Influences of polysilicic acid in Al13 species on floc properties and membrane fouling in coagulation/ultrafiltration hybrid process

Effect of polysilicic acid (pSi) on pre-formed Al13 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. Al13 produced stronger flocs than PACl, while pSi enhanced the floc strength factors for both Al13 and PACl. However, Al13–Si and PACl–Si led to smaller recovery factors (Rf) of aggregates than Al13 and PACl. Additionally, the flocs formed by Al13 presented compacter degree (Df = 2.34) than those formed by PACl (Df = 2.22); while both Al13–Si and PACl–Si gave rise to much loosely constructed flocs with similar Df values of 2.15 and 2.12, respectively. The results of ultrafiltration experiments implied that Al13 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 Al13–Si and PACl–Si on flux declines and filtration resistances were similar.

Normalized flux declines of effluents coagulated by Al13, PACl, Al13–Si and PACl–Si and the corresponding resistance analysis.Figure optionsDownload as PowerPoint slideHighlights
► PSi enhanced floc aggregation and strength but militated against floc Rf and Df.
► PSi additive diminished the superiorities of Al13 in floc Sf and Df over PACl.
► PSi was more beneficial to PACl than Al13 in terms of filtration fluxes.