Antifouling nanofiltration membranes for membrane bioreactors from self-assembling graft copolymers

Ultrafiltration (UF) membranes, often employed in membrane bioreactors (MBRs), exhibit high susceptibility to fouling by extracellular polymeric substances (EPS). As potential alternatives, commercial polyvinylidene fluoride (PVDF) UF membranes were coated with the amphiphilic graft copolymer poly(vinylidene fluoride)-graft-poly(oxyethylene) methacrylate, PVDF-g-POEM, to create thin film composite (TFC) nanofiltration membranes. Pure water permeabilities up to 56 L/m2 h MPa were obtained at pressures of 0.21 MPa (30 psi). The new TFC NF membranes exhibited no irreversible fouling in 10-day dead-end filtration studies of model organic foulants bovine serum albumin, sodium alginate and humic acid at concentrations of 1000 mg/L and above. Dead-end filtration of activated sludge from an MBR (1750 mg/L volatile suspended solids, VSS) resulted in constant flux throughout the 16 h filtration period. Fouling performance of the TFC NF membrane and effluent water quality were substantially improved in all cases over that for the base PVDF UF membrane. Utilizing the atomic force microscope (AFM) colloid probe technique, the measured interaction force profiles indicated the presence of repulsive steric interactions, which likely prevent the attachment of foulants to the TFC NF membrane. Similarly, the adhesion (pull-off) curves reveal the absence of foulant adhesion to the TFC NF membrane surface, even in the presence of divalent calcium ions. In contrast, when such force measurements are carried out with the base PVDF UF membrane, substantial adhesion forces are registered.