Reverse osmosis is a separation process that uses pressure to force a solvent through a membrane that retains the solute on one side and allows the pure solvent to pass to the other side, meaning it allows the passage of solvent but not of solute. In reverse osmosis, the idea is to use the membrane to act like an extremely fine filter to create drinkable water from salty water.
The membranes used for reverse osmosis have a dense barrier layer in the polymer matrix where most separation occurs. In most cases the membrane is designed to allow only water to pass through this dense layer while preventing the passage of solutes (such as salt ions). This process requires that a high pressure be exerted on the high concentration side of the membrane, usually 30-250 psi for fresh and brackish water, and 600-1000 psi for seawater.
Reverse osmosis (RO) is used to reduce dissolved solids from feed waters. Municipalities and industrial facilities are able to use RO permeate as a consistently pure drinking water supply and to transform drinking water to high purity water for industrial use at microelectronics, food and beverage, power, pharmaceutical facilities, and can be used for removing bacteria, pyrogens, and organic contaminants.
RO systems are also now extensively used by marine aquarium enthusiasts, as the domestic water supply contains substances that are extremely toxic to most species of saltwater fish. In the production of bottled mineral water, the water passes through a RO water processor to remove pollutants and microorganisms, including the smallest microbe known, archaeobacteria.
In the United States, house hold drinking water purification systems, including a reverse osmosis step, are commonly used for improving water for drinking and cooking.