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Retorts

Water for Auckland

Five-sevenths of the planet we live on is covered with water. This substance supports life on both land and sea. Experts tell us our way-back ancestors came from the sea. We can certainly taste the salt in our blood.

The main difference between sea and rainwater is the salt content. Depending on location, 3-5% of a given quantity of seawater will be composed of dissolved salts. When the sun turns seawater into vapour it leaves the salt behind. Here we find an interesting paradox. Most of the salt we find in the sea comes from the land, brought down by rivers and streams. Yet the average percentage of salt in rivers and streams is so low at any particular moment these water sources are seen as fresh by the authorities. The sun is the great concentrator of salt in seawater each time it lifts clouds high into the sky.

The city of Auckland, we are told, has a water crisis. Yet Auckland, like all our major cities, is surrounded by water, untold millions of gallons of it. Why can't this water be used to flush toilets, wash cars and cook vegetables, etc? There's no particular reason why this can't happen. We've become so used to taking fresh tap water for granted that we never think seawater could be used on a lot of the jobs we use fresh water today.

If we visit some Middle East countries we find the main source of drinking water is the sea. Huge desalination plants extract the salt from an unlimited supply of seawater. Expensive? Yes. But then many of these countries make lots of money from selling oil, so costs tend to balance out. It depends too on how the salt is extracted. Distillation using artificial heat would be the most expensive way of removing the salt. Using the sun would be cheaper.

If weather patterns are changing, as some experts claim, cities like Auckland may have to seriously consider reorganising their water supplies so that they can take advantage of the unlimited supply of water on their doorsteps. For vital supplies of drinking water they may need to seriously consider emulating Middle Eastern countries and build suitable desalination plants.

None of these ideas are impractical. What is required is long-term planners who can visualise the problem in its totality and be aware that science has already come to grips with these problems in other parts of the world.

R.D. Coyle, Wellington