I heard a little while ago that one can actually purify water by sealing it in a clear plastic container and putting it in the sun for a time. Something about the sun's rays shining through the plastic being capable of killing the bacteria. Is this true? If so, how does it work, and how long does the water need to sit in the sun?

Check out this thread:
http://www.equipped.org/ubbthreads/showt...=true#Post16853

And yes it does work. Pete

Pete:

I followed the thread you indicated. Amazing stuff. I also googled several of the products and terms I found, for even more interesting info. Since the SODIS (Solar Water Disinfection) precess relies on a dark bottle to heat the water to 70 deg. C, (or a clear bottle on a dark surface or reflective surface, here is my question - wouldn't this process also work with a US GI canteen (dark green) or British MoD water bottle (black)?

Thoughts most welcome. Doug? Chirs? MybrotherPeanut?

.....CLIFF

NO ! you needs clear bottle's because solar Water Disinfection works by heat AND UV radiation. there are a couple other links on the link paramedic posted, read them 2...

If that is the case then a puddle of water on some black top should be safe to drink. Because it is warm and the water is clear, how can that be? I know UV kill some bacteria but I don't think I would risk the two or three weeks down time caused by giardia

I think that there are probably a number of things that make the clear plastic bottle on the dark surface uniquely useful for this.

1) clear to UV - rules out opaque bottles and glass bottles which are opaque to UV

2) Heat concentration - rules out metal containers

3) insulative properties of plastic - rules out metal containers

4) sealed from recontamination - rules out open puddles

5) food-grade plastic won't introduce chemical contaminants. - rules out black-top

6) Time. This process takes 1/2 day or so because it doesn't really get that hot.

If you set up a double layer window on a very shallow slant, paint the underside black, let water flow very slowly between the panes, you will get considerable condensation on the upper pane which will flow down to the bottom for collection. The condensate is distilled and drinkable. If you separate the panes enough you can rig a trough at the bottom of the upper pane to catch and divert the condensate. If you further make a highly convoluted path for the water to flow down the lower pane by dribbling silicone caulking or some similar clear substance on the glass you can get the flowing water to move accross the surface extremely slowly thereby increasing the heating and subsequent condensation. Don't allow any air to flow between the panes of glass. This simple still is equivalent to those used in solar desalination plants. Eventually you will need to open it up and clean residue from the bottom pane.

You are correct. My bad. What about water in a Zip-Loc bag, or even.... a condom?

Traditional their are three ways to purify water:
1.) boil it
2.) filter it
3.) chemically purify it

You can try to make a solar still if you like but it's not the most productive way of gathering water. My advice is to store a bottle of potable aqua in your kit. It will work a lot better than leaving a clear bottle of water out in the sun.

From:
http://www.sodis.ch/Text2002/T-FAQ.htm#Whichorganisms

"Which microorganisms in the water are destroyed by using SODIS?
Research has shown that pathogenic bacteria and viruses are destroyed by SODIS. The inactivation of the following microorganisms has been oberserved:

Bacteria: Escherichia coli (E.coli), Vibrio cholerae, Streptococcus faecalis, Pseudomonas aerugenosa, Shigella flexneri, Salmonella typhii, Salmonella enteriditis, Salmonella paratyphi
Viruses: bacteriophage f2, rotavirus, encephalomyocarditis virus
Yeast and Mold: Aspergillus niger, Aspergillus flavus, Candida, Geotrichum
The inactivation rate of E.coli has a similar magnitude as the inactivation rate of Str.faecali and enterococci. Hence E.coli are appropriate indicators for the efficiency of SODIS on the inactivation of bacteria of faecal origin.

The inactivation of spore and cyst forming organisms such as protozoa and helminths by solar water disinfection has not systematically been assessed yet.
However, the thermal death point of amoebic cysts for example is about 50°C. If contaminated water reaches a temperature of at least 50°C during an hour, this would ensure the destruction of the amoebic cysts as well as other microorganisms. (Compare with table)

Thermoresistance of microorganisms
Microorganism Time and Temperature for 100% destruction
1 Min 6 Min 60 Min
Enteroviruses - - 62°C
Rotaviruses 63°C for 30 Min
Faecal coliforms (E.coli) 80°C - -
Salmonellae - 62°C 58°C
Shigella - 61°C 54°C
Vibrio Cholerae - - 45°C
Entamoeba, Histolytica cysts 57°C 54°C 50°C
Giardia cysts 57°C 54°C 50°C
Hookworm eggs and larvae - 62°C 51°C
Ascaris eggs 68°C 62°C 57°C
Schistosomas eggs 60°C 55°C 50°C
Taenia eggs 65°C 57°C 51°C "

This process is also known as solar-oxygen treatment and relies on a combination of UV and oxygenation provided by agitation. You really need either a clear glass or clear plastic PET bottle for it to work well, such as a two-liter soda bottle. A new supply must be treated each day as UV treatment provides no residual anti-microoorganism effect. The process is described in detail in The Ultimate Desert Handbook by Mark Johnson (www.ultimatedeserthandbook.com)