Tennessee Wastewater Operator Practice Exam

What is one method to prevent further pH drop in an aerobic digester?

1. Increase DO concentration

2. Lower DO concentration

3. Stop mixing

4. Add chemicals

The correct answer is: Lower DO concentration

In an aerobic digester, controlling pH is crucial for maintaining optimal conditions for microbial activity and digestion processes. One method to prevent further pH drop is to lower the dissolved oxygen (DO) concentration. This may seem counterintuitive, as oxygen is necessary for aerobic digestion, but managing DO levels can influence microbial metabolic activity. When the DO concentration is excessively high, certain bacteria can become overly active, leading to an increased metabolism that can produce more acidic byproducts, thereby contributing to a further decrease in pH. By lowering the DO concentration, the metabolic rate of these microorganisms can be moderated, which can help stabilize the pH and reduce the production of acids. This approach can assist in maintaining a balanced environment for the aerobic digestion process, where the microbial community can function effectively without overwhelming acid production. In contrast, increasing DO concentration would usually promote higher activity levels in bacteria, potentially worsening pH drop if acid production is increased. Stopping mixing can lead to stratification and poor distribution of nutrients and oxygen, while adding chemicals may not directly address the bacterial activity causing the pH drop.