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Mathematical models are useful tools to optimize the performance of granular sludge reactors. In these models, typically a uniform granule size is assumed for the whole reactor, even though in reality the granules follow a size distribution and the granule size as such affects the process performance. This study assesses the effect of the granule size distribution on the performance of a granular sludge reactor in which autotrophic nitrogen removal is realized through one‐stage partial nitritation–anammox. A comparison is made between different approaches to deal with particle size distributions in one‐dimensional biofilm models, from the use of a single characteristic diameter to applying a multiple compartment model. The results show a clear impact on the conversion efficiency of the way in which particle size distribution is modeled, resulting from the effect of the granule size on the competition between nitrite …