Solar Water Purification

[Kevin Tjaden]

I was reading an article in National Geographic this week about fresh water and it discussed that water in a clear plastic bottle on a reflective metal surface would be purified by UVa in 2 hours. Hmmm, I thought, that's the same amount of time I usually wait for chemical treatments to work. So I thought why not use a piece of mylar and a platy bottle to purify my water in the sun when both time and sunshine are available. Weight would be for a 6" x 9 " piece of mylar which my scale won't measure. See this photo for how it would work. In the photo I used tinfoil an a sport drink bottle, but I am thinking a platy and mylar for the trail. Even if I could only use this 1/3 of the time it would be of benefit. An additional benefit would be to preheat my cooking water to save fuel. Any thoughts or is anybody already doing this?

[Bob Gross]

In the NG article, were there any statements about how good of a job this method did on water?

There are many anecdotal reports about water treatment, but not many that break it down into hard numbers.

For example, it would be neat if you could get a maximum water temperature reading from the bottle. The kill temperature for Giardia is 175 F, so if this could produce 180 F, it would be a good thing. Suppose it produces only 100 F. What would that prove?

–B.G.–

[Steve M]

One could also reach pasteurization temperatures (+160F) by slightly modifying your setup—use a black bottle inside a clear bag with the Mylar reflector behind it. Besides killing off the bad stuff this can also save on stove fuel—assuming you have enough time/sun.

PS Yes I know–UV and heat are two different methods. Unlike UV, solar heat has the potential advantage of reducing fuel needs.

[John Whynot]

It's the UV-A that does the purification. Description of technique.

[Kevin Tjaden]

The principal purification method I would assume is UVa rays similar to the concept behind a Steripen (UVc) rather than relying on heat for sterilization. Though one could assume the two in combination could be necessary. Outside on a cold windy day the water temperature only rose by 15 degrees in two hours. If heat were the primary goal I would assume a more absorptive surface (Black) would be better. The article states 99% effectiveness, but that sounds like a generalization and I don't know if it is backed up by data.

[W I S N E R !]

The N.G. article actually says it takes six hours.

Heat has nothing to do with this- it's UVA radiation mutating/scrambling DNA faster than the host's cells can repair it.

Solar Death Ray!

Yeah, it's doing that to our skin too!

[Bradley Danyluk]

Steripen claims that UV rays are incapable of penetrating clear plastic, which is why it's safe to use their purifying light turned on inside one and look at it from outside.

Or is it that Steripen uses UVC which can't, and the sun's UVA will get through, but somehow still effectively purify the water?

Anyway… if it was sunny enough to do this, I would probably feel better myself by making a solar still instead, which is definitely always effective.

[James D Buch]

The energetics of a solar still aren't really attractive. It takes about 600 calories to vaporize a gram of water which is then condensed back to liquid. True, some of that heat could be diverted to the heating process of getting water from ambient temperature up to the vaporization temp..

The heating to boiling only takes 80 to 100 calories per gram, depending upon the initial starting temperature.

The solar heater to heat to just below boiling then too would take only about this much energy. So, the simple heating/sterialization process could perhaps give you nearly six times the treated water as the solar still – for the same amount of captured solar energy.

Recently, I was party to a discussion on distilled water, and the bother and cost of buying it. Yes, someone brought up the idea of a solar still. Then someone else got some data on small affordable commercial solar stills and the amount of distilled water you could easily generate from a sunny window wasn't very much. The discussion just tapered off to nothing after that. I believe that people just went back to the bother and cost of buying distilled water in the relatively small quantities needed, about a cup or so per day.

I was initially excited about the use of solar UV as a water treatment, and still have an interest.

[Kevin Tjaden]

I am not sure how I read that as 2 hours as the article clearly states 6 hours. Must be wishful thinking. 6 hours would mean it would have to be carried and be exposed most of the day, which might be possible in desert situations, but not in many others.

I still will use this to raise the water temp for cooking where possible. Just to see if the fuel consumption difference is of value. Though for this purpose something more along the lines of a solar shower may be more effective. Does anyone have experience with pre-heating their cooking water?

[Travis Young]

The SODIS web site (http://www.sodis.ch/index_EN) lists a technique using water bags that will purify water in just one hour at the right temperature. See SODISTechnical Note #17:
SODIS Bags and Temperature Sensors at http://www.sodis.ch/methode/anwendung/factsheets/bag_tempsensor_e.pdf.

[Javan Dempsey]

People in africa use solar ovens to purify water on a daily basis in some areas.

180deg for 20mins pasteurizes the water, as I recall, which pretty much any variety of solar cooker can easily handle.

[Trevor Greenwood]

Funny that I read this post as I had just posted an experiment I did on preheating water on a different forum. I took a platy and darkened one side with a very large black permanent marker. I then took an unmodified platy and filled both with 57 degree water and placed them side by side in the sun for 1 hour. Outside air temp was about 64 degrees.

Unfortunately the shade moved over the bottles for an unknown amount of time. After the hour was up I tested the water and got just under 100 degrees for the dark one and 82 degrees for the unmodified one.

You can't always count on sun but on most trips with the weather report you could maybe take a bit less fuel for boiling water. Also nice for cleaning up with some nice hot water.

-Skippy

[Michael Moccia]

I've been referencing the SODIS technique in my survival classes since 2002. We offer it as an alternative when chemical and mechanical purification means are not available. Good to do if you can let the container sit still for at least 4 hours. You have to drill down on the web site but there is a manual that describes field expedient methods.

I’ve seen it in action is small towns near the equator and it did reduce the cases of gastrointestinal related illness.

You can built a demonstrator using a 2 liter platy bag by painting one side with black flexible tool handle paint. I made one 8 years ago and it’s still going strong. I carry it on a regular basis as a hydration / shower bladder. Under the right conditions the water can get hot enough to make FD coffee (as tested in TX and VA on a sunny day). That’s hot enough to pasteurize the water along with the UVA treatment.

Mike M

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