The invention relates to a method and a system for purifying water whereby: the water to be purified is pressurized (13); a pressurized flow of said water is directed onto at least one selective permeability membrane to divide (11) the flow of pressurized water into a permeate flow and a retentate flow; the permeate flow is electrodeionized (12) to produce a flow of purified water consisting of the electrodeionized permeate flow; the flowrate of the retentate flow (19) is reduced; a substantially constant predetermined pressure (21) is maintained on the selective permeability membrane(s); and a substantially constant predetermined permeate flowrate is maintained. It also relates to a tangential filtration module suitable for the above kind of system.