Surface water nanofiltration incorporating (electro) coagulation–microfiltration pretreatment: Fouling control and membrane characterization

Bench-scale experiments were performed to evaluate the capability of four different advanced pretreatment processes to reduce fouling of a commercially available thin-film composite nanofiltration (NF) membrane during surface water treatment. Specifically, microfiltration (MF) alone, chemical (alum) coagulation–MF, electrocoagulation–MF, and electroflotation–MF were compared in terms of their ability to maintain specific flux during NF of Lake Houston water. It was empirically determined that NF productivity decline was best controlled by electroflotation–MF. Attenuated total reflection-Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy revealed polysaccharides and proteins as major NF foulants. Coagulation induced subtle differences in protein foulants in terms of their secondary structures as inferred from second derivative analysis of the amide I band. For example, electro- and chemical-coagulation decreased intramolecular β structures and α helices but increased 310-helices and unordered structures indicating conformational changes of protein foulants with pretreatment type. Alcian Blue staining established the presence of acidic polysaccharides in the fouling layer. Similar air–water contact angles for virgin and fouled nanofilters provided further evidence that foulants were predominantly hydrophilic in nature. Fouling caused only a slight increase in surface roughness suggesting that natural organic matter complexation with divalent cations (soft feed water) was relatively insignificant leading to the deposition of a thin foulant layer. Further, the RMS surface roughnesses of fouled nanofilters were all statistically similar suggesting negligible effect of pretreatment type on the physical morphology of NF foulant layers.

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► Comparison of four different advanced pretreatment processes for surface water NF.
► Electroflotation–MF outperformed other pretreatments in terms of NF fouling control.
► Spectroscopic, microscopic, and physicochemical characterization of NF foulants.
► Hydrophilic NOM components (proteins and polysaccharides) were major NF foulants.
► Coagulation pretreatment changed conformation of proteinaceous foulants.