Partial nitritation-anammox process is an interesting process option for biological nitrogen removal from wastewater. Partial nitritation needs to provide about equimolar ammonium and nitrite concentrations as required for the Anammox conversion. This study analyzes the equilibrium points of a stand-alone partial nitritation process as part of a two-reactor partial nitritation-anammox configuration. Conditions for the feasibility and stability of the equilibrium points are obtained in the general case of a reactor with biomass retention and controlled oxygen concentration. This knowledge is subsequently applied to achieve the desired reactor behavior, namely suppressing nitrite oxidizing bacteria (NOB) and obtaining the desired effluent nitrite:ammonium ratio. The optimal oxygen concentration to achieve these two objectives is determined. Moreover, a control algorithm is proposed to drive the process to the desired operating point. Its steady state behavior is assessed for changing influent ammonium concentrations and biomass retention times. The dynamic performance of the controller is tested through simulation as well, prioritizing NOB suppression in case the two objectives are not compatible. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.