"Retrieve A discarded water bottle. Tear off the label and fill with any water that's not too murky from a creek, standpipe, or puddle. Place the bottle on a piece of metal in full sun. In six hours the UVA radiation will kill viruses, bacteria, and parasites in the water, making it safe to drink"

National Geographic, April, 2010

Known by the acronym SODIS, this is evidently used in the third world to provide economical pure water. The article further states that PET plastic is preferable and "water from sun-warmed bottles is safe to drink."

I have never tried this, but I have heated up water many times is a solar water shower, and the water can get uncomfortably hot after a full day's exposure. This is certainly a technique that is simple and cheap, although it won't be very useful if you must travel. I have been in situations where this technique would have been very helpful. I would consider NG a generally reliable source.

Hmmm. What about under the cap? What about on the outside? Where is there enough UV? Where is there NOT enough UV?

I don't think there is sufficient info given to make the technique safe.

consider it an emergency technique for your repertoire. Plastic water bottles are ubiquitous wherever litter is to be found.

You can also make a solar still with two bottles joined mouth-to-mouth, set in the sun at about a 45 degree angle.

“Solar disinfection is an example of another measure with proven health impact that requires little capital investment on the part of end-users, and is thus appropriate for the very poor.” WHO, 2007

“UNICEF promotes a variety of treatment methods such as user-friendly filtration, simple solar water disinfection (SODIS) and home chlorination. These are all low-cost, effective and manageable at the household level.” UNICEF, 2009

Red Cross Prize, 2006: "The jury considers SODIS an impressive way of contributing by the simplest means to making water supplies better and safer, thereby reducing diarrhoea and other diseases like it, and mortality in developing countries." Red Cross, 2006


I googled SODIS (solar disinfection) and got bunches of hits, including several research articles.

It has been discussed before
google SODIS site:forums.equipped.org
http://forums.equipped.org/ubbthreads.php?ubb=showflat&Number=119538


it is basically solar pasteurization (UV+HEAT)
to be sure pasteurization has been achieved,
pack a WAPI in your kit
http://en.wikipedia.org/wiki/Water_Pasteurization_Indicator
http://solarcooking.wikia.com/wiki/Water_Pasteurization_Indicator

It does work, but there are some limitations:

1. "The most favourable region for SODIS lies between latitudes 15°N/ S and 35°N/ S... The second most favourable region lies between the equator and latitude 15°N/ S..."

35°N is approximately Oklahoma City. The limitations may not be so limited in more northern climates if the day is warm and clear, and the temperature of the water rises to at least 113F (45 C).

2. The optimum size bottle is 1.5 liters, the max. is 2 liters, and the plastic must be clear, not tinted (the bluish PVC bottles are not as effective as the clear PET).

3. The water has to be fairly clear: fill the bottle, set the bottle on a newspaper headline, and sight down through the uncapped bottle -- if you can read the print through the bottle, it's clean enough.

4. SODIS won't do anything about chemically-contaminated water.

Sue

It works well at high altitude, even when cold, or high latitude
as the UV rays are more powerful.

It is a recommended technique for brackish water when used
after cloth filtering and settling, or after water is treated
with a biosand filter.

It is not technically sterilizing, but does make water safer to
drink. More so than some tap water or bottled water.

It works, though you need good intense sunlight

I'm inclined to believe it's the temperature rise rather than the UV, otherwise the top 6" of every stream and lake would be sterile.

The top 6" of streams is constantly being mixed with the water below it.

It'd be easy enough to test if it were the heat or the UV by taking to samples of the water, subject one to heat matching the temperature that the one in the sun reaches.

-Blast