Abstract: The effect of in-line coagulation on treated water quality and membrane fouling for immersed ultrafiltration(UF) was investigated in pilot-scale study using polyaluminium chloride as the coagulant,and the effluent water quality from the hybrid process was compared with that by direct UF and that by the conventional treatment process in an existing water treatment plant.According to the experiments,the removal efficiency for turbidity and pathogens of the immersed-UF was not affected by in-line coagulation,the effluent turbidity of direct UF and the hybrid UF process were both lower than 0.1 NTU and the total bacteria count were both no more than 5 CFU/mL.Compared with direct UF of raw water,the removal efficiency of the hybrid UF process for COD_Mn increased 8.1% and 14.3%,respectively,with the coagulant dosage of 20 mg/L and 30 mg/L and that for UV₂₅₄ increased 19.4% and 26.5%,respectively.Compared with direct UF of raw water,in-line coagulation with fewer dosages of coagulant than conventional treatment process can effectively mitigate membrane fouling,and higher coagulant dosage cannot improve membrane fouling better.