Low pressure operation of nanofiltration process ranged from 0.3 bar to 5 bar was examined. The lower rejection of nitrate compared to that of chloride was found in all cases especially in lower pressure operation. A set of extended Nernst-Planck equations was applied to describe the rejection difference theoretically. The original extended Nernst-Planck equation failed to explain the difference in rejection between nitrate and chloride though it can explain the rejection difference between monovalent ions and divalent ions. The difference in rejection between chloride and nitrate was well explained by introducing a new parameter which describes partitioning of ions at the membrane-solution interfaces. The new parameter might be related to materials of membranes and to the affinity of ions on the membrane surface.

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