Texas Wastewater Class B Practice Exam

What should the Dissolved Oxygen (DO) levels in the aeration tank of an activated sludge plant be maintained at?

1. 0 to 1 mg/L

2. 1 to 2 mg/L

3. 2 to 4 mg/L

4. 4 to 6 mg/L

The correct answer is: 2 to 4 mg/L

The appropriate range for Dissolved Oxygen (DO) levels in the aeration tank of an activated sludge plant is 2 to 4 mg/L. Maintaining DO levels within this range is crucial for the effectiveness of the aerobic microorganisms responsible for breaking down organic matter in wastewater. Inactivation of these microorganisms can occur at lower levels, such as in the 0 to 1 mg/L and 1 to 2 mg/L ranges, where there may not be sufficient oxygen for optimal biological activity. At levels exceeding 4 to 6 mg/L, though oxygen saturation is not usually harmful, it often indicates an inefficient use of oxygen, which can lead to unnecessary operational costs. Therefore, the 2 to 4 mg/L range strikes the right balance, ensuring that the microorganisms remain active and capable of effectively treating the wastewater.