When ammonia is oxidized during nitrification, hydroxides contribute to alkalinity. Alkalinity is needed to neutralize these hydrogen ions. Without sufficient alkalinity, the pH of the system will drop, and nitrification will slow down. Nitrification works best when the pH is between 6.5 and 8.5. The process slows considerably at pH values outside this range.
Every plant operator is aware that nitrification efficiency decreases as the temperature falls.
Many plants have severe nitrification problems during the colder winter period. Likewise, fewer nitrification problems are experienced during the warm summers. Therefore, many treatment plants are designed with two stages. The first stage is for BOD removal. The second stage is for nitrification.