Abstract Wastewater treatment plants are highly energy consuming and can contribute up 1 % of a citys energy consumption. A high amount of this energy can be saved by applying an appropriate operation mode. Anaerobic digestion of influent organic matter can produce biogas and the overall energy consumption can this way be reduced. Remaining sludge after anaerobic digestion is dewatered and results in an effluent, called reject water. Reject water or Sludge Dewatering Effluent (SDE) needs further treatment, since it still contains a high amount of ammonium, which needs to be removed. Pretreatment of reject water in the side-stream can save additional energy. The potential for the energy saving lies in a higher aeration efficiency in side-stream compared to main-stream. Moreover, when stopping the conversion process from ammonium to nitrite (nitritation) and further to nitrate (nitratation) at the first conversion step, less COD is required in the denitrification process of a 2-stage wastewater treatment plant. A higher amount of COD in the sewage sludge leads to a higher biogas yield in anaerobic digestion. The present thesis compares a lab-scale Continuously-Stirred-Tank-Reactor (CSTR) and a Moving-Bed-Biofilm-Reactor (MBBR) regarding their nitritation performance. It will mainly focus on the influencing parameters like sludge age and free ammonia/free nitrous acid inhibition in order to achieve partial nitritation in SDE side-stream treatment. Partial nitritation is achieved in CSTR down to a temperature of 15C. MBBR is able to treat up to 3 times the ammonium load of a CSTR and also showed stable nitritation with even lower susceptibility towards changing operation conditions.