Process for depleting monovalent cations from a water intended for nutritional purposes

A process for depleting monovalent cations from water comprising subjecting the water to reverse osmosis, so that the retentate from this reverse osmosis has a higher ionic concentration, and subjecting said retentate to electrodialysis, so as to recover a water depleted in monovalent cations. The water may, for example, be intended for nutritional purposes. The may also, for example, contain monovalent and divalent cations, In one embodiment, the permeate from the reverse osmosis is added to the water depleted in monovalent cations, so as to obtain a water with a controlled mineral content.

EXAMPLES Procedure Use is made of the process according to the diagram represented by FIG. 1 . The water intended for nutritional purposes ( 1 ) is subjected to a reverse osmosis ( 7 ). The reverse osmosis retentate ( 2 ) is subjected to an electrodialysis ( 8 ), so as to obtain a solution which is highly concentrated in monovalent cations ( 6 ) and a solution which is depleted in monovalent ions ( 4 ). The reverse osmosis permeate ( 3 ) is subsequently reincorporated at the electrodialysis outlet ( 7 ) in this solution ( 4 ), in order to adjust the ionic concentration of the final water ( 5 ) and to obtain a good water yield. The pressure of the reverse osmosis is 0.5 MPa. The membrane surface area involved in the reverse osmosis is 53 m2. Results The water flow rate and the concentrations (in mg/l) of Na&plus;, K&plus;, Ca2&plus; and Mg2&plus; ions present at each stage in the process during the treatment of 1 m3/h of water intended for nutritional purposes, with a membrane surface area employed in the electrodialysis of 25.4 m2, are presented in the following table 1. 1 TABLE 1 1 2 3 4 5 Flow rate (m3/h) 1 0.204 0.796 0.204 0.999 Na&plus;(mg/l) 120 563 4.5 71 19 K&plus;(mg/l) 10 46 0.4 4 1.2 Ca2&plus;(mg/l) 20 94 0.8 60.4 13.1 Mg2&plus;(mg/l) 10 47 0.5 35 7.6 The water yield is 100% apart from feeding the electrodialyzer (1% maximum). It is found that the concentration of sodium cations has decreased by 84% and that the concentration of potassium cations has decreased by 88%. As regards the divalent cations, the loss of calcium cations is 34.5% and that of magnesium cations is 24%. The combination of the reverse osmosis at standard pressure and of the electrodialysis made possible the preferential reduction in the content of monovalent cations in a water intended for nutritional purposes while not significantly modifying the content of divalent cations, ions having a proven health benefit. The water obtained had a controlled mineral content and is suitable for individuals who, for medical reasons, wish to restrict their consumption of sodium while retaining a good nutritional balance. The water and calcium yields at the outlet of the process ( 5 ) as a function of the membrane surface areas employed in the electrodialysis (AED) are presented in table 2. 2 TABLE 2 Ca2&plus; Yield (%) Water Yield (%) AED (m2) 65 100 25.4 69 92.6 26 70 91 26.1 72.4 86.5 26.5 76 80.7 27 83.9 69.9 28 93.2 60 29 The sodium concentration at the outlet ( 5 ) is equal to 19 mg.l-1 in all cases. The process according to the present invention made possible to obtain a very good yield of water and of divalent cations. Furthermore, no regeneration may be necessary, in contrast to a treatment with ion-exchange resins.