Influence of floc structure on coagulation–microfiltration performance: Effect of Al speciation characteristics of PACls

Microfiltration performance in coagulation–microfiltration hybrid systems (without settling process) has strong correlation with the floc structure. This study investigated the effect of Al speciation distribution (Ala, Alb and Alc) of PACls (polyaluminum chlorides) on floc structure and the overall membrane performance. The floc properties, analyzed with small angle laser light scattering (SALLS), of different aluminum based coagulants, AlCl3 (PACla, with high Ala species), polyaluminum chloride (PAClb, with high Alb species), commercial polyaluminum chloride product (PAClp, with similar amount of each Al species) were compared using synthetic waters with added humic acid and kaolin particles. The coagulated waters were then fed through a dead-end microfiltration system using an unstirred batch cell with 0.22 μm PVDF flat sheet membrane. The results show that the property of flocs, largely depended on aluminum species, is the most influencing factor for membrane filtration behaviors. The floc sizes from different coagulants are in the following order PACla > PAClp > PAClb, which is consistent with the amount of Ala species in the PACls. The fractal dimension values are very similar for each PACls. The floc strength follows the order of PACla > PAClb > PAClp. Cake made of PACla-flocs shows a noticeable flux advantage followed by PAClp-flocs and PAClb-flocs, which agrees well with the order of floc size. However, the flux decline of PAClp-flocs is much worse than PACla-flocs although both of them have similar final steady-state floc size. The difference between the flux of PAClb-flocs and PAClp-flocs is not as significant as the difference between their floc sizes. This can be explained that the PAClp-flocs are much more fragile than PAClb-floc due to its weak floc strength. These results also suggest that floc strength is another very important factor influencing the filtration performance.