Nitrite (NO₂^-) is an inorganic nitrogen compound. It is formed as an intermediate of nitrification (nitritation), where it is formed by oxidation from ammonium:
2NH₃⁺ + 3O₂ -> 2NO₂^- + 2H⁺ + 2H₂O. Autotrophic microorganisms (Nitrosomonas) oxidize ammonium via several intermediates to nitrite. Upon further oxygen supply and presence of corresponding bacteria (Nitrobacter) nitrite is oxidized to nitrate.
Nitrite is highly toxic and is considered to be toxic to fish. Therefore, a complete nitrification is of particular importance in a sewage treatment plant.


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