Aerobic biofilm systems are increasingly used for household wastewater treatment, but little is known about their potential to emit nitrous oxide in response to different loading conditions. We studied nitrification and nitrous oxide production in a biofilm reactor that was continuously fed with three different mixtures of household wastewater. Higher proportions of blackwater increased nitrification activity, which resulted in enhanced nitrite accumulation and nitrous oxide emissions. Applying a conceptual biofilm model together with the results of ancillary batch incubations suggested that this was caused by a higher proportion of slowly degradable compounds in blackwater. Increasing amounts of blackwater would result in less oxygen depletion by heterotrophic degradation in outer biofilm layers, leading to nitrite accumulation by enhanced ammonia oxidation as well as electron limitation of denitrification in anoxic biofilm layers. Under such conditions, nitrifier denitrification and incomplete heterotrophic denitrification would be the prevailing sources of nitrous oxide emission. These assumptions are supported by an exponential increase in the determined emission factor (nitrous oxide relative to oxidized ammonia), which accounted for 0.7 with 20 % blackwater, 1.1 with 50 % blackwater, and 8.5 with 100 % blackwater in the wastewater load.