Membrane surface modification and backpulsing for wastewater treatment

Using a novel photoinduced grafting method, hydrophobic polypropylene (PP) membranes were rendered hydrophilic by grafting monomers of poly(ethylene glycol 200) monomethacrylate (PEG200MA), dimethyl aminoethyl methacrylate (DMAEMA), or acrylic acid (AA), to produce a neutral, positive, or negative charge, respectively, on the membrane surface. Using both unmodified and modified PP membranes, as well as a hydrophilic cellulose acetate (CA) membrane, the effects of backpulsing and surface chemistries on membrane fouling were investigated for crossflow microfiltration of bentonite clay suspensions and crude oil emulsions. Without backpulsing, the permeate volumes collected over 60 min were not strongly dependent on membrane surface chemistry and morphology for the filtration of either clay or oil. With backpulsing, however, 5-fold and 6-fold permeate enhancements were obtained by backpulsing alone and by a combination of backpulsing and surface modification, respectively, for clay filtration. The recovered water fluxes after backwashing the PP membranes fouled without backpulsing were approximately 80% of the initial water flux, showing that bentonite fouling is primarily nonadhesive. However, the recovered fluxes with backpulsing were slightly less than those without backpulsing, due to internal fouling after each backpulse. For the filtration of crude oil, 1.3-fold and 2.7-fold permeate volume enhancements were obtained by backpulsing alone and by a combination of backpulsing and surface modification, respectively. More than a 3-fold permeate volume enhancement was obtained upon adding an anionic or cationic surfactant to the oil mixture when the modified membrane was similarly charged. The recovered flux after backwashing the unmodified PP membrane fouled without backpulsing or surfactant was only about 30% of the initial water flux, indicating adhesive primarily fouled by oil, but the recovered flux doubled when the membranes were rendered hydrophilic by surface modification.