Tennessee Wastewater Operator Practice Exam

What action can be taken to increase DO concentration in primary cells of a parallel three pond system?

1. Decreasing water temperature

2. Increasing recycle flow from the settling pond

3. Reducing sludge blanket depth

4. Adding chemical aeration

The correct answer is: Increasing recycle flow from the settling pond

Increasing the recycle flow from the settling pond into the primary cells of a parallel three pond system can enhance the dissolved oxygen (DO) concentration for several reasons. Recycle flow introduces more water and possibly more microorganisms into the primary cells, which can stimulate biological activity. This increased biological activity can lead to higher oxygen demand, thereby encouraging the growth of aerophilic microorganisms that consume oxygen in the process of breaking down organic matter. Moreover, the movement of water from the settling pond to primary cells can help mix the contents, promote gas exchange, and enhance aeration naturally. Maintaining or increasing flow can reduce stagnation, leading to better oxygen circulation and distribution throughout the ponds. As a result, the overall health of the ecosystem in the primary ponds improves, leading to a higher concentration of DO. In contrast, other options like decreasing water temperature or reducing sludge blanket depth do not directly contribute to increasing DO concentration. While adding chemical aeration could theoretically help, it often has limitations in natural systems and can alter the ecosystem's balance, which can be counterproductive.