The objective of this study is to improve knowledge on the IFAS system designed for nitrogen removal. Biofilm growth and its contribution to nitrification was monitored under various operating conditions in a semi-industrial pilot-scale plant. Nitrification rates were monitored in biofilms developed on free-floating media and in activated sludge operated under a low sludge retention time (4 days) and at an ammonia loading rate of 45 to 70gNH¬4-N/kgMLVSS/d. Operational conditions, i.e. oxygen concentration, redox potential, suspended solids concentration, ammonium and nitrates, were monitored continuously in the reactors. High removal efficiencies were observed on carbon and ammonium at high loading rate. The contribution of biofilm to nitrification was determined as 40% to 70% of total NOx-N production under the operating conditions tested. This study was able to define optimal conditions to optimize process compacity. The tested configuration responds especially well to winter and summer nitrification conditions. These results help provide a deeper understanding of how autotrophic biomass evolves through environmental and operational conditions in IFAS systems.