Tennessee Wastewater Operator Practice Exam

Which method might help prevent a drop in alkalinity and pH in an aerobic digester?

1. Cycling between aerobic and anoxic conditions

2. Increasing solids retention time

3. Adding chemical buffers

4. Decreasing airflow rates

The correct answer is: Cycling between aerobic and anoxic conditions

To prevent a drop in alkalinity and pH in an aerobic digester, the method of cycling between aerobic and anoxic conditions is beneficial because it enhances the microbial population's diversity and activity. In aerobic conditions, certain bacteria process organic material and produce carbon dioxide, which can contribute to maintaining a stable pH. Alternating to anoxic conditions allows for denitrifying bacteria to thrive, which can help remove excess nitrogen compounds, thereby reducing acidification processes. This cycling also encourages a more balanced microbial ecosystem that can better manage fluctuations in pH and alkalinity, preventing significant drops that would otherwise affect digestion efficiency. In contrast, while increasing solids retention time can theoretically allow for more complete digestion of solids, it does not directly address the pH and alkalinity dynamics. Adding chemical buffers is indeed a common approach to stabilize pH, but it does not enhance the biological processes occurring within the digester as effectively as cycling would. Decreasing airflow rates might reduce the risk of excessive pH elevation due to over-aeration, but it could also inhibit the aerobic digestion processes necessary for effectively managing alkalinity. Thus, cycling between conditions is the most holistic method of maintaining stability in these critical parameters.