Part of the show Naked Science Questions and Answers
Gordon in Suffolk asked:
With the ever increasing shortage of water in reservoirs, why can't we purify seawater to overcome the problem?
The simple answer is that we can. However the problem is that it uses loads and loads of energy. There are two ways of doing it. You can either boil up the water into steam and then recondense it into water. Obviously that's going to use loads of energy, as you can see how expensive it is to boil a kettle. The other way of doing it is something called reverse osmosis. This is when you take a really fine filter and push the water through it. The salt then stays on the other side. This is a lot more efficient than boiling it, but it's still very energy expensive. Considering that the greenhouse effect is a big problem because we're using too much energy already, if started producing water by desalination in one way or another, we'd really be in trouble. There's no such thing energetically as a free lunch and what you're trying to do is take salt water which has a great concentration of minerals in it and separate those minerals into a strong concentration of minerals and a strong concentration of water. In other words, you've got to do work to sort the wheat from the chaff. That work comes at a high price. If we do it, we have to burn a fossil fuel in some way or another. The sun does that almost all the time all around the Earth. The sun is hitting the surface of the ocean, evaporating some water and leaving behind the minerals in the sea. The water forms clouds and then comes to Earth as precipitation. So the sun is desalinating all the time, but then the sun has money to burn.