Abstract: This invention relates to a reverse osmosis membrane module for separating a selected fluid component from a feed stock. The module comprises a mandrel having an axial passageway forming a flow path for the selected fluid component through the mandrel. A flow guide means is attached to the mandrel and forms a channel for the selected fluid component as it flows through the membrane module. A plurality of generally rectangular membrane envelopes are joined to the flow guide mandrel and are formed from a plurality of elongated permeate collector sheets each forming a flow path for the selected fluid component from the outer periphery of the envelope to the flow guide. A plurality of folded sheets of support material with each sheet having predetermined membrane areas where reverse osmosis occurs are joined to the collector sheets so as to form the membrane envelope. Each sheet of support material is formed such that there are no fold lines on the membrane areas where reverse osmosis occurs.