Experimental Investigation On Nanofiltration Membranes Used In Desalination Treatment

In order to predict NF membrane performance,a systematic study on the filtration performance of selected commercial NF membranes is presented in this paper. Nanofiltration(NF) membranes are new kinds of membranes that have properties between those of ultrafiltration(UF) and reverse osmosis(RO). The application of nanofiltration becomes increasingly popular. Up to now , the applications and researches of nanofiltration membranes have been still at experimental research stage, and many research efforts are needed.In this work, the efects of operation parameters on rejection and flux of permeate were tested with a lab rig . Investigations have revealed the permeation performance of electrolytes in the separation process by nanofiltration membranes in salt solutions.The researches were carried out with the orthogonal experiment of 4 factors. The effect factors influencing separation performance of the membrane were operating pressure, temperature of experiment, concentration of feed solution, and velocity of flow. Each factor varied at four different condition levels. The experimental results show that operating pressure and temperature of experiment are two very prominent factors influencing flux of permeate. Increasing operating pressure or temperature of experiment, flux of permeate would increase. Concentration of feed solution also has influence on flux of permeate. The higher the concentration of feed solution is, the lower the flux of permeate. In the experiments, velocity of flow is the least prominent factor. Its change has little influence on flux of permeate. The prominent factor influencing rejection is operating pressure. The effects of other factors are not so significant. The rejection increases with increasing operation pressure. On the other hand, the higher the feed concentration is, the lower the rejection. And making temperature and flow velocity higher, respectively, would result in increasing rejection although rejection increases fluctuantly.Investigations have revealed the permeation performance of electrolytes in the separation process by nanofiltration membranes in single salt solutions. However, there is still not much information about the separation process of mixed electrolyte solutions, and by now most of t he investigations have been on solutions containing three kinds of ions , in which anions are mostly Cl- and SO42- , and cations are mostly Na+, K+, Ca2+ and Mg2+. In this work, permeation experiments of nanofiltration membranes (NF90-2540) for binary electrolyte solutions (NaCl, Na2SO4; NaCl MgCl2; NaCl, CaCl2; Na2SO4, K2SO4) were carried out. The result showed that the ion with better permeation performance would permeate first, thus depressing the rejection of it self and enhancing the rejection of the co-ion with poor permeation performance.Last , the effects of membrane fouling on separation performance of nanofiltration membrane were discussed. The cleaning efficiency and the fouling mechanism were thus investigated. In the cleaning experiment of nanofiltration membrane, the flux was almost recovered after cleaning with chemical medicine, which is a strong chelating agent for calcium.This research shows the different factors influence the nanofiltration in different way. It offer the theories for building the membranes model. A new rejection model was built up to predict the rejection of binary electrolyte solutions.