Comparison of FeCl3 and alum pre-treatment on UF membrane fouling

•The increase rate of TMP was greatly higher for the FeCl3–UF system than alum–UF.•Smaller primary particles and nano-scale particles were evident in FeCl3–UF system.•The lower porosity and greater thickness of the cake layer in FeCl3–UF system. Coagulation pre-treatment can enhance the performance of membrane filtration by reducing fouling effects, and the choice of coagulant is one of the key variables. In this study a comparison has been made between two conventional, metal-ion coagulants, alum and FeCl3, with respect to their impact on ultrafiltration (UF) membrane fouling. Extensive laboratory tests were conducted to characterize the nature of the coagulant flocs (size, charge and fractal dimension), the membrane cake layer (thickness and porosity), and the resulting UF performance (NOM removal, rate of TMP increase). The results have shown that for the same coagulation conditions, the rate of increase of TMP was significantly higher for the FeCl3–UF system than the alum–UF system. Various differences were observed between the two systems, principally: (a) that larger floc sizes, with more active surface and a lower fractal dimension, were found with the FeCl3–UF system which corresponded with a thicker membrane cake layer; (b) smaller primary particles and nano-scale particles were evident in the FeCl3–UF membrane tank which are more able to block the pores of the cake layer on the surface of the membrane, thereby producing a higher density/lower porosity of cake layer. The lower porosity and greater thickness of the cake layer in the FeCl3–UF system is the likely cause of the larger hydraulic resistance (rate of TMP increase) observed.