Nitrifying biologically active filters (BAFs) have been found to be high emitters of nitrous oxide (N2O), a powerful greenhouse gas contributing to ozone layer depletion. While recent models have greatly improved our understanding of the triggers of N2O emissions from suspended-growth processes, less is known about N2O emissions from full-scale biofilm processes. Tertiary nitrifying BAFs have been modeled at some occasions but considering strong simplifications on the description of gas-liquid exchanges which are not appropriate for N2O prediction. In this work, a tertiary nitrifying BAF model including the main N2O biological pathways was developed and confronted to full-scale data from Seine Aval, the largest wastewater resource recovery facility in Europe. A mass balance on the gaseous compounds was included in order to correctly describe the N2O gas-liquid partition, thus N2O emissions. Preliminary modifications of the model structure were made to include the gas phase as a compartment of the model, which significantly affected the prediction of nitrification. In particular, considering gas hold-up influenced the prediction of the hydraulic retention time, thus nitrification performances: a 3.5% gas fraction reduced ammonium removal by 13%, as the liquid volume, small in such systems, is highly sensitive to the gas presence. Finally, the value of the volumetric oxygen transfer coefficient was adjusted to successfully predict both nitrification and N2O emissions.