Osmotic cleaning to control inorganic fouling of nanofiltration membrane for seawater desalination

The inorganic fouling of NF membrane caused by long-term operation as a pretreatment process seawater desalination cannot be ignored. In this study, an osmotic cleaning with NaCl solution was proposed for controlling NF membrane inorganic fouling in seawater desalination. The effects of cleaning solution concentration, cleaning time and cross-flow velocity of cleaning solution were investigated in this study and the results showed that the cleaning efficiency increased with the cleaning time, the cleaning solution concentration, and the cross-flow velocity of cleaning solution. The water flux of NF membrane for synthetic seawater filtration was recovered to 99.8% of the initial water flux after 5 min of osmotic cleaning with 17 wt% NaCl solution at 0.69 m·s-1 of cross-flow velocity and 25 oC, and reached to 100.0% after 10 min of cleaning. The repeated fouling and cleaning experiment indicated that osmotic cleaning shows high efficiency and long-term stable performance of water flux recovery. The microstructure analysis indicated that CaSO4 and CaCO3 deposited on the membrane surface were efficiently removed, and the main mechanism is that the backwash flux creates synergy effect with ion-exchange to removes the inorganic fouling on the membrane surface. By comparing the changes in water flux and permeate water quality after cleaning with different cleaning methods, it was found that osmotic cleaning has better cleaning efficiency and less membrane damage. The results indicated that the osmotic cleaning is a process combines with dissolution, backwashing and tangential scouring and shows highly efficient performance to control inorganic fouling on NF membrane.