Biological nitrogen removal (BNR) as dinitrogen (N2) by simultaneous nitrification and denitrification (SNdN) could be accompanied by side pathways that emit N2O. A biofilm was developed that could be used for BNR in a stream with a high ammonium content, e.g., the liquid fraction from digestate. The BNR reactions through the depth of the biofilm were integrated into a scheme. The objective was to achieve the oxygen‐limited autotrophic nitrification‐denitrification process in a biofilm with minimum emissions of N2O with respect to those of N2 by SNdN. This was shown in a laboratory rotating biological contactor, in which ca. 53 % of the influent ammonium ions was removed by anaerobic ammonium oxidation. Furthermore, less than 3 % ammonia loss was attributed to side autotrophic denitrification as N2O.