SCIENCE AND TECHNOLOGY: "Cheaper desalination: Current thinking; A fresh way to take the salt out of seawater," The Economist, 31 October 2009.

Saltworks Technologies, founded by two Simon Fraser U (Vancouver) MBAs, says they can reduce the usual cost of desalinization from 3.7kilowatt hours of energy to less than 1kwh.

The key? Using solar heat to evaporate much of the water and increase the concentration of salt before it's sent into the desal unit, where a sort of electrical circuit is created by taking advantage of the fact that salt is made of two ions (positive sodium and negative chloride).

This part gets complicated, so I quote at length:

These [ions] flow in opposite directions around the circuit. Each of the four streams of water is connected to two neighbors by what are known as ion bridges. These are pathways made of polystyrene that has been treated so it will allow the passage of only one sort of ion--either sodium or chloride. Sodium or chloride ions pass out of the concentrated solution to the neighbouring weak ones by diffusion through these bridges (any chemical will diffuse from a high to low concentration in this way). The trick is that as they do so, they make the low-concentration streams of water electrically charged. The one that is positive, because it has too much sodium, thus draws chloride ions from the stream that is to be purified. Meanwhile, the negative, chloride-rich stream draws in sodium ions. The result is that the fourth stream is stripped of its ions and emerges pure and fresh.

The mag says this is a "simple idea" that can be used on a grand scale or in small rooftop units the size of fridges. Lotsa "clever engineering" required, but the low-pressure nature of the work means you can use plastic pipes vice steel ones.

But the calling card is the low-energy usage:

... the only electricity needed is the small amount required to pump the streams of water through the apparatus. All the rest of the energy has come free, via the air, from the sun.

Damn! And I was so hoping for resource wars here, assuming human ingenuity would fail completely!