Effect of shear conditions on floc properties and membrane fouling in coagulation/ultrafiltration hybrid process—The significance of Alb species

Impacts of increased shear during coagulation on ultrafiltration permeate flux in coagulation–ultrafiltration (C–UF) hybrid process were investigated in this study. Three Al-based coagulants, alum, PACl and PACl–Alb, were applied to explore the role of Alb species in the C–UF process. Floc characteristics, including floc size, strength, re-formation ability and fractal structure under different coagulation conditions, were studied using a laser diffraction particle sizing device. Additionally, resistance analyses were conducted to investigate the membrane fouling mechanisms. The results indicated that alum generated the largest flocs with loosely bonded structures before breakage, which were, however, the weakest and with poor re-growth abilities. PACl–Alb contributed to the strongest flocs with the best re-growth abilities and the highest compact degree. The results of ultrafiltration experiments showed that for conventional coagulation without breakage, alum led to a flux reduction of 34%, while the reduction for PACl and PACl–Alb was 39% and 49% respectively. However, under the increasing breaking shears, the adsorption and cake layer resistances for alum coagulation increased and fluxes were markedly aggravated; PACl–Alb coagulation effluent displayed the least resistances and flux variations. An extension in breakage time led to much severer flux decline than short breakage period. PACl–Alb led to the smallest changes in flux declines.