The invention relates to a method for the biological treatment of nitrogen in the form of ammonium in wastewater, by nitritation in a biological reactor, comprising: at least one step a of aerating the biological reactor containing the wastewater to be treated, at least one step b of eliminating at least part of the nitrites produced in step a, and a step c of extracting, from the reactor, a fraction of the sludge resulting from steps a and b. The invention also concerns a method for the biological treatment of nitrogen in wastewater, by nitritation/denitritation and/or deammonification, wherein the nitritation is implemented using the nitritation method according to the invention.