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This work systematically studied the role of pre-coagulation in the performance of ultralow pressure membrane system for algae-laden water treatment. The membrane performance with/without pre-coagulation was compared in terms of membrane permeate flux, water quality and membrane fouling. Ultralow pressure membrane system can effectively reduce TOC of Microcystis aeruginosa -laden water from 5.8 to 2.1 mg/L, and pre-coagulation removed most large inorganic particles but few small organic particles. Interestingly, pre-coagulation aggravated the fouling of ultralow membrane system which is generally acknowledged method to alleviate the ultrafiltration membrane fouling. According to Extended Derjaguin-Landau-Verwey-Overbeek theory (XDLVO), the interaction energy of membrane-foulants (ΔG fm TOT = − 41.95 mJ/m2), and foulant-foulant (ΔG ff TOT = − 30.15 mJ/m2) with coagulation were higher than those without coagulation (ΔG fm TOT = − 36.54 mJ/m2) and (ΔG ff TOT = − 15.73 mJ/m2) suggesting greater adherence between membrane and foulants & foulant and foulant after coagulation, which well agreed with SEM results. Membrane fouling models were also applied to analyze the fouling mechanism of ultralow-pressure membrane filtration. Based on above analysis, the possible fouling mechanisms for membrane filtration with/without precoagulation were proposed and then confirmed by pre-filtration experiment, where large inorganic particles played important roles. Our study could be indicative for membrane fouling control of ultralow-pressure membrane filtration for the treatment of algae-laden water. Unlabelled Image • Ultralow pressure membrane system effectively treated algae-laden water • Pre-coagulation aggravated the fouling of ultralow pressure membrane system • Interaction energy of membrane-foulants and foulant-foulant increased after coagulation. • Membrane fouling mechanisms with/without coagulation were proposed and confirmed • Compact and adhesive fouling layer caused severe fouling after coagulation. [ABSTRACT FROM AUTHOR]