RO units require only electrical power to operate and, therefore, are well suited to both diesel and gas turbine ships. One major advantage of reverse osmosis is that pure water is made within 5 minutes of system start up, which is much faster than other systems. An RO plant requires no heat and involves no phase change of the seawater. They are significantly lighter in weight, less complex, and more energy efficient than other processes in Navy ships. The RO process is similar to a filtration process - pressurized seawater is passed over a semipermeable membrane that passes pure water but excludes salt.
Reverse osmosis is the opposite of normal osmosis.
If two solutions are separated by a semipermeable membrane, the weaker solution tends to pass through the membrane diluting the stronger solution. This is normal osmosis. The fresh water dilutes the seawater. The pressure the fresh water exerts on the membrane is called the osmotic pressure and is about 350 psig. If a pressure is applied to the seawater side, reverse osmosis will take place where seawater flows to the pure waterside.