A filter assembly for removing a solute from a solvent includes a first annular filter element and a second annular filter element disposed radially inwardly and concentrically aligned with the first filter element together defining a common axis. The first annular filter element is defined by a first annular coil of a flat wire and the second filter element are each defined by a second annular coil of a flat wire being generally helical in the axial direction. A filter membrane is cylindrically shaped and concentrically disposed between the first and second annular filter element. The filter membrane is porous having aperture size of less than a nano-particulate size of the solute, but greater than a nano-particulate size of the solvent. The second annular filter includes adjustable porosity for selectively preventing particles from reaching the filter membrane and selectively cleaning the membrane by reversed flow of solvent through the membrane.