The effects on structure, morphology, composition and characteristics of reverse osmosis membrane fouling were investigated by an effluent with different Ca/Al ratios of membrane bioreactor in a biopharmaceutical enterprise.The results demonstrated that calcium and aluminum combined with organic matter formed a composite fouling layer on the membrane surface,and the change of Ca/Al ratio resulted in significantly different membrane fouling characteristics.The form of Al-O-H in the solution was preferentially combined with the organic functional groups by polar force,and it was co-deposited with the coagulation of the hydrophobic organic components under the condition of low aluminum content and high Ca/Al ratio.It had a far greater effect on the formation of membrane fouling than the Ca component. According to the extended Young's equation,the total surface tension of the system was calculated as:53.0 mJ/m² (low Ca low Al),69.5 mJ/m² (low Ca high Al),70.4 mJ/m² (high Ca high Al).It showed that the change of aluminum content mainly affected the polar force between the solution and the membrane material, and the change of the calcium content had almost no corresponding effect.The results of membrane cleaning indicated that the membrane fouling caused by calcium-aluminum components were stratified in spatial structure,and aluminum was tightly bound to the bottom layer of membrane fouling by the polar force between the molecules on the membrane surface, forming irreversible fouling.Calcium mainly existed in the contaminated filter cake layer which can be easily recovered by chemical cleaning.Therefore,it is of great significance to accurately control the dosage of aluminum salt in the coagulation pretreatment stage to prevent membrane fouling.