Reverse osmosis (RO) is a water purification technology that uses a semi-permeable membrane in which an applied pressure is used to overcome osmotic pressure. Reverse osmosis can remove many types of molecules and ions from solutions, including bacteria and is used in both industrial processes and the production of potable water. The result is that the solute is retained on the pressurized side of the membrane and the pure solvent is allowed to pass to the other side.

Pretreatment is important when working with reverse osmosis and nano-filtration membranes due to the nature of their spiral-wound design. The material is engineered in such a fashion as to allow only one-way flow through the system. As such, the spiral-wound design does not allow for back pulsing with water or air agitation to scour its surface and remove solids. Since accumulated material cannot be removed from the membrane surface systems, they are highly susceptible to fouling (loss of production capacity). Therefore, pretreatment is a necessity for any reverse osmosis.