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Praise for Ed's previous novel, Lost in Translation:

"Edward Willett has arrived, and SF is the richer for it." -  Robert J. Sawyer, Hugo Award-winning author of Hominids

"A believable, absorbing, thought-provoking and highly enjoyable read." - Kathy Tyers, Author of the Firebird trilogy, Star Wars: The Truce at Bakura, and Star Wars: Balance Point

Copyright 1995 by Edward Willett  

"Water, water, everywhere,
and all the boards did shrink;
Water, water, everywhere
Nor any drop to drink."

--Samuel Taylor Coleridge, The Rime of the Ancient Mariner

It's one of the ironies of nature that although three-quarters of the Earth's surface is covered by water, only a very small percentage of that water is drinkable--as the Ancient Mariner and his shipmates discovered in Coleridge's famous poem.

And it's not just seawater that suffers from a surfeit of salt. Water isn't potable unless it contains fewer than 500 parts per million of minerals, but there are lots of brackish wells and lakes that contain 1,000 to 4,500 parts per million of minerals. (Although that's not even in the same league as seawater, which contains 35,000 parts per million.)

However, it is possible to make salt water sweet. This is called desalination, and it was first carried out on a widespread basis on pre-Second World War ships. Heat from the ship's engines boiled salt water; the resulting steam could be condensed back into fresh water.

Desalination moved to land after the war, especially in the Middle East, where there's very little fresh water available but the sea is never far away. Israel built its first seawater distillation plant in 1965 to serve the new desert town of Eilat. Arab countries soon built even larger plants for their desert communities. (Key West, Florida, became the first North American city to get its drinking water from the sea when its desalination plant opened in 1967.)

These desalination plants all used "flash evaporation," a variation of the original ship-board system. Seawater is first heated, then pumped into a low-pressure tank. Since the boiling point of water drops with the decrease in air-pressure, the water vaporizes almost instantly, "flashing" into steam. The steam is condensed into pure water.

This process is expensive because it takes lots of energy, but the high price of fuel oil isn't exactly a big concern in Middle Eastern countries. As well, desalination plants have become more energy-efficient with time: today they use incoming sea water to cool the condensers, which is the same thing as using the condensers to heat up the incoming sea water--whichever way you look at it, it saves energy.

Flash evaporation is not the only method of desalinating water. Another is freezing. Salt water has a lower freezing point than salt water, so lowering its temperature below the freezing point of fresh water but above the salt-water freezing point produces fresh-water ice crystals in the salt water, which can be removed, washed free of salt, and melted into fresh water.

Still another method is electrodialysis. When salt is dissolved in water, it splits into positive and negatively charged ions. By running an electrical current through the water you can draw these ions out. Buckeye, Arizona has had all its water supplied by its own electrodialysis-desalting plant since 1962. Port Mansfield, Texas, also has an electrodialysis desalination plant.

But the "technology of choice" these days is reverse osmosis, a process used in home water purification systems as well as in large commercial plants like the one serving Cape Coral, Florida, which produces 15 million gallons of desalinated ocean water a day.

In the reverse osmosis process, water is forced under pressure through a "semi-permeable membrane" which will only allow pure water molecules to pass. Salt, dirt, bacteria, heavy metals and other contaminants are precluded.

For a homeowner, this isn't a terribly efficient process, because only 10 to 25 percent of the water that goes through a reverse osmosis filter is actually purified. The rest simply goes down the drain. This can mean high water bills, so a water purification system of this kind really only makes sense in a location where the water has unacceptably high levels of dissolved solids, lead, or other inorganic contaminants. Reverse osmosis systems aren't good at removing some hardness chemicals (especially calcium), either, so they can't replace a water softener. In a large-scale plant, however, the fact that only a portion of the water passed through the system is desalinated isn't a problem, because there's no water meter attached to the ocean.

Reverse osmosis has also found a place on ocean-going vessels. Today's mariners can install a shipboard "watermaker" which turns salt water into fresh water for crew and passengers, reducing the need, especially on smaller vessels, to ration water on long voyages.

Or, as Coleridge might have put it:

"Water, water everywhere,
Oh, what will be our fate?
Water, water everywhere,
Relax! Desalinate!"

These weekly columns on science appear in the Regina (Saskatchewan) Leader Post and Red Deer (Alberta) Advocate..  They are available for one-time publication or regular syndication to any interested newspapers, magazines or on-line publications.  E-mail me for details.

Posted May 1, 2001

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