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How Desalination Works

Desalination at Work

There's more than one way to separate salt from water, but nearly 90 percent of the time, only one of two methods are used: multistage flash and reverse osmosis [source:].

Remember why it's so bad to drink salt water? When your cells pass water through the outer membrane to keep you from dehydrating, osmosis is occurring. By moving the water through the membrane, the cell is attempting to equalize its high internal salt concentration with a low external salt concentration. That's called osmosis. Reverse osmosis occurs when, for example, you put salt water on one side of a semi-permeable membrane and pressure moves the water molecules through the filtering membrane as the larger molecules -- including salt molecules -- stay trapped behind. For salty sea or ocean water, a considerable amount of pressure is required to move the water through a filter, where each pore is just a fraction of the size of a human hair [source: American Chemical Society]. This means a series of pumps are usually in play, all exerting pressure on the water [source:].

Unlike reverse osmosis, which relies on a membrane to filter out salt molecules, the multistage flash method uses heat to convert salt water into fresh water. Why such an unusual name? "Flash" refers to rapidly bringing the water to a boil, and this happens multiple times, or in stages. As the salt water enters each stage of the conversion unit, it is subjected to externally supplied steam heat and pressure. During each stage, water vapor forms and is collected. This water vapor is fresh water and the left-behind salty concentrate is known as brine. In multistage flash distillation -- as with reverse osmosis -- chemicals or water softening agents are not usually added [source: Organization of American States].

So if desalination is possible, why aren't large-scale plants quenching the world's thirst for fresh water? Only about 15 billion gallons -- two-tenths of a percent of the fresh water consumed around the globe each day -- is desalinated salt water [source: Schirber]. On the next page, we'll explore where the newest generation of salty water converters are cropping up.

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