A full scale demonstration project of an Integrated Fixed Film Activated Sludge process (IFFAS) with floating biofilm carriers has been operating at the Lakeview wastewater treatment plant (Ontario, Canada) since the fall of 2003. In this study, 18 months of data collected on site from July 2005 to December 2006 are analysed and compared to the performance of a conventional activated sludge (CAS) operated in parallel as a control. Both trains had similar loading and suspended solids concentrations, but the IFFAS had 50% more biomass. In the winter, the conventional train operated at the nitrifier washout limit and had unstable partial nitrification. The IFFAS had more stable ammonia removal, and a doubled average nitrification capacity. In the summer when the suspended SRT was less limiting, the difference between the IFFAS and the CAS nitrification performance was marginal. There are indications that the IFFAS train had slightly improved settling characteristics, but it cannot be established if they were due to the anoxic selector installed at the beginning of the train or to the fixed biomass itself. In this demonstration study, the performance benefits of the IFFAS technology were hampered by recurring operational issues including impaired carrier mixing, seasonal foaming in the winter, challenging carrier transfer during tank maintenance, and carrier containment failures.