The performance of a biofilm system, single-stage flexible fibre biofilm reactor (SS-FFBR) treating milk processing wastewater (MPW) is evaluated under various process and operational conditions. The system behavior is analyzed with different biological and physical parameters. Results show that the high COD removal efficiency of 95% is obtained at a low CODin concentration of 809 mg/L. However, the COD removal is slightly decreased to 91.7% once the CODin concentration incremented to nearly 4000 mg/L. The effect of organic loading rate (OLR) on the SS-FFBR performance is examined as total suspended solids removal efficiency, dissolved oxygen, and turbidity. The SS-FFBR showed considerable performance, so that 89.9% and 89.7% removal efficiencies in terms of COD and TSS removals, respectively, obtained at the highest OLR of 11.7 kg COD/m3d. TSS removal efficiency of 96.7% is obtained at a low OLR of 1.145 kg COD/m3d. A linear relationship between the OLR and COD removal rate was revealed. The COD removal rate was incremented from 1.08 to 10.68 kg COD/m3d as the OLR increased from 1.145 to 11.7 kg COD/m3d. Finally, the operating system is a promising technique recommended to treat various industrial wastewaters with high OLR.