Treatment of real-life industrial oily wastewater using surface-functionalized PVDF membranes via low-temperature radio-frequency nitrogen plasma for high-performance filtration

The present study investigates the real-life application of membrane filtration for the treatment of industrial oily wastewater collected from a super thermal power plant. Flat-sheet polyvinylidene fluoride membranes were surface-treated via radio-frequency plasma discharge of nitrogen gas to enhance their antifouling properties. The modification aimed to implant hydrophilic functional groups onto the surface while preserving the structural integrity of the membrane. Various characterization techniques, including XPS, SEM, AFM, and universal testing machine, were used to investigate the plasma-induced physical and chemical changes on the membrane surface. Even after 325 days of ageing, the membrane maintained a significantly lower water contact angle of around 22° ± 1.5° compared to the pristine one (90° ± 3.5°), indicating only ~14% recovery in the contact angle. For filtration experiments, a batch cell setup was employed to treat industrial oily wastewater under higher transmembrane pressures. After plasma exposure, the membrane permeability increased by about 2.6 times, and the oily water flux was enhanced by ~3.7-fold at 689.5 kPa (6.8 bar). The flux decline ratio showed ~20% reduction at 517 kPa (5.1 bar), and the flux recovery ratio reached ~100% (at both 517 kPa and 689.5 kPa), indicating excellent cleaning efficiency after surface treatment of the membrane. The Hermia's model was applied to investigate the fouling mechanisms; the permeability predictive transport model for RF-exposed membranes was used to validate the experimentally observed results; and a theoretical estimation of the critical pressure was developed to assess oil droplet penetration at the pore mouth on the membrane surface.