Rejection of salts in nanofiltration of skim milk

The authors carried out nanofiltration of skim milk and analyzed the salt rejection data using the extended Nernst-Planck equation. The following observed rejections of major constituents in salts were obtained. Na and It: 50 to 60 %, Cl: -5 to 15%, Ca, Mg and P : > 97%. It is considered that since Ca and Mg are mostly combined with phosphate and citrate in milk, they were rejected by a sieving effect, thus producing high rejections. Electroneutrality in permeate, and no-electric-current condition in solute flux are observed, respectively. In the subsequent analysis, the authors used a set of proposed assumptions, 1) a complete rejection of solute except Na, K and Cl by the membrane, 2) existence of a fixed charge in skim milk due to impermeable ions of proteins, etc. Rejection of Na, It and Cl calculated from the analysis are in good agreement with those of the experimental data.