Tennessee Wastewater Operator Practice Exam

What could cause settled sludge to rise back to the top of the settleometer?

1. Poor sampling technique

2. Denitrifying and production of nitrogen gas

3. Heavy rainfall

4. Excessive aeration during testing

The correct answer is: Denitrifying and production of nitrogen gas

The phenomenon of settled sludge rising back to the top of the settleometer can indeed be attributed to the denitrification process and the production of nitrogen gas. During denitrification, certain bacteria convert nitrates into nitrogen gas as part of their metabolism. This gas can accumulate in the sludge and create buoyancy, causing the settled particles to rise. As the gas bubbles form within the floc or mass of sludge, they can lift the solids back into suspension, counteracting the effects of gravity that would typically keep them at the bottom of the settleometer. This behavior is critical in wastewater treatment processes since it indicates the presence of biological activity and potential issues with sludge blanket stability. Poor sampling technique, heavy rainfall, and excessive aeration might impact the testing process or conditions but are less directly related to the specific phenomenon of gas production impacting the settled sludge within the context of a settleometer. Understanding the role of denitrification helps clarify the interactions occurring in wastewater systems.