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Water from Water: Desalination

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Water, water, everywhere,
And all the boards did shrink,
Water, water everywhere,
Nor any drop to drink.

From: “The Rime of the Ancient Mariner”, by Samuel Taylor Coleridge.

Earth is commonly known as “the Blue Planet”, with oceans and seas covering almost 71% of its surface, yet ironically many of its inhabitants suffer from water shortage. Even though we are continuously surrounded by an abundance of water, we are unable to drink it, as the above stanza by Coleridge expresses.

This paradox, which is well known to all sailors, has got humans trying to make fresh water out of salt water for centuries. Even though some ancient sailors have succeeded in their endeavors to desalinate seawater with simple boilers on their ships, it was never enough to quench the thirst of the entire crew, let alone the entire world.

That is because the energy required for this distillation process, also called “thermal desalination”, makes it prohibitively expensive on a large scale. As a result, a lot of the current market for this method has been in oil-rich, water-poor countries in the Middle East.

Of course the thermal method is now different than how ancient sailors used to do it, but the concept is more or less the same, as both are based on evaporation and the subsequent condensation of the steam.

The most common thermal desalination process used nowadays is vacuum distillation—essentially the boiling of water at less than atmospheric pressure and thus a much lower temperature than normal. Thus, because of the reduced temperature, low-temperature "waste" heat from electrical power generation or industrial processes can be used decreasing energy costs.

A Multi-Stage Flash distillation system (MSF) is commonly used in vacuum distillation, which works by flashing a portion of the water into steam in multiple stages of what are essentially countercurrent heat exchangers. Multi-stage flash distillation plants produce about 60% of all desalinated water in the world.

The principal competing desalination process to thermal desalination is Reverse Osmosis Desalination. This process uses semi-permeable membranes and pressure instead of heat to separate salts from water, applying the principle of reverse osmosis.

A reverse osmosis plant membrane system typically uses less energy than thermal distillation, which has led to a reduction in overall desalination costs over the past decade.

Even with membranes, large amounts of energy are needed to generate the high pressure that forces the water through the filter. Current methods require about 14 kilowatt-hours of energy to produce 3,800 liters of desalinated seawater.

Researchers are however trying to tackle the energy problem by developing new types of membrane materials. The goal is to cut in half the energy required for desalination.

As we are essentially running out of water from other resources, desalination will continue to become an increasingly attractive option, and hopefully governments will begin to support more research into reverse osmosis membranes and other cost-effective desalination methods.

So we can finally have water from water.

References
scientificamerican.com
livescience.com
wabag.com

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