Ammonia is toxic to aquatic organisms and the levels of ammonium often discharged with treated wastewater may well be exceeding upcoming Swedish environmental protection standards. A biofilm process was investigated combining nitrification after conventional biological treatment (by i.e. activated sludge) and nitrification of peak flows occurring at rain weather conditions, that would normally be by-passed biological treatment. A pilot plant was operated comprising two moving bed biofilm reactors (MBBRs) operated in parallel (average HRT 1.2 h). The reactors were fed biologically treated wastewater although one of them was intermittently fed with pre-settled, biologically un-treated, wastewater for part of the time. The average effluent ammonium nitrogen could be reduced to 0.2 mg/L. The higher periodic loading to the seconds reactor resulted in 2-5 times higher nitrification rate upon sudden increase in loading. However, this came at the expense of higher effluent ammonium level and formation of nitrite. The nitrification rate was mainly influenced by the loading rate and intermittent loading regime of nitrogen available for nitrification. The proposed process has potential to be a compact alternative for reaching low ammonium levels. Further optimization is currently being conducted with focus on minimized effluent concentrations while monitoring nitrous oxide emissions.