Method for the biological treatment of nitrogen in the form of ammonium in waste water by nitritation in a biological reactor comprising ammonium-oxidizing bacteria and nitrite-oxidizing bacteria, said reactor being a microbial cultured activated sludge reactor with dissociation of the hydraulic residence time and the residence time of the sludge, the waste water being urban or domestic effluents containing nitrogen at least partly in the form of ammonium, the process comprising: • at least one step a of controlled aeration of the biological reactor containing the waste water to be treated to maintain the dissolved oxygen concentration in the reactor at an average value equal to or greater than 2 mg/L, so as to oxidize at least part of the ammonium to nitrite by the ammonium-oxidizing bacteria present in the reactor, characterized in that the process further comprises: • at least one step b of removal of at least some of the nitrites produced in step a by: - a b1 sampling of treated water at stage a outside the reactor, and/or by - a biological transformation b2, into anoxia, implemented by heterotrophic denitritation in the presence of carbon and/or de-ammonification by Anammox, steps a and b being implemented successively and cyclically so that the nitrite concentration in the reactor at the beginning of stage a is less than 2 mg N-NO2/L or at least 2 times lower than the ammonium concentration in the reactor, and • a step c of extraction of a fraction of the reactor sludge resulting from steps a and b, per unit time, calculated so that an effective aerated sludge age of the reactor is less than or equal to a theoretical minimum aerated sludge age required for nitrification as defined by an exponential law decreasing as a function of the temperature defined by the equation: A theoretical = A = SF × 3.4 × 1.103 (15-T°C), with T°C temperature in degrees Celsius and SF a correction factor of 1.45 to 1.8 depending on the size of the biological nitrogen treatment plant in the effluent, such that nitrate production is minimized relative to nitrite production in the reactor by suppressing the activity of nitrite-oxidizing bacteria by said stage a aeration, said stage b nitrite removal and said effective stage c aerated sludge age; and Steps a, b and c are repeated until the ratio of nitrite concentration to the sum of nitrite and nitrate concentrations at the end of aeration step a is greater than 0.8. • at least one step a of controlled aeration of the biological reactor containing the waste water to be treated to maintain the dissolved oxygen concentration in the reactor at an average value equal to or greater than 2 mg/L, so as to oxidize at least part of the ammonium to nitrite by the ammonium-oxidizing bacteria present in the reactor, • at least one step b of removal of at least some of the nitrites produced in step a by: - a b1 sampling of treated water at stage a outside the reactor, and/or by - a biological transformation b2, into anoxia, implemented by heterotrophic denitritation in the presence of carbon and/or de-ammonification by Anammox, steps a and b being implemented successively and cyclically so that the nitrite concentration in the reactor at the beginning of stage a is less than 2 mg N-NO2/L or at least 2 times lower than the ammonium concentration in the reactor, and - a b1 sampling of treated water at stage a outside the reactor, and/or by - a biological transformation b2, into anoxia, implemented by heterotrophic denitritation in the presence of carbon and/or de-ammonification by Anammox, • a step c of extraction of a fraction of the reactor sludge resulting from steps a and b, per unit time, calculated so that an effective aerated sludge age of the reactor is less than or equal to a theoretical minimum aerated sludge age required for nitrification as defined by an exponential law decreasing as a function of the temperature defined by the equation: A theoretical = A = SF × 3.4 × 1.103 (15-T°C), with T°C temperature in degrees Celsius and SF a correction factor of 1.45 to 1.8 depending on the size of the biological nitrogen treatment plant in the effluent, such that nitrate production is minimized relative to nitrite production in the reactor by suppressing the activity of nitrite-oxidizing bacteria by said stage a aeration, said stage b nitrite removal and said effective stage c aerated sludge age; and