Biological active filters (BAFs) are widely developed for nitrogen removal in Europe especially in large cities. Recent studies have shown that nitrifying BAF can be N2O hotspots. Indeed, measuring campaigns performed at the Seine Aval WRRF (Paris, 5.5 million PE) reported high N2O emission (from 2% to 4% NH4-Nremoved) which contributed up to 80% of the carbon footprint of the biological nitrogen removal stage of the WRRF (Filali et al. 2017). Experimental and modelling studies on full-scale biofilm reactors are still rare (Ni & Yuan 2015). A comprehensive full-scale BAF model describing N2O production was recently developed and calibrated by Fiat et al. (2019). Multiple metabolic pathways for N2O production were considered with appropriate description of concentration gradients and mass transfer mechanisms in gas, liquid and biofilm phases. The purpose of this work is to scrutinise the key mechanisms underlying N2O production and to evaluate different control strategies for nitrifying BAFs. The calibrated model was used to investigate the production rate over a two-year period and to identify major influencing factors triggering N2O production. Then, it was used to evaluate different control strategies for N2O emissions reduction.