Effects of polydimethyldiallylammonium chloride (PD) on coagulation behaviors of different Al-based coagulants were investigated in coagulation-ultrafiltration (C-UF) hybrid process, regarding the impurity removal efficiency, floc properties and membrane foulings. Floc characteristics, including floc size, compact degree, strength and re-growth ability were studied using a laser diffraction particle sizing device. Resistance analyses were implemented to explore the membrane fouling mechanisms. The results indicated that PD aid could increase the purification efficiency of C-UF, especially at low coagulant doses. PD-alum/PD-PACl contributed to large flocs, while alum-PD/PACl-PD gave rise to flocs with high R-f and D-f values. The results of ultrafiltration experiments showed that conventional coagulant, i.e,, alum led to a flux reduction of 52%; while the reductions for alum-PD and PD-alum were 53% and 34%, respectively. The flux reductions for PACl, PACl-PD and PD-PACl were 60%, 57% and 39%. Flux declines became more severe when the coagulated suspensions were exposed to increased shears and the suspensions coagulated by aluminum plus PD resulted in the least reductions in fluxes. (C) 2014 Elsevier B.V. All rights reserved.