Tennessee Wastewater Operator Practice Exam

If the solids concentration in an aerobic digester is 3.5% and no supernatant layer forms, what is the most likely reason?

1. Insufficient mixing

2. Maximum concentration is already achieved

3. Contamination with chemicals

4. Low temperature

The correct answer is: Maximum concentration is already achieved

When the solids concentration in an aerobic digester is at 3.5% and no supernatant layer forms, this suggests that the system has reached a maximum concentration of suspended solids. In aerobic digestion, the breakdown of organic matter by microorganisms occurs, and a certain concentration of solids can limit the system's ability to retain additional solids. Once the maximum concentration is approached, any additional solids introduced into the digester may not be able to settle out and form a distinct supernatant layer because the system is already operating at capacity. The absence of a supernatant indicates that there isn’t enough liquid volume to accommodate additional solids without having them remain suspended. This makes it evident that the maximum concentration has been achieved, rendering the occurrence of a supernatant layer unlikely. Understanding this concept emphasizes the importance of monitoring solid concentrations in an aerobic digester to ensure optimal operation and efficiency for waste treatment processes. In contrast, factors like insufficient mixing, contamination with chemicals, or low temperature could potentially affect the digester’s operation, but in this specific scenario, they would not directly result in the absence of a supernatant layer given the solid concentration level detected.