Biological nitrification is the biochemical oxidation of ammonia nitrogen by means of autotrophic aerobic bacteria. This process consists of two stages that involves two distinct groups of bacteria, each responsible for one of the stages.
NITROSOMONAS bacteria, which oxidize ammonia nitrogen (N-NH4+) to nitrite (N-NO2-):
2NH4+ + 3O2 ==> 2NO2- + 4H+ + 2H2O
NITROBACTERS bacteria, which oxidize nitrite (N-NO2-)to nitrate (N-NO3-):
2NO2- + O2 ==> 2NO3-
Therefore, the total oxidation reaction is
NH4+ + 2O2 ==> NO3- + 2H+ + H2O
The nitrification reaction takes place simultaneously to the oxidation of BOD in the same aeration basin, using the dissolved oxygen insuffled by micro air bubbles. The speed of growth ("yield") of the nitrifying bacteria is much slower than the heterotrophic bacteria, for which the designed installations for this type of treatment are characterized by higher values of sludge age and higher hydraulic residence time.
The nitrification process is influenced by several environmental factors, such as pH, presence of toxic substances and metals, concentration of free ammonia:
- The most critical factor is the pH, due to the potential oscillations of nitrogen level. At pH8.5 the free ammonium causes inhibition of Nitrosomonas.
- The activity of nitrifying bacteria is inhibited by also low concentrations of toxic substances; also, these bacteria are positioned on the external part of the flock, for which, in case of increased toxicity in the inlet, the bacteria are more exposed to environmental conditions.
- Instead, temperature affects differently the rate of growth of Nitrosomonas and Nitrobacter; for example at a room temperature the Nitrobacter grow faster, while its growth is inhibited to a higher temperature.
The nitrogen removal can also be reached by chemical-physical treatments, such as stripping with air, chlorination at point of rupture or selective ion exchange. The advantages of these processes, compared with biological treatment, are linked to the less sensitivity to such factors, which limit the efficiency of the bioprocess, such as the presence of toxic substances, abrupt drop in temperature etc. However, they also present some drawbacks such as fouling of calcium carbonate in the stripping towers, atmospheric pollutions for any leaks of ammonia, making great quantities of byproducts at high concentration, very difficult to dispose of, which in fact limit the possibility of their use.
Currently in the most part of the systems installed, nitrogen removal is performed through the biological processes of nitrification - denitrification.
This technology is an integral part of the treatment process of a Biological plant.