Tennessee Wastewater Operator Practice Exam

What is a side effect of operating aerobic digesters at DO concentrations below 1.0 mg/L?

1. Increased nitrification

2. Enhanced sludge production

3. Decreased VS destruction

4. Increased pH levels

The correct answer is: Decreased VS destruction

Operating aerobic digesters at dissolved oxygen (DO) concentrations below 1.0 mg/L can lead to decreased volatile solids (VS) destruction. This occurs because a lower DO level creates an environment that is not optimal for the aerobic microorganisms responsible for breaking down organic materials in the sludge. These microorganisms require sufficient oxygen to effectively metabolize the organic matter, leading to a reduction in the amount of VS present. When DO levels drop below the necessary threshold, the efficiency of the digestion process diminishes, resulting in less effective decomposition of volatile solids. In contrast, other processes that might be suggested, such as nitrification or sludge production, are not directly enhanced or favored under low DO conditions. Nitrification relies on specific aerobic bacteria that thrive at higher DO levels, while enhanced sludge production typically occurs under optimal aerobic conditions. Additionally, pH levels are influenced by various factors in the digestion process and may not directly correlate with the DO levels in the same straightforward manner.