The study addresses the prolonged start-up phase in Moving Bed Biofilm Reactors (MBBRs), a key limitation that hampers the widespread adoption of biofilm-based wastewater treatment technologies despite their high efficiency in removing nitrogen and organic pollutants. This delay is primarily due to the slow establishment of biofilms on carrier media. To overcome this, a pilot-scale study was conducted to evaluate the influence of carrier media physical properties—shape, density, voidage, and hydraulic efficiency—on biofilm development using real municipal wastewater. The system, comprising anaerobic, anoxic, and aerobic zones, was operated over 90 days with four different media types, each with distinct geometrical and surface area characteristics. Results showed that spherical carriers with higher voidage and lower density promoted significantly faster biofilm growth, achieving stable biofilm accumulation in 35–45 days. Media 2, a cylindrical type, achieved the highest attached biomass at 10.12 g TS/m² and a biofilm growth rate of 10.12 g TS/m²/day, while Media 1 (spherical) demonstrated enhanced start-up dynamics due to its superior hydraulic properties despite lower total surface area. COD and BOD₅ removal efficiencies improved correspondingly, achieving effluent concentrations as low as 18–20 mg/L (COD) and 4–7 mg/L (BOD₅). The study suggests that traditional measures that rely only on the percentage of protected surface are inadequate. Rather, a composite variable—diameter-to-void ratio divided by hydraulic efficiency—better over hydraulics than predicts biofilm performance. The work described in this study demonstrates a new method for assessment of the carrier media effectiveness, which can be readily utilized as a tool for minimizing the MBBR start-up time, optimizing the media design and improving the scalability of moving biofilm systems for sewage treatment.