In wastewater treatment, ammonium removal is a key step which can be done biologically. One method is by coupling a partial nitritation with the Anammox process. The partial nitritation goal is to convert half of the ammonium into nitrite, so ammonium and nitrite can be later converted into dinitrogen gas by Anammox bacteria. To obtain a stable partial nitritation, ammonium oxidizing bacteria (AOB) have to prevail over nitrite oxidizing bacteria (NOB) so as to avoid further conversion of nitrite into nitrate. Two control objectives can be identified for partial nitritation: the repression of NOB and the regulation of the effluent to obtain the required nitrite to ammonium ratio. In this work, the equilibrium points of the partial nitrification process are analyzed to find operating conditions for AOB prevalence over NOB. Based on this analysis, a feeding strategy is proposed to regulate the effluent nitrite to ammonium ratio at the value required by Anammox. The study is based on the process dynamical model in a reactor with biomass retention.