A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Why can't we use a giant magnifying glass to convert salt water to fresh water?

July 23, 1999

Dear Cecil:

Why is it such a problem to convert salt water to freshwater? I remember doing an experiment in eighth-grade science where we put a magnifying glass to salt water, added sunlight, and through the magic of heat and condensation, presto, fresh water! Why can't a giant magnifying glass be put over a section of the ocean, and the condensation be collected for all of the drought-stricken countries?

Cecil replies:

Now, Thirsty, surely you can appreciate that there might be some difficulty in scaling up from an eighth-grade science experiment. You don't see a lot of power companies plugging electrodes into giant potatoes. Similar problems attend the implementation of planet-sized magnifying glasses. But don't worry, desalinating seawater isn't as tough as you think.

Desalination was one of those wacky concepts that the popular-science writers of the 1950s told us would change the world. Space colonies! Undersea farms! Computers so small you could hold one in your lap! OK, so some of those ideas actually came to pass. Desalination of seawater kinda did too, but since it never made a huge splash in the U.S. (as it were) we might think the technology flopped. We wouldn't think this if we lived in Riyadh. Even though it's hundreds of miles inland, Saudi Arabia's capital gets half its water supply from seaside desalination plants.

Desalination these days isn't confined to desert nations with pots of dough. Only about 2.5 percent of the world's H2O supply is freshwater, and 99 percent of that is inaccessible (polar ice caps, deep groundwater, etc). Because of population growth, there's increasing pressure to make use of water that was previously considered marginal, such as the brackish (slightly saline) water found in parts of the southeastern U.S. Already there are 12,000 desalination plants in operation worldwide, handling about six billion gallons per day, enough to supply 60-120 million people at the consumption rates we're accustomed to in the developed world. Annual investment in new desalination plants is estimated at about $1.5 billion.

It'd be nice if all this could be accomplished using solar energy, but even with giant magnifying glasses the sun's rays are too diffuse to be harnessed economically (at least that's the common view of experts in the field). But there's been steady pressure to use energy more efficiently. For example, most desalination processes based on evaporation (the technique used by the magnifying glass) are multistage — the warm-water vapor output from one stage is used to heat the incoming seawater in the next. At the same time — you physics PhDs will appreciate this — the ambient air pressure is lowered at each stage, maintaining the rate of evaporation.

An even more efficient technique is "reverse osmosis," in which seawater is forced under pressure through special membranes that filter out the salt. Though evaporative processes still dominate the industry, RO is said to be the coming thing.

You may consider this more technological detail than you need to know, but that's because you're not seeing the big picture. A mere 12 years from now the first baby boomers will reach retirement age. Retirement homes in warm climates are likely to be in big demand … but lack of water will be a major obstacle. Whoever figures out how to supply the Arizonas of the world with water, through desalination or other means, ain't gonna have to worry about whether his social security check got lost in the mail.

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