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Pre-oxidation is widely used to reduce ultrafiltration membrane fouling.However,the variation in the composition of microbial communities and extracellular polymeric substances (EPSs) accompanying pre-oxidation in drinking water treatment has received little attention.In this study,hydrogen peroxide (H2O2) was used in a coagulation-ultrafiltration process with Al2(SO4)3·18H2O.A long-term reactor experiment (60 d) showed that pre-oxidation alleviated membrane fouling,mainly due to its inhibition of microbial growth,as observed by flow cytometry measurements of the membrane tank water.Further analysis of the formed cake layer demonstrated that the corresponding levels of EPS released from the microbes were lower with than without H2O2 treatment.In comparison to polysaccharides,proteins dominated the EPS.2D-electrophoresis showed little difference (p ＞ 0.05,Student’s t-test) in the composition of proteins in the cake layer between the treatments with and without H2O2.The molecular weights of proteins ranged from approximately 30-50 kDa and the majority of isoelectric points ranged from 6 to 8.High-throughput sequencing showed that the predominant bacteria were Proteobacteria,Bacteroidetes,and Verrucomicrobia in both cake layers.However,the relative abundance of Planctomycetes was higher in the cake layer with H2O2 pre-oxidation,which was likely probably due to the strong oxidative resistance of its cell wall.Overall,our findings clarify the fundamental molecular mechanism in H2O2 pre-oxidation for ultrafiltration membrane bio-fouling alleviation in drinking water treatment.