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Desalination: Definition & Process

Instructor: Dina El Chammas Gass

Dina has taught college Environmental Studies classes and has a master's degree in Environmental and Water Resource Engineering.

Desalination is a water treatment process that turns salt water into fresh water. Learn why this could be useful, and examine the two broad processes involved in desalination systems.

What is Desalination?

Picture yourself in ancient Phoenician times, planning to go on a long trip with two hundred or so of your closest friends. Imagine how much food and water you'd need for the trip. Two hundred people, two gallons of drinking water per day, for weeks. That's a lot of water!

But then one of your friends says you don't need to worry about packing water; after all, it will be all around when you're on the ship.

'Drinking salty water doesn't sound too appetizing,' you say.

'No, we'll be drinking fresh water the whole trip. I have a desalination unit,' your friend replies enthusiastically.

Desalination is the process of removing dissolved minerals from water so that people can use the treated water for applications that usually require fresh water. Desalinated water can be used for agriculture, industry and domestic use. It might need further treatment to be deemed drinkable, but if no storage is required, drinking it on the spot should be just fine.

This desalination plant is located in Barcelona, Spain.

Key Desalination Processes

Desalination is used in areas where fresh water is scarce, like on ships in the ocean or in water-starved areas like the Middle East or Africa. When large quantities of fresh water are needed, desalination might not be as cost effective as using fresh water sources because the process is very energy intensive. But if an area is scarce of water, desalination might prove more economical than transporting water from very far away.

The removal of dissolved minerals, or salts, from water can occur through different processes. Salts are usually left behind when water changes from one phase to another. But salts can also be removed from water without any phase change.

Phase Transition Processes

When you freeze salty water, an interesting phenomena occurs. The water freezes, but the salts remain behind. You don't get salty ice cubes. You get ice cubes that taste like ice cubes!

The same thing happens when you boil or distill water. When water changes from liquid to a gas, the salts are too heavy to carry in gas form, and they remain behind. You get salt-free steam that you can later collect by allowing the steam to condense on a cold surface. This is the process behind a solar still, which is a device that uses energy from the sun to evaporate water and has a surface that the water vapor can condense on. The condensed water that's collected is pure and free of any minerals.

Membrane-Based Processes

The other way to desalinate water is to pass it through very fine filters under high pressure. These filters must have pore openings that are bigger than the size of a water molecule, but smaller than the size of the salts. This is the most widely used process to desalinate water in large quantities and is called reverse osmosis.

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