Nitritation is the oxidation of the nitrogen inside the ammonium molecule to nitrite. This process is performed by aerobic organisms AOB (Ammonium Oxidizing Bacteria).
It is essential that a further oxidation of nitrite to nitrate (see nitrification) is avoided by means of a stringent process control. To achieve this, ideal living conditions for the AOBs must prevail and NOBs (nitrite oxidizers) must be sufficiently inhibited or flushed out. In practical terms, this can only be done by controlling the sludge retention time because the AOBs grow slightly faster than the NOBs. The process of nitritation can be depicted by the following equation:

2NH₃⁺+ 3 O₂ -> 2 NO₂^- + 2 H⁺ + 2 H₂O

The conversion from ammonium to nitrite takes place in several steps:

NH₃ -> NH₂OH -> N₂O -> NO -> NO₂

Nitritation releases H+ ions, which consumes alkalinity in the biological system.
The targeted nitritation requires significantly less energy than the complete nitrification to nitrate. It is particularly advantageous that in accordance with the lower energy requirement for nitritation, denitritation also requires less carbon than denitrification.


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