Study On Direct Osmosis Cleaning Of Nanofiltration Membrane Used In Seawater Desalination Process

As a pretreatment of reverse osmosis in seawater desalination, nanofiltration can effectively retain particles, organic and divalent ions. It also can alleviate the pollution of reverse osmosis, reduce the cleaning frequency, extend service life. But the pollution problem should not be ignored. At present, the commonly used cleaning method for nanofiltration membrane is acid(hydrochloric acid, citric acid) and alkali(sodium hydroxide). Acid washing and alkali washing can effectively remove the pollutants and restore the water flux. But it also has the disadvantages of high cost, easy to make the second pollution and the damage to the membrane structure. Therefore, this paper puts forward a new cleaning method for the nanofiltration membrane fouling in the process of seawater desalination. Using artificial simulated seawater as pollutants and NaCl solution as the draw solution. Through laboratory experiments, mainly researched the fouling process and influence factors of the nanofiltration membrane in seawater desalination, the main influence factors and the influence law of osmotic cleaning, as well as the comparison of the effect of osmotic cleaning and other cleaning technology. The main results of this research are as follows:(1) Pressure, concentration and temperature are important factors affecting desalination membrane fouling. Increasing the pressure will increase the water flux, at the same time, it will speed up the membrane fouling. Increasing the concentration of pollutants would improve the liquid osmotic pressure and decrease the transmembrane pressure, then water fluxes decrease. Too high concentration will enable concentration polarization serious and accelerate the membrane fouling. Increasing temperature could make the membrane pore size expansion, and increase the water flux. But it can also increase CaSO4 and CaCO3 inorganic salting out, accelerate contamination.(2)The concentration of the draw solution and operation pressure greatly influenced the osmotic cleaning. Increasing the concentration of draw solution can accelerate the reverse flow of water and enhance the scouring effect on the membrane hole and surface. Increasing operating pressure will reduce the total cross membrane pressure, and the reverse flow rate of water slows down, the cleaning effect will decrease. From the microscopic point of view, 17% of the NaCl draw solution at 25 ℃, operating pressure was 0, cleaned 5 min,the pollutant was removed.(3) The effect of the osmotic cleaning is better than acid wash or alkaline wash. Compared with the acid and alkaline cleaning, it is not limited by the types of pollutants. Osmotic cleaning has a high removal rate on both organic and inorganic materials. And the cleaning process has no damage to the nanofiltration membrane, which does not affect the performance of the membrane.