This is the reverse of the normal osmosis process, which is the natural movement of solvent from an area of low solute concentration, through a membrane, to an area of high solute concentration when no external pressure is applied. The membrane here is semipermeable, meaning it allows the passage of solvent but not of solute.

The membranes used for reverse osmosis systems have a dense polymer barrier layer in which separation takes place. Since Reverse Osmosis does not occur naturally, it must be created by applying pressure to the high solids water in order to force it through the membrane, with pressures from 8 – 14 bar for fresh and brackish water, and 40 – 70 bar for seawater, which has around 24 bar (350 psi) natural osmotic pressure which must be overcome.

Advantages

• RO performs a separation without a phase change. Thus the energy requirements are low
• RO systems are compact and space requirements are less than with another desalting system eg: distillation
• Many RO systems are fully automated and designed to start up and shut down automatically through interlocks. Thus RO plants usually require little labour
• Due to their modular design, maintenance is easy. Scheduled maintenance can be performed without shutting down the entire plant. The modular design also makes expansion an easy option