Abstract: A method and system for processing a solution is provided. In at least one first membrane unit, a feed flow is separated into a permeate flow and a retentate flow. According to the invention, at least part of the retentate flow is fed to at least one second membrane unit. In the second membrane unit, retentate is located on one side of a membrane. Located on the other side is a solution in which the concentration of dissolved substances of which is lower than that of the retentate. Thus solvent of the solution passes through the membrane and thins the retentate.