Large wastewater treatment plants have to deal not only with the influent wastewater, but also with the highly concentrated reject water coming from anaerobic digestion. The aim of this work was to verify the suitability of using encapsulated nitrifiers in polyvinyl alcohol carrier (so called Lentikats Biocatalyst) at temperatures between 5 and 30 °C. For laboratory nitrification batch tests synthetic wastewater with ammonia nitrogen (Namon) concentration 10-800 mg L(-1) was used. The system has been proved to operate at the temperature of 10 °C, but not at 5 °C. It was found that the highest specific nitrification rates were observed at 30 °C and with ammonia nitrogen concentrations above 250 mg L(-1), which means that separate treatment of reject water by using encapsulated biomass seems to be an effective tool for wastewater treatment plant optimization.