Nitrification is the continued oxidation of ammonium to nitrite (see Nitritation) and subsequently to nitrate. The two-step process is performed by aerobic organisms AOBs (Ammonium Oxidizing Bacteria) and NOBs (Nitrite Oxidizing Bacteria). Both processes run almost parallel as natural processes.
The bacterial groups involved are ammonium oxidizers (Nitrosomonas) and nitrite oxidizer (Nitrobacter) and are together summarized under the term of nitrifying bacteria.
A technical implementation of the nitrification in the wastewater treatment plants takes place in an aeration stage (supply of dissolved oxygen). The conversion of ammonium to nitrate is associated with a large oxygen demand. To oxidize 1g of ammonium nitrogen, about 4.6g of O2 are required.
Nitrification consumes alkalinity.
In an activated sludge plant, the largest energy demand is caused by the consumption of aeration energy. The aeration energy is primarily needed for nitrification and only secondarily for carbon removal.



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