Abstract: A method and apparatus for removing dissolved solids from a liquid which utilizes the technique of reverse osmosis (RO). The liquid to be treated is directed into a pressure vessel which contains a plurality of filter elements positioned therein. The filter elements have a layer of hollow RO fibers around a center core area such that the liquid flows in a direction from the outside of the filter elements towards the center core areas. Pure permeate liquid permeates into the center bores of the fibers and concentrate liquid passes into the center core areas. The permeate liquid from the center bores passes into longitudinally extending permeate collection channels which communicate with the center bores over substantially the entire length of the layers of hollow RO fibers. Various structural embodiments of the filter elements are disclosed.The method and apparatus provide for the backwashing of the filter elements when they become fouled.