Layer-by-layer Self-assembly Preparation Of PVDF/TiO₂-Fe（OH）₃ Modified Membrane And Its Application In Membrane Bioreactor

Membrane bioreactor（MBR）,which organically combines membrane separation technology and activated sludge method,has received wide attention in the field of water treatment and reuse,and it is considered as a water-saving technology with good economic,social and environmental benefits.However,membrane fouling has always been a key difficulty limiting the development and application of MBR,and surface modification of membranes can achieve effective control of membrane fouling.The application of modified membranes in new MBR systems for treating municipal domestic wastewater can result in higher quality effluent.The PVDF/TiO₂-Fe（OH）₃ modified film was prepared by depositing positively charged titanium dioxide（TiO₂）and negatively charged iron hydroxide（Fe（OH）₃）alternately on the surface of sulfonated polyvinylidene fluoride（PVDF）film using electrostatic layer-by-layer self-assembly method.The chemical composition and morphology of the PVDF membrane and modified membrane surfaces were characterized by infrared spectroscopy,X-ray diffractometer,energy spectrometer and scanning electron microscopy,and it was determined that TiO₂ and Fe（OH）₃ were successfully deposited on the PVDF surface,and it was observed that the pore size of the modified membrane gradually decreased and particulate matter existed inside the membrane pores.The water contact angles of both PVDF and modified membranes were determined to show that the hydrophilic properties of the modified membranes were better than those of the original membranes,and the contact angles decreased gradually with the increase in the number of deposited layers.In addition,the pure water flux and the anti-pollution property of bovine serum protein（BSA）of PVDF and modified membranes were measured by terminal filtration device,and the results showed that the water flux tended to increase and then decrease with the increase of deposited layers,and the anti-pollution property gradually increased.The photocatalytic oxidation effect of the modified membranes was investigated with methylene blue as the target contaminant,and the PVDF/6TiO₂-5Fe（OH）₃ modified membranes had good photocatalytic performance.The hydrophilic,anti-pollution and photocatalytic properties of the modified membranes with different deposited layers were combined,and it was concluded that the PVDF/6TiO₂-5Fe（OH）₃ modified membranes had the best comprehensive performance.A moving bed membrane bioreactor（MBMBR）and a moving bed membrane bioreactor with intermittent irradiation of UV lamps（MB-UVPMBR）were constructed using PVDF membrane and PVDF/6TiO₂-5Fe（OH）₃ modified membrane as membrane modules to investigate the treatment effects of MBMBR and MB-UVPMBR on municipal domestic wastewater under the same operating conditions.The two systems showed excellent removal of organic matter,with MB-UVPMBR removing 96.66%and 94.13%of total organic carbon（TOC）and chemical oxygen demand（COD）,respectively,which were higher than 95.37%and 90.78%of MBMBR,indicating that the photocatalytic oxidation of TiO₂ in cooperation with microbial degradation of organic matter was more effective.Comparing the removal effects of MBMBR on nitrogen and phosphorus,the same improvement was observed in MB-UVPMBR.By comparing the removal rates of pollutants in the two systems,it is clear that MB-UVPMBR is more effective for municipal wastewater and has higher effluent quality.The membrane resistance analysis was performed for PVDF membrane in MBMBR and PVDF/6TiO₂-5Fe（OH）₃ modified membrane in MB-UVPMBR after 26 days of parallel operation of the two systems,respectively.The results showed that the total membrane fouling resistance of PVDF was 11.25×10⁹ m^-1,which was much higher than that of 5.04×10⁹ m^-1 in MB-UVPMBR,and the resistance of all parts of the modified membrane was lower than that of the PVDF membrane.The pore blockage resistance of the modified membrane decreased from 25.51%to 21.2%of the total resistance,while the percentage of the cake layer resistance of the modified membrane increased from 55.54%to 62.30%of the total resistance,and the percentage of irreversible resistance of the membrane decreased and the percentage of reversible resistance increased,which prolonged the service life of the membrane.Scanning electron microscope characterization of the membrane revealed that the fouling layer covered by PVDF membrane was compact denser than the modified membrane,so the oxidation of TiO₂ and flocculation of Fe（OH）₃on the membrane surface could reduce the membrane fouling and increase the membrane flux.Under the same operating conditions,the membrane fouling rate of the modified membrane is slower than that of PVDF membrane,and the cleaning cycle of the modified membrane is 2.74 of that of PVDF membrane,which prolongs the cleaning cycle.After cleaning the membrane surface fouling layer,the water flux recovery rate of the modified membrane was 1.72 times that of PVDF membrane;the flux recovery rate of the modified membrane increased up to 81.82%after UV irradiation,while there was almost no change in the original membrane,indicating that the modified membrane has certain self-cleaning ability.Therefore,deposition of TiO₂ and Fe（OH）₃ on the membrane by electrostatic layer self-assembly method can not only realize functional modification of the membrane,but also effectively alleviate membrane pollution.