You Asked:
How do they desalinate water to make it drinkable? — Reid, Raleigh, NC

Marshall Answered:
Salt water from the ocean is undrinkable. The same holds true for brackish water. But 97% of the water on earth has salt in it. And more than half of the fresh water is locked in icecaps and glaciers. So people who need fresh water often look at the ocean and wonder if there is an easy way to remove the salt.

There are a number of ways to separate the salt from the water to desalinate it and make it drinkable:

1) It is possible to filter the salt out using reverse osmosis. Tampa bay has a reverse osmosis desalination plant that produces 25 million gallons of fresh water per day using reverse osmosis:

2a) It is possible to boil salty water with heat and condense the resulting steam. This is traditional distillation. The heat needed to boil the water might come from solar energy. A solar tower is one way to accumulate large amounts of solar heat:

2b) Or the heat might come from a boiler in a cogeneration power plant. A traditional coal-fired power plant or nuclear power plant uses heat to create steam to power a steam turbine that spins a generator. As long as you are boiling water anyway, you might as well boil salt water and then condense the steam after it produces power.

3) A more modern way to distill fresh water from salt water is to use flash distillation, where the water boils in a vacuum chamber. Power is required for flash distillation, but it is less power than is required to boil the water using heat at normal atmospheric pressures. The link mentions that the power requirements are 17 kilowatt-hours per cubic meter (264 gallons) of water. If electricity costs ten cents per kilowatt-hour, that’s less than a penny per gallon in energy costs. See also this and this.

4) If you need to desalinate a small amount of water for personal use, the watercone may be useful: