such as increased turbidity in the digester. While agitation of sludge, stabilization of pH levels, and temperature management are all important factors in the treatment process, the physical settling of solids directly correlates with the digestion process's success, making it the most appropriate indicator for terminating air-off cycles in an aerobic digester.