Osmosis, turned around
Osmosis is a natural effect. If fresh water and salty water are separated by a thin membrane that lets water through but blocks most dissolved salts, water tends to move toward the salty side. The French clergyman and physicist Jean-Antoine Nollet described the movement of liquids through a membrane in the 1740s, and later scientists studied it carefully in the 1800s.
Reverse osmosis does the opposite. If pressure greater than the natural osmotic pressure is applied to the salty side, water is forced through the membrane to the fresh side, leaving most of the salt behind. For seawater, the pressure required is many times normal atmospheric pressure, which is why the equipment is heavy and the pumps are a significant cost. Unlike distillation, no boiling is involved, so in principle less energy is needed.
The membrane problem
The principle was understood well before it was useful. Early membranes either passed too little water to be practical or let too much salt through. For decades, desalination by membrane remained a laboratory topic. During the 1950s, American government programs funded research on desalination broadly, motivated by concerns over water supplies in the arid Southwest and by Cold War interest in science and development.
The breakthrough usually cited came at the University of California, Los Angeles, in the late 1950s and early 1960s. Researchers Sidney Loeb and Srinivasa Sourirajan developed a cellulose acetate membrane with an unusual structure: a very thin, dense skin on top of a thicker, porous support layer. The skin did the filtering, while the support gave strength and allowed water to pass at a useful rate. This “asymmetric” membrane made flows high enough to be practical. Other research groups were working on membranes in parallel, so credit is commonly shared, but the UCLA design is regarded as the key step.
First plants
Early commercial use focused on brackish water, which is less salty than seawater and so requires less pressure. A demonstration plant at Coalinga, California, in the mid-1960s is often cited as an early example of reverse osmosis supplying a small town. Early installations had short membrane lifetimes and were costly, and engineers had to deal with fouling, where particles and microbes clogged the surface.
Developments during the following decades made the technology more dependable:
- Spiral-wound modules. Rolling flat membrane sheets into compact cylinders packed a large surface area into a small space.
- Hollow fiber designs. Fine hollow fibers offered another compact arrangement, used in several systems.
- Thin-film composite membranes. In the late 1970s, researchers including John Cadotte developed membranes made of a very thin polyamide layer on a support. They rejected salt better, worked at lower pressures and lasted longer than cellulose acetate.
- Pretreatment. Filtering and conditioning the water before it reaches the membrane became standard, protecting the membranes.
- Energy recovery. The high-pressure leftover brine still holds energy. Devices that capture it and pass it back to the incoming feed water, developed and improved from the 1980s onward, cut the electricity use of seawater plants substantially.
Cheaper and more efficient pumps, many driven by electric motors, were also essential, since pressure is the main energy cost.
Why it spread
By the 1980s and 1990s, seawater reverse osmosis plants were being built around the world, and in new projects they gradually displaced large thermal plants in many places, because they generally use less energy for each unit of fresh water. Countries including Spain, Australia, Israel and the United States built large plants, and many Gulf states added membrane capacity to their older thermal systems. The same technology became common on a smaller scale in ships, island resorts, industrial plants and household systems.
Reverse osmosis is also used far from the sea. Industries use it to prepare very pure water, and it plays a part in water recycling, where membranes polish treated wastewater to a high standard. Aircraft and spacecraft support systems, including space stations, use related membrane and treatment technologies, though they are designed very differently.
What it does and does not solve
Desalination adds to supply and can reduce pressure on rivers and groundwater, but its effect on “water preservation” depends on how it is used. The main concerns are:
- Energy. Membrane plants use less energy than thermal ones but still consume a large amount of electricity. Where that power comes from affects the environmental result. Some projects pair plants with wind turbines or other renewable sources.
- Brine. The concentrated salt stream must be disposed of, usually back into the sea, with local environmental effects that depend on how it is released and diluted.
- Cost. Desalinated water is typically more expensive than water from rivers or aquifers, so it tends to be used where cheaper options are limited.
- Membrane care. Membranes need cleaning and replacement, and plants must be operated carefully.
Because conditions differ from place to place, claims about exact costs or energy figures should be checked against recent project data and not taken from general summaries.
A short summary
Reverse osmosis was not invented in a single moment. A well-known natural effect was understood for centuries, a workable membrane arrived around 1960, and engineering improvements over the next several decades made it economical. The result is the most widely used method for new desalination capacity today.