At a high pH, most of the ammonia in solution is in the unionized form, whereas at a low pH the ammonia is mostly in the ionic form. Since the unionized form is toxic to the aquatic organisms in the receiving body of water and since both the ionized and unionized forms exert an oxygen demand, effluent limits now often include a maximum limit on the total ammonia. This technical note considers the fundamentals of nitrification and the removal of ammonia in aerated lagoons.
Nitrification is defined as the oxidation of ammonia to nitrate.
The oxidation occurs in two steps the oxidation of ammonia to nitrite by the bacterium Nitrosomonas followed by the oxidation of nitrite to nitrate by the bacterium Nitrobactor . Below a pH of 8.5, almost all of the ammonia in solution will exist as the ammonium ion. If the pH of the wastewater is less than 8.3, which is typical for domestic wastewaters, the hydrogen ions produced are neutralized by bicarbonate ions in the wastewater. This reaction results in the decrease in bicarbonate alkalinity as well as an increase in the carbon dioxide concentration, both occurrences of which lowers the pH. If the wastewater has a relatively low alkalinity, the change in pH can be dramatic. In turn, the low pH can significantly reduce the rate of nitrification.