What is the ideal pH range for nitrification/denitrification processes?

• A. 6-7
• B. 7.8-9
• C. 9-10
• D. 5-6

Answer: (B)

Maintaining pH in near-neutral to mildly alkaline conditions keeps the enzymes of both nitrification and denitrification working efficiently. Nitrifying bacteria, which oxidize ammonium to nitrate, perform best around 7.8 to 9 because their cellular machinery functions optimally in this range and because enough buffering capacity (alkalinity) is available to counter the acid produced during nitrification. Denitrifying bacteria, which reduce nitrate to nitrogen gas under anoxic conditions, also prefer a neutral to slightly alkaline environment, enabling metabolism and enzyme activity needed for nitrate reduction.

If the pH is too low, nitrification slows dramatically or stops because the enzymes become inactive and proton balance shifts unfavorably. If the pH is too high, above about 9, nitrification efficiency can decline and ammonia can begin to strip from solution, complicating control. So, the 7.8 to 9 range provides a balance that supports both processes, maintaining stable nitrogen conversion.