Role of visible light photocatalysis in alleviation and mechanism transformation of ultrafiltration membrane fouling caused by natural organic matter

•UF membrane fouling was alleviated depended on pre-photocatalysis time.•UV254 reduction played an important role in the mitigation of membrane fouling.•HMW NOM was degraded into LMW-Acids, with increased hydrophilicity.•Fluorescent humic and protein/polyphenol components led to reversible and irreversible fouling, respectively.•Partially NOM fractions hindered or even inhibited the formation of the cake layer.