The phenomenon of reverse osmosis occurs when pure water flows from a concentrated saline solution through a membrane into a dilute saline solution by the effect of a pressurization of the concentrated side. The phenomenon of reverse osmosis is illustrated in figure. A semi-permeable membrane is placed between two compartments.Semi-permeable means that the membrane is permeable to some species, and not permeable to others. Assume that this membrane is permeable to water, but not to salt. Then, place a salt solution in one compartment and pure water in the other compartment. By a pressure gradient effect the membrane will allow water to permeate through it to either side. But salt cannot pass through the membrane. Reverse osmosis (RO) membrane technologies are widely recognized to offer the most effective and economical process options currently available.

From small scale systems, through to very large scale desalination, RO can handle most naturally occurring sources of brackish and sea waters or by useful pretreatment process to treat industrial waters for water recovery or reuse. Permeate waters produced satisfy most currently applicable standards for the quality of drinking waters or process waters for water reuse.
RO can reduce regeneration costs and waste when used independently, in combination or with other processes, such as ion exchange. They can also produce very high quality water, or, when paired with thermal distillation processes, can improve asset utilization in power generation and water production against demand .