What is the recommended dissolved oxygen range during nitrification/denitrification?

• A. 0.5-1 mg/L
• B. 2-3 mg/L
• C. 8-9 mg/L
• D. 0-0.5 mg/L

Answer: (B)

Dissolved oxygen in a system that does both nitrification and denitrification is kept at a level that supports nitrifying bacteria while allowing denitrification to occur in low-oxygen zones. Nitrifiers need oxygen, and they respond best when DO is around a couple of milligrams per liter. In plants designed to do both processes, you maintain roughly 2–3 mg/L in the aerobic zones to sustain nitrification, and the design provides anoxic pockets or separate stages for denitrification where oxygen is very low. Keeping DO in this mid-range avoids wasting energy on over-aeration and prevents denying denitrifiers the low-oxygen conditions they need. Too little DO would throttle nitrification; too much DO would hinder denitrification. So, 2–3 mg/L is the best target for balancing both processes.