One of the largest wastewater treatment plants in the Paris conurbation (240,000 m3/d) has been studied over several years in order to provide technical and economical information about biological treatment by biofiltration. Biofiltration systems are processes in which carbon and nitrogen pollution of wastewater are treated by ascendant flow through immersed fixed cultures. This paper, focused on technical information, aims: (1) to compare performances of the three biological treatment layouts currently used in biofiltration systems: upstream denitrification (UD), downstream denitrification (DD) and combined upstream-downstream denitrification (U-DD) layouts; and (2) to describe in detail each treatment step. Our study has shown that more than 90% of the carbon and ammoniacal pollution is removed during biological treatment, whatever the layout used. Nitrate, produced during nitrification, is then reduced to atmospheric nitrogen. This reduction is more extensive when the denitrification stage occurs downstream from the treatment (DD layout with methanol addition), whereas it is only partial when it is inserted upstream from the treatment (UD layout – use of endogenous carbonaceous substrate). So, the UD layout leads to a nitrate concentration that exceeds the regulatory threshold in the effluent, and the treatment must be supplemented with a post-denitrification step (U-DD layout). Our work has also shown that the optimal ammonium-loading rate is about 1.1–1.2 kg N-NH4+ per m3 media (polystyrene) and day. For denitrification, the optimal nitrate-loading rate is about 2.5 kg N per m3 media (expanded clay) and day in the case of DD with methanol, and is about 0.25 kg N-NO3− per m3 media and day in the case of UD with exogenous carbonaceous substrate.