Abstract: A module containing a reverse osmosis membrane for a water purification system is provided. A membrane assembly is mounted in the center of a pressure vessel with a seal surrounding the membrane assembly between the outside of the assembly and the inner wall of the pressure vessel. An inlet port passes through the pressure vessel and enters the vessel between the seal and one end of the pressure vessel in close prximity to the seal. An outlet port passes through the pressure vessel at a point between the seal and the other end of the pressure vessel, also in close proximity to the seal. The water pressure differential between the two ports maintains the integrity of the seal and their location insures a turbulent water flow throughout the module.