Denitrification occurs under what conditions and DO range?

• A. DO not a factor
• B. Anoxic or near anaerobic conditions with DO less than 0.5 mg/L
• C. Fully aerobic with DO > 2 mg/L
• D. Completely anaerobic with no DO

Answer: (B)

Denitrification happens when the bacteria have little or no oxygen to breathe, so they can use nitrate instead as the terminal electron acceptor. This process occurs in anoxic or near-anoxic zones where dissolved oxygen is very low, typically less than about 0.5 mg/L. Oxygen, if present at higher levels, shifts microbial metabolism to aerobic respiration and denitrification slows or stops, even if nitrate is available. For denitrification to proceed effectively, you also need nitrate in the water and a carbon source to supply the electrons. So the best operating condition is anoxic or near anaerobic with DO under about 0.5 mg/L.