Effect of coagulation mechanism on membrane permeability in coagulation-assisted microfiltration for spent filter backwash water recycling

Coagulation is an effective pre-treatment process in membrane filtration for recycling spent filter backwash water (SFBW). To optimize the operation of the coagulation/filtration process for SFBW recycling, it is important to understand the critical role of coagulation mechanism on membrane filtration. In this study, SFBW samples were coagulated with polyaluminum chloride (PACl), followed by a dead-end microfiltration (MF), and the mean permeate flux and permeate quality were determined. The results showed that pre-coagulation improved the flux decline and the degree of the improvement was closely related to coagulation mechanisms. The permeate flux was enhanced most effectively by precipitation charge neutralization (PCN) coagulation, followed by charge neutralization and then sweep flocculation. Close examination of cake properties indicated that the reduced cake resistance (Rc) was subjected to the coagulated SFBW flocs in response to different coagulation mechanism. Smaller flocs, induced by PCN, were more compact and stronger, forming less compressible cake, which facilitated membrane filtration, while slower membrane filtration was observed when more compressible cake formed from larger flocs of looser and weaker structure. Coagulation mechanism also governed the size distribution of SFBW floc, which is strongly related to the subsequent cake compressibility as well as membrane permeability in membrane filtration.