Effect of polysilicic acid (pSi) on pre-formed Al-13 polycation ([Al13O4(OH)(24)(H2O)(12)](7+)) was investigated regarding the coagulation efficiency, floc properties and subsequent impact on membrane fouling in coagulation-ultrafiltration (C-UF) hybrid process. Also, the effect of pSi on the commonly used coagulant, polyaluminum chloride (PACl) was studied for comparison. Characteristics of aggregates pre-coagulated by different coagulants were investigated using a laser diffraction particle sizing device. And membrane fouling was investigated using a dead-end batch ultrafiltration unit. The results of this study indicated that the introduction of pSi into pre-hydrolyzed Al coagulants could obviously increase the coagulation efficiency, especially when the Si/Al ratio was 0.05. Al-13 produced stronger flocs than PACl, while pSi enhanced the floc strength factors for both Al-13 and PACl. However, Al-13-Si and PACl-Si led to smaller recovery factors (R-f) of aggregates than Al-13 and PACl. Additionally, the flocs formed by Al-13 presented compacter degree (D-f = 2.34) than those formed by PACl (D-f = 2.22); while both Al-13-Si and PACl-Si gave rise to much loosely constructed flocs with similar D-f values of 2.15 and 2.12, respectively. The results of ultrafiltration experiments implied that Al-13 produced less membrane fouling than PACl. Incorporation of pSi into Al coagulants could significantly alleviate the cake layer and adsorption resistance and thus the membrane fouling. The effects of Al-13-Si and PACl-Si on flux declines and filtration resistances were similar. (C) 2011 Elsevier B.V. All rights reserved.