Surface modification of polypropylene microporous membranes to improve their antifouling property in MBR: NH3 plasma treatment

To improve the antifouling property of polypropylene hollow fiber microporous membranes (PPHFMMs) in a membrane bioreactor (MBR) for wastewater treatment, PPHFMMs were subjected to surface modification by NH3 plasma treatment. Structural and morphological changes of the membranes were characterized by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Water contact angle, which reflects the hydrophilicity of the membrane surface, was measured by the sessile drop method. Results of XPS clearly indicated that the plasma treatment introduced nitrogen- and oxygen-containing polar groups on the membrane surface. The water contact angle of the modified membrane reduced with the decrease of the initial NH3 pressure in the plasma reactor. To assess the relation between the plasma treatment and the membrane fouling in an MBR, membrane filtration for activated sludge was carried out using synthetic wastewater. PPHFMMs after NH3 plasma treatment showed better flux recovery after cleaning than that of the unmodified membrane. Under the operating conditions with 8-12 h hydraulic retention time and 6.2 g/L sludge concentration in the MBR, the removal efficiency of COD and NH3-N was above 95% and around 60%, respectively, for a 350 mg/L COD and a 10-15 mg/L NH3-N fed to the bioreactor.