What is the minimum recommended dissolved oxygen concentration in the aeration basin?

• A. 2 mg/L
• B. 0.5 mg/L
• C. 6 mg/L
• D. 9 mg/L

Answer: (A)

Dissolved oxygen in the aeration basin must stay above a threshold so aerobic microorganisms can efficiently oxidize organic matter and, if present, enable nitrification. About 2 mg/L provides enough oxygen for stable biological activity while keeping energy use reasonable. Levels as low as 0.5 mg/L risk anaerobic pockets and poor treatment performance, while 6 or 9 mg/L are unnecessarily high and wasteful with no extra benefit in typical activated-sludge systems. So, 2 mg/L is the appropriate minimum.