This work presents a new control system for the nitritation of high-strength ammonium wastewater as reject water from sludge dewatering. It is based on three independent feedback control loops: (i) DO control by manipulating the aeration flow-rate, (ii) pH control with the addition of solid Na2CO3 and (iii) control of NH4--N concentration in the reactor using the influent flow-rate as the manipulated variable. Its application in an activated sludge configuration with one reactor and a settler, demonstrated: (i) capability to achieve stable effluent composition with proper NO2--N/NH4--N ratio for anammox treatment and ii) possibility to obtain an effluent with full nitritation suitable for heterotrophic denitrification only modifying the ammonium setpoint. A nitrogen loading rate (NLR) up to 5.0 +/- 1.0 gN L-1 d(-1) was stably treated using real reject water (T= 30 degrees C, pH = 7.5) with a NO2--N/(NO2--N + NO3--N) ratio of 99%. NLR reached up to 9.3 +/- 0.5 gN L-1 d(-1) with synthetic wastewater. (C) 2013 Elsevier B.V. All rights reserved.