Fractal structure and permeability of membrane cake layers: Effect of coagulation–flocculation and settling as pretreatment steps

• Inline, direct, and conventional pretreatments for ultrafiltration are studied.
• New method for quantifying fractal dimension of membrane cakes is proposed.
• Average porosity of membrane cakes increases with filtration time.
• Fractal dimensions of flocs and membrane cakes weakly correlate.
• Broad range of DflocDfloc values maps onto a narrow range of DcakeDcake values.

Effect of pretreatment on the flux performance of ultrafiltration membranes is studied and interpreted in terms of structural characteristics of suspended flocs and membrane cakes formed from these flocs. Coagulation–flocculation with different values of G‾tmix and optional pre-settling are applied to approximate pretreatment conditions of inline, direct, and conventional filtration processes. A new method for experimentally determining fractal dimension of membrane cakes, DcakeDcake, based on permeate flux data and Happel permeability model is proposed and employed to show that DcakeDcake correlates with the fractal dimension of flocs, DflocDfloc. The broad range of DflocDfloc values (1.95 < DflocDfloc < 2.56) is found to map onto a narrow range of DcakeDcake values (2.92 < DcakeDcake < 2.99) indicating breakup and restructuring that flocs undergo as they are incorporated into a growing cake. Information on the structure of flocs and membrane cakes complements flux data and may be useful in optimizing feed pretreatment in terms of the tradeoff between the mass of the cake, its permeability and responsiveness to hydraulic and chemical cleaning.

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