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• The moderate KMnO 4 /Fe(Ⅱ) pre-oxidation process was quantified. • The KMnO 4 /Fe(Ⅱ) pretreatment delays the formation of cake layer blockage. • In-situ formed MnO 2 has the best effect for organic removal. • In-situ formed Fe(III) has the best effect on algal aggregation. Moderate KMnO 4 /Fe(II) pre-oxidation is a promising technique to mitigate membrane fouling and avoid the environmental risk brought by the breakage of algae cells, which often happens in the practical application of pre-oxidation techniques. Various components, i.e., KMnO 4 , in-situ formed MnO 2 and in-situ formed Fe(III), are involved in this technique, and how they mitigate membrane fouling is still unclear. In this work, we investigated their performances on the mitigation of membrane fouling and their influences on the physiochemical characteristics of the organic foulants and fouling layers individually. It was found that Fe(III) formed in situ showed the best performance in controlling membrane fouling among the various components, and the total fouling resistance was reduced by 87.6%. MnO 2 formed in situ was most effective in enhancing the organic matter removal performance of ultrafiltration, and the removal rate of the dissolved organic matter reached 39.9%. In addition, after the KMnO 4 /Fe(II) pretreatment, the electrostatic repulsion between algal foulants was decreased, facilitating the aggregation of foulants. Based on the physicochemical changes of the characteristics of foulants and fouling layers on the membrane surface, we concluded that KMnO 4 /Fe(II) alleviated the algae-derived membrane fouling mainly in two ways: (1) enhancing the interception effect of ultrafiltration membrane on hydrophobic organic matter, which reduced the blockage of membrane pores; (2) enhancing the sparseness of the cake layer on the membrane surface, which reduced the cake fouling resistance. Our work not only shows that the KMnO 4 /Fe(II) pretreatment technique has considerable applicative potential in alleviating the membrane fouling caused by algae-laden water, but also gives a comprehensive understanding of the fouling mitigation mechanisms. [ABSTRACT FROM AUTHOR]