When operating a plant, an operator notices the secondary clarifier sludge blanket is higher than normal, and the effluent appears turbid. The operator should:

• A. Decrease DO
• B. Increase DO
• C. Decrease the return sludge rate
• D. Increase the return sludge rate

Answer: (B)

Maintaining adequate dissolved oxygen in the aeration basin is key to forming strong, settleable activated sludge flocs. When the sludge blanket in the secondary clarifier is higher than normal and the effluent is turbid, it usually means the solids aren’t settling well, often because the flocs are weak or filamentous growth is interfering with settling.

Raising the DO improves microbial activity and floc structure, producing larger, denser, and more cohesive flocs that settle more readily. With better settling, the sludge blanket drops toward normal and the clarified effluent becomes clearer.

Lowering DO would worsen settling and turbidity because the biomass would struggle to form good flocs and maintain aerobic conditions. Increasing the return sludge rate can move more solids back to the aeration basin and may worsen effluent quality rather than fix the underlying settling issue. Decreasing the return sludge rate reduces solids recycling and can destabilize the aeration basin without addressing the poor settleability observed in the clarifier.