Removal Of Natural Organic Matter In Water With Modified Ultrafiltration Membrane

Natural organic matter （NOM） is commonly found in surface and ground waterand considered to react with the major disinfectants to produce a host of disinfectionby-products （DBPs）. Therefore, the removal of NOM effectively is of significantimportance in meeting the stringent DBPs regulations and providing safe drinkingwater. Ultrafiltration （UF） allows the removal of particle, colloids, microorganism,variety of water born viruses, and certain amount of the dissolved organic matter. Ithas proved to be one of the best alternatives replacing conventional drinking watertreatment technologies and is increasingly used in drinking water treatment. However,UF is not very effectively to remove NOM due to the comparatively larger pore sizecompared with the size of NOM. Membrane fouling is another factor which restrictsits widespread application.In this study, the effect of charge modification of regenerated cellulose （RC）membrane was studied by discussing NOM removal and flux decline with neutral andnegatively charged version of RC membranes. Effects of the membrane charge,molecular weight cut-off （MWCO）, membrane material and NOM fractions on NOMremoval and flux decline with charged UF membranes were also investigated. Inaddition, solution environment, including pH, ionic strength and calcium ionconcentration, affecting NOM removal and flux decline were evaluated. Furthermore,the performance of charge modified UF membrane and nanofiltration （NF） membraneon NOM removal and flux decline was compared.In order to get UF membranes with better durability and anti-fouling properties,the modified polyvinylidene fluoride （PVDF） membranes were prepared andcharacterized by adding different amounts of polyethylene glycol （PEG）, LiCl andTiO₂particles. The performances of different kinds of modified PVDF membranes were investigated by discussing NOM removal and flux decline in both dead-end andcross-flow filtration experiments. The effect of photocatalytic degradation onTiO₂-doped PVDF membranes was also evaluated.Results indicated that the appropriate charge modification on the neutral RCmembrane could be an effective way for better removal of NOM and reduction of themembrane fouling due to the electrostatic interaction with the combination effect ofmembrane pore size. Charge modified RC membrane could be potentially used toreplace NF membrane on the aspect of NOM removal. The membrane charge,membrane MWCO，membrane material and NOM fractions had some importanteffects on charged UF process. Besides, the extent of flux decline and removal on thecharged RC membrane was also found to be strongly governed by solutionenvironment. Though the degrees of the effects were not the same, the trends of theeffect of solution environment on the NOM removal and fouling were same on bothneutral and negatively charged membrane. The pH value would affect the net chargeof membrane and NOM molecules through the protonation effect. The ionic strengthwould affect the physical and chemical properties of NOM molecules and theelectrostatic shielding on membrane surface. The effect of calcium concentration onNOM removal and flux decline was due to both the electrostatic interaction and thepermeability of the cake layer formed on the membrane. In the application of UFprocess for NOM removal, one has to consider not only the properties of membraneitself （including MWCO, membrane charge, hydrophobicity and material）, but alsothe solution environment, to achieve better NOM removal rate and less membranefouling.Results also indicated that the appropriate TiO₂-doped modification on PVDFmembrane could be an effective way for better removal of NOM and reduction ofmembrane fouling at the same time. The addition of TiO₂nanoparticles can improvethe hydrophilicity of PVDF membrane and increase the removal rate of testingmolecule pepsin. Compared with PVDF membrane, TiO₂-doped PVDF membraneincreases NOM removal and at the same time decreases the flux decline, which is due to smaller membrane pores, and more hydrophilic and smoother membrane surface.TiO₂-doped PVDF membrane has self-cleaning ability offered by photocatalysis. Asto TiO₂-doped PVDF membrane, an equilibrium might be reached between thefoulants deposited on the membrane surface and photocatalytic degradation of thefoulants during cross-flow UF coupled with photocatalysis. UV irradiation might be anew kind of membrane cleaning method for TiO₂photocatalytic membranes.Compared with conventional physical and chemical cleaning methods, this novelmembrane cleaning method provides advantages of energy saving, high efficiency andno second-pollution, which indicates great potential for applications.