One of the things that got left out of my recent interview with the water experts was a question about desalination. A reader just emailed to ask about it, so I thought I’d share:

DR: With water shortages everywhere, and so many big players jostling for water, why hasn’t desalination taken off? 

PG: The short answer is, up until now it’s been too expensive for all but the highest-value uses in places that are truly, truly water short — the Persian Gulf, certain islands, Kurasao — that have no other options and are willing to pay the price. That’s the reason it hasn’t taken off. There is some belief that it’s going to take off soon because the price is coming down.

DR: Are there technological advances? Or just faith?

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PG: There are technological advances. There are two ways of desalinating water. One is distillation, where you effectively boil water. Another is membrane reverse osmosis, where you have membranes that selectively pass salts, and so you push water with salt against the membrane and either fresh water goes through and leaves salts behind, or salts go through and leave the fresh water behind, depending on the membrane. And membranes have gotten more efficient and cheaper and more reliable and more productive, and so the price partly has come down.

DR: Do you personally think it will ever reach the point where it will make a real impact on the world water situation?

PG: It’s viable now in some places. It will be viable in more places over time. But it’s never going to provide huge amounts of water for irrigation, for example, because it’s just far too expensive.

It will slowly become more cost-effective for municipal and industrial use, but that’s a thousandth of a percent of the total water use worldwide now.

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The problem’s not technology; the problem’s economics. Fifty percent of the cost of desalination is energy. It takes energy to take salt out of water. And so as long as we’re paying high prices for energy, we’re going to pay high prices for desalinated water.