Reverse osmosis is a process that finds application in the purification of water. Reverse osmosis systems consist of a series of tubes containing stacks of spiral wound membranes. These tubes are mounted on high-pressure containers. The membrane stack consists of two very long semi permeable membranes with a mesh in between, sealed along the sides. This is wound up in a spiral tube with another mesh to separate the outside of the stack. The spiral winding provides a very large surface area for transfer. Between each membrane layer is a mesh separator that allows pure water to flow without obstructions. Water is forced through one end of the spiral cylinder and out through the other end. The resulting pressure forces the water through the membrane and collects in the space between the membranes. Pure water flows around the spiral and is collected in the center of the tube.

A typical reverse osmosis system consists of a holding tank with level controls that feeds the reverse osmosis pump. It also contains a reverse osmosis water storage tank with level controls and duplex pumps for water pressurization. High-pressure gauges are fitted on the reverse osmosis output pump and the concentrate pump. Pressure switches are fitted on the reverse osmosis feed and flow monitors on the concentrate, permeate and recycle stream. The cleaning cycle is automated in larger systems with automatic valves. Pretreatment is required for all systems which are designed to eliminate suspended solids, water hardness, chlorine and other oxidizers. Deposits of calcium and magnesium can plug the membrane. Chemicals or water softeners are added to the water to keep the solids in suspension. A water softener is used to remove the hard ions and replace them with sodium.

To continually perform well, reverse osmosis systems require regular maintenance and replacement of various components. Pre-filters and post-filters need to be replaced on a regular basis. The length of time between changing pre-filters will depend on the water quality, especially the concentration of solids. The contaminant concentration, membrane rejection percentages and efficiency of activated carbon removal, determine when post-filters should be replaced.