Studies On Membrane Evolution Mechanism Of Nanofiltration Performance For Seawater Softening

The Nanofiltration membrane (NF) has distinct advantages in removing divalent ions such as Ca2+, Mg2+ and SO42-, can be used as pretreatment for desalination process. However, the NF membrane is chosen mainly by experience and experiments in practical engineering applications, lacking systematic comparison and research. The experiment and theoretical foundation of engineering design are also limited. In practical engineering, the water quality standards are mainly concerned. The effects of inorganic ions and organic matter in the seawater on membrane structure and properties are often overlooked, but the erosion effect on membrane is a serious problem, which can severely decrease the economics of NF membrane engineering applications.In this paper NF membranes were chosen in seawater softening experiments. The effects between seawater and membranes were studied. This work will provide experimental and theoretical references for design and operation of NF membrane seawater softening engineering.Three flat NF membranes were used to study the softening performance and membrane fouling in seawater and simulated seawater. Experiments were carried out in seawater system at first. The total recycle experiments results showed that sulfate rejection of three negative-charged NF membranes approached to 100% when pressure is over 2.0MPa. The IR, SEM and EDX results showed that surface crystal of calcium sulfate was the main reason for the membrane flux decline. The complexation effect between bivalent cations and organic matter in seawater was negligible. In seawater concentration experiments, the turbidity change of enrich solution accorded with the theoretical model results.Based on the seawater system experiments, the NF membrane experiments for the treatment of the simulated seawater were carried out. The effect of pressure on membrane flux was studied in inorganic simulated seawater. In simulated seawater system containing humic acid substances, the experiments results indicated that the higher the concentration of humic acid was, the more severe the flux decline was. The reason was that the organic substances deposited on the surface or inside the membrane pore, deteriorated membrane pollution and eventually leaded to flux decline.The analysis methods such as IR, SEM and EDX were carried out in order to study complexation effect between divalent cations and organic substances in simulated seawater. The results showed that complexation effect is negligible when humic acid concentration was low. When increased the humic acid concentration, the most of humic acid and divalent cations formed complex salt and deposite on the membrane surface.