Reverse osmosis, also called hyperfiltration, is mainly used to remove salts and minerals and therefore to reduce the conductivity. Another effect is that reverse osmosis partly blocks other substances, such as pesticides, heavy metals, medicine residues and more. Osmosis is a natural process involving flow through a semi-permeable membrane. When pure water of the same temperature is present on both sides of a membrane and the pressure on both sides is the same, no water will flow through the membrane. When salt is dissolved in the water on one side, it will stimulate a water flow through the membrane from the pure water to the water with salts. Nature tries to eliminate the difference in concentration, as it were. When pressure is applied to the side where the salts have been added, a new equilibrium is created. Due to this pressure, water will pass through the membrane whereas the salts cannot. This phenomenon is called reverse osmosis. The driving force behind reverse osmosis is the applied pressure minus the osmotic pressure. The energy consumption of reverse osmosis is directly related to the salt concentration, given that a higher salt concentration produces a higher osmotic pressure.

This technique is used, among other things, for the production of process water and boiler feed water and the preparation of drinking water (from brackish or salt water), demineralised water and ultra-pure water. The possible options also include the softening and decolourisation of water, the upgrading of process flows to a higher quality and the purification of waste water aimed at reuse.