The aim of this study was to investigate the efficacy of a hybrid biofilm pilot-scale treatment plant, designed with a novel configuration by the integration of a fixed-film system, to improve nitrogen removal. The pilot-scale system was established at a wastewater treatment plant in Istanbul and operated based on stream separation following a process consisting of Bio-P and primary sedimentation units in which carbonaceous compounds were entrapped/incorporated in settled biomass. The ammonia-rich supernatant was directed to a moving bed biofilm (MBBR) nitrification tank to obtain an efficient nitrification with the reduced organic loading after the primary sedimentation. The conventional activated sludge process, for which the net specific growth rate ((mu) over cap (NH)-b(NH)) was measured to be 0.26day(-1) at 15 degrees C, exhibited a low nitrification capacity. However, the pilot-scale hybrid biofilm system secured nitrification performance up to 1.8 gN/m(2)/day ammonia loading, providing a competitive advantage over the conventional single sludge systems. The proposed hybrid configuration enables removal efficiencies of 80% and 85% for total nitrogen and phosphorus. It was possible to entrap organic matter by mixing 30% of return activated sludge (RAS) with raw wastewater. Simulation-based design study showed that the use of the hybrid biofilm system reduces the environmental footprint and aeration requirement of the nutrient removal by about 50% and 19%, respectively. Economic analyses highlighting the benefit of hybrid biofilm over conventional BNR system are illustrated.