Reverse Osmosis is a process where water is demineralized using a semipermeable membrane at high pressure. Reverse osmosis is osmosis in reverse. To reverse this process, you must overcome the osmotic pressure equilibrium across the membrane because the flow is naturally from dilute to concentrate. To do this, the pressure on the salty side of the membrane is increased to force the water across. The amount of pressure is determined by the salt concentration. As water is forced out, the salt concentration increases requiring greater pressure to produce more pure water. Industrial reverse osmosis units use spiral wound membranes mounted in high-pressure containers. The membrane stack is two very long semipermeable membranes with another spacer to separate the outside of the stack. The spiral winding provides a very high surface area for transfer. Between each membrane layer is a mesh separator that allows the pure water to flow. Water is forced in one end of the spiral cylinder and out the other end. Back pressure forces the water through the membrane where it is collected in the space between the membranes. Clean water then flows around the spiral where it is collected in the center of the tube.