It has been revealed that the use of reverse-osmosis (RO) membrane units for water treatment is complicated by the buildup of their concentrate waste needed to be disposed. A technology for recycling the concentrate, consisting of the removal (crystallization) of calcium carbonate present in the concentrate by seed crystals, has been proposed and experimentally tested. By this process using RO units, it is possible to obtain from natural water highly desalinated (softened) water and water with reduced concentrations of calcium and bicarbonate ions having the same salinity as the source water. The technology can be effectively used in the production of drinking water and integrated treatment of feed water for steam boilers and heat supply systems.