But technological advances in recent years have altered the equation. The most common form of desalination is reverse osmosis; it involves forcing water through cartridges that contain thin-film composite polyamide membranes, which trap salt and other impurities but allow the fresh water through.
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Randy Truby, comptroller of the International Desalination Association, says that advances in manufacturing processes have allowed 450 sq ft of membrane to be crammed into each cartridge, compared with 300 sq ft when they first came on the market. But treating seawater still requires pressure of about 80 bar, 40 times more than car tyres. That is why treating seawater is more energy-intensive than brackish or waste water, which require less force.
The location of a seawater desalination plant also makes a difference, Truby adds: while the salt content of water off the coast of California is about 34,000 parts per million, the figure in the Middle East is more like 40,000.