This study evaluated the roles of suspended flocs and attached biofilms from integrated fixed-film activated sludge (IFAS) systems in their overall contribution toward organic compound removal in oil sands process-affected water (OSPW). The batch experiments were operated with suspended flocs and biofilm carriers from our continuous laboratory-scale IFAS reactors for the OSPW treatment. A distinct difference between IFAS flocs and biofilms for the chemical oxygen demand (COD), ammonium and acid extractable fraction (AEF) removal was demonstrated. Compared to the biofilms, the flocs demonstrated considerably higher removal rates for COD and ammonium, whereas, biofilms had better performance on the AEF removal than flocs. Meanwhile, the results also revealed that the biodegradation was the principal removal mechanism whereas, the biosorption contributed little to the OSPW organic compounds and the ammonium removals in the IFAS system. Microbial analysis from q-PCR revealed that the abundances of nitrifiers and denitrifiers genes were significantly higher in flocs than in biofilms in both raw and ozonated-OSPW IFAS reactors. The microbial communities analysis from MiSeq sequencing showed that Proteobacteria, Acidobacteria, Nitrospirae and Bacteroidetes were dominant phyla in both flocs and biofilms. (C) 2016 Elsevier B.V. All rights reserved.