BACKGROUND Ultrafiltration (UF) technology is efficient in treating wastewater. However, membrane fouling during filtration process affects the performance of UF to a large extent. Pretreatment prior to UF is important for enhancing membrane performance, reducing membrane fouling and extending the membrane lifetime. RESULTS In this study, two pretreatment technologies, including coagulation and Fenton, were applied individually before UF for treating secondary effluent (SE) from recycled paper mill. Results exhibited that direct membrane filtration got the highest flux loss and worst membrane fouling compared with performances of those conducting pretreatments. Compared to coagulation + UF, Fenton + UF had superior performance in reducing dissolved organics with a COD rejection of 91.81%, and mitigating membrane fouling with a stable flux of 133 J m(-2) h(-1). GPC results further showed that Fenton pretreatment degraded macromolecules into smaller organic compounds. In addition, the classical blocking filtration laws were employed to scrutinize the fouling alleviation mechanism of pretreatments. CONCLUSION Pretreatments prolonged the transition from pore blocking to cake filtration, thereby reducing fouling. Among them, Fenton was a promising pretreatment method prior to UF for treating recycled paper SE. (c) 2018 Society of Chemical Industry