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• The γ-Fe 3 O 4 -N-BC improved the adsorption properties of biochar effectively. • Nutrient removal was improved by inoculating γ-Fe 3 O 4 -N-BC in MBR. • Effective membrane fouling alleviation was achieved in γ-Fe 3 O 4 -N-BC-MBR. • Increase of sludge size and zeta potential in 500-MBR reduced gel layer formation. • The reduction PN/PS ratio in 500-MBR contributed to alleviate membrane fouling. In this study, a new type of iron–nitrogen doped biochar (γ- Fe 3 O 4 -N-BC) was developed to improve nutrient removal efficiency and mitigate membrane fouling in combination with MBR. The characteristion of γ-Fe 3 O 4 -N-BC, N-BC, and BC revealed that the BET surface area (S BET) of γ-Fe 3 O 4 -N-BC, which was 1320.86 m2/g, was greater than that of N-BC (702.75 m2/g) and BC (1033.63 m2/g). The excellent S BET exhibited by γ-Fe 3 O 4 -N-BC compared to that of the other biochars is attributed to hydrogen bonding and electrostatic interactions. Compared to the addition of N-BC or BC or the absence of biochar, the addition of γ-Fe 3 O 4 -N-BC in the membrane bioreactor (MBR) resulted in superior wastewater treatment performance, for which the average removal efficiencies of chemical oxygen demand (COD), ammonia nitrogen (NH 4 +-N), and total phosphorus (TP) were 94.65 %, 82.56 %, and 65.90 %, respectively. Moreover, after inoculation with γ-Fe 3 O 4 -N-BC, the TMP increase rate and time required to reach 42.37 kPa were 2.35 kPa/d and 18 d, respectively. The investigation of the effect of the γ-Fe 3 O 4 -N-BC concentration on MBR performance showed that the nutrient removal efficiency in the 500-MBR greater than that in the 350-MBR, 650-MBR, and 800-MBR, and that the membrane fouling was more ameliorative. A mechanistic investigation demonstrated that the 500-MBR had a beneficial influence on sludge biomass growth and helped to improve the nutrient removal capacity of the MBR. The 500-MBR improved the flocculability and stability of the sludge flocs in the MBR by increasing particle size and zeta potential. Changes in the extracellular polymeric substance (EPS) concentration and the smaller protein (PN) / polysaccharide (PS) ratio in 500-MBR also had significant impacts on alleviating the adhesion and accumulation of fouling layers on the improved membrane surface. Consequently, this study provides a new method for nutrient removal and membrane fouling mitigation in the MBR process. [ABSTRACT FROM AUTHOR]

• 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]