How do desalination plants work?

Our planet is covered in water, but countries the world over struggle with drought because over 96 per cent of this precious liquid is found in the oceans and is therefore completely undrinkable.

Seawater is more than three per cent salt. If we try to consume it our kidneys go into overdrive, filtering out the excess sodium and passing it out as urine. But there’s a problem — drinking water straight from the ocean makes you more dehydrated than drinking nothing at all. Kidneys can’t make urine as salty as seawater, and to get rid of the salt from one glass you need to produce more than one glass of pee.

The solution is desalination, a process that removes salt from seawater to make it drinkable, either using a boiling technique called multistage flash or a filtration method known as reverse osmosis.

Multi-stage flash uses the same principle as a solar still: as water boils, pure vapour evaporates, leaving salt crystals behind. The vapour can then be collected, condensed and used for drinking.

Reverse osmosis filters the water to remove the salt, putting the liquid under high pressure against a membrane that only allows the water molecules to pass. Water is forced across, leaving a salty brine on one side of the membrane and clean water on the other.

According to the International Desalination Association, there are now more than 18,000 desalination plants worldwide, supplying over 86 billion litres of water to 300 million people in more than 150 countries every day.

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Article by Laura Mears, originally published in issue 105 of How It Works.

Opening image by dolvita108

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