BACKGROUND The objectives of this study were to provide a detailed qualitative and quantitative characterization of different reject water types and to evaluate the effect of temperature and type of external organic carbon source on short-cut nitrification/denitrification process performance for the treatment of reject water with a high ammonia content. RESULTS CONCLUSION According to the results reject water originating from the sludge dewatering unit is characterized by a high ammonium-nitrogen content which can be further increased upon pre-treating the sludge in a thermal-hydrolysis unit, a low carbon to nitrogen ratio, a low biodegradability and contributes approximately 75% of the total ammonium-nitrogen load of the recycled flows. It is anticipated that the process can be sustained even at low temperatures (15 degrees C). The temperature effect on both nitritation/denitritation rates is described by an Arrhenius equation with activation energies equal to 37.6 and 39.6 kJ mol(-1) respectively. The addition of an organic carbon source rich in easily biodegradable compounds can provide for the achievement of high denitritation rates (in the order of 10 mgNO(2)-N gVSS-1 h(-1)). An interesting low cost option to reserve on-site an easily biodegradable organic carbon source by integrating different processes in a wastewater treatment plant is primary sludge hydrolysis. (c) 2018 Society of Chemical Industry