Tuesday, August 20, 2013

Water Treatment: Desalination

Large swathes of southern California are virtual desert, parks and lawn made possible only by water piped in from far away. One of my concerns should disaster strike is that the system for getting water from the Colorado river to my tap will be interrupted. I have some water stockpiled, and with a little warning I can fill up our WaterBob in the bathtub, but I’m keenly interested in alternate sources of water.

Lo, what sound is that that falls on my ear? The sound of waves breaking on the beach. I’m within a very short walk of the Pacific Ocean—more water than I could possibly drink in a thousand lifetimes. Unfortunately, that abundant supply of water is not in a form one can drink. It needs to be treated to get out the excess salt and other minerals; it needs to be desalinated to be made potable.

Thinking about water from this perspective, distillation and reverse osmosis deserve a second look. I rejected these methods of water treatment in favor of just about every other method, but that was assuming I had a supply of fresh water in the first place. If all one has is saltwater or brackish water, none of those other methods can help.

In distillation, vapor from evaporating saltwater or steam from boiling saltwater leaves the salt behind in the solution, so that the condensation you collect is fresh. This is a credible solution that could save your life. However, distilling water by boiling generally consumes a lot of fuel, while it could take days with a solar still before the sweat of putting one together is replaced with fresh water.

It is common for people in ships to be surrounded by saltwater, and marine desalination solutions (“watermakers”) have been developed. Unfortunately, these systems typically require electrical power to regulate the pressure of the water through reverse osmosis membranes—and it’s hard for me to imagine being cut off from water in southern California while still having plenty of power.

But wait! Sailors also prepare to be stranded in a lifeboat without power, yet persist in wanting their fresh drinking water. Emergency solutions such as the SeaPack have been developed—which takes 5 hours to produce .5 liters of potable water, once a day for 10 days for almost $75, or a one-time-use system for almost $50. This is low-power, but also low-output and very expensive.

Katadyn makes a lifeboat device that would be perfect for me: the Katadyn Survivor 35 Desalinator is hand-powered and produces a gallon of fresh water in an hour. So I’d be all set, assuming I had the time to spend 1-2 hours per day per person in my party pumping the little device. While the device costs more than $2,000, and membrane replacements cost more than $300, if I’m going to depend on desalinating water this looks like the way to go. (Katadyn also makes a lower-capacity Survivor 06 for about $1,000,) With desalinators it is especially important to properly maintain one’s equipment, so I'd want to inspect mine at least once a year, prepared if need be to replace the membrane.
Keeping it real, there are many other items or supplies on which I might better spend my cash. $2,000 will buy a lot of water storage. I doubt I'll acquire a desalinator any time soon; for now I'll continue to stockpile water at home and keep an eye out for other local fresh water sources.

Related Articles

This article is part of a series I am writing dealing with treating water for drinking.

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