Membrane fouling can be significantly mitigated through circulating coagulation, floc breakage and re-growth under appropriate conditions. In this study, unified membrane filtration index (UMFI) was adopted to analyze the membrane fouling. The effect of circulating coagulation pretreatment on both flocs' average diameter and membrane fouling were investigated under different concentrations of coagulant and reflux flocs. When the concentrations of coagulant and reflux flocs were 6 and 10 mg/L, respectively, the average diameter of flocs and UMFI were 565.416 μm and 0.0004. Floc breakage and re-growth existed in the aeration cleaning; circulating coagulation was conducive to the flocs' re-growth and recombination. The recovery rate of flocs' average diameter could reach 90%. In circulating coagulation–membrane filtration process, flocs underwent re-growth and the characteristic of flocs changed which made size distribution uniform, slowed down the growth of transmembrane pressure and mitigated membrane fouling.

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