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The membrane technique of nanofiltration (NF) was used to carry out partial demineralization of whey and milk ultrafiltration permeate (MUP). The experiments were carried out in a completely automated NF pilot plant. An aromatic polyamide spiral-wound membrane supplied by Osmonics (USA) was selected (model DK2540C). Feed, permeate and retentate were analyzed for lactose, protein, ashes, ions (Ca2+, Na+, K+, Mg2+, P, Cl!) and total dry extract content. Different operating conditions were considered. By means of NF total salt content was reduced at least by 30%, depending on the operating conditions and volume concentration ratio. Monovalent ions showed lower retention than divalent ones. Higher salt retention was obtained when whey was used as the feed stream. However, permeate fluxes were higher for MUP due to the lower amount of proteins, which can form a gel layer on the membrane surface. Protein and lactose permeation was observed to be negligible.

NF was observed to be an effective method to perform partial demineralization of whey and MUP. The degree of mineral salt removal was 27–36% and was dependent on the value of the VCR. Monovalent ions showed higher permeation than divalent ones, while chloride showed the highest permeation due to the Donnan effect. Losses of lactose and protein in the permeate stream are negligible. Permeate flux was lower when whey was nanofiltered, probably due to the presence of proteins, which can form a gel layer on the membrane surface. The same reason can be used to explain the higher ion retention that was observed in the nanofiltration of whey compared to the nanofiltration of MUP.