As the population of the world increases, it becomes increasingly difficult to provide fresh water suitable for drinking.   This situation is particularly true in semiarid regions, such as the Middle East.   Scientists have proposed various schemes to reduce the concentration of salts in seawater and water from other natural sources, making it fit for human consumption.   One means of purifying water, a process called reverse osmosis, is the subject of continuing research.

Reverse osmosis is the transport of water from a solution on one side of a semipermeable membrane to pure water on the other side, by applying a pressure greater than the osmotic pressure to the solution.  We have seen that no net transport of water across a semipermeable membrane occurs when the osmotic pressure is applied to the solution.  If a pressure greater than the osmotic pressure is applied to the solution, however, net transport of the water from the solution side to the pure water side occurs.  Because the osmotic pressure of natural seawater is fairly high, one of the problems in the design of a reverse osmosis apparatus is to make a membrane that is thin enough to allow rapid transport of the water yet is strong enough to withstand high pressures without bursting.

The reverse osmosis method of water purification.

Reverse osmosis is used for water purification in Saudi Arabia.  In 1991 this water source was threatened when the Iraqis, who were at war, dumped oil from a well in Kuwait into the Persian Gulf.  The reverse osmosis plants were shut down, because the oil would have ruined the semipermeable membranes in the equipment.

The U.S. Navy has developed small, portable, manually operated units to desalinize seawater, for use in life rafts.  Such units are capable of producing 5 L of drinkable water per hour, which is sufficient to keep several people alive.  These units are replacing the bulky containers of fresh water now stored on Navy lifeboats.

Reger/Goode/Mercer:  Chemistry Principles and Practice,  2/e,  p. 502