Effect of pre-coagulation using different aluminium species on crystallization of cake layer and membrane fouling

Abstract Pre-coagulation could mitigate the membrane fouling, and thus we used different Al coagulants as a pre-treatment for ultrafiltration to explore their effects on the morphology of the membrane cake layer and fouling. Parallel bench-scale tests, using three different species of Al (AlCl 3 , PACl 15 , and PACl 25 ), with and without humic acid, were operated continuously for a long period to investigate the effects of floc aging (~13 days). Specifically, the presence of humic acid affects the cake layer by influencing the rate and extent of floc crystallization, as greater crystallization leads to greater fouling (bio-fouling was excluded in this study). The fouling rate (indicated by the trans-membrane pressure at constant flux) varied with Al species and was found to increase as follows: PACl 15 < PACl 25 < AlCl 3 . The presence of humic acid also intensify membrane flouling. The results showed that three species of Al induced different sizes of primary nanoparticles and fractal dimensions of flocs, and therefore produced cake layers with different thickness/structure. Analysis of flocs with different ages indicated a crystallization process in the cake layer. Crystallization exacerbate membrane fouling by decreasing the porosity of the cake layer, and the situation became severe in the presence of humic acid.