I found an interesting PDF tonight on solar water disinfection that looks like it could be useful in times of real need. From my reading it is not useful for everything at all times. It does appear to be dependant on the temps you can acheive.
http://www.sodis.ch/methode/anwendun...l/manual_e.pdf
The technique is used in a lot of 3rd world countries, in particular, areas that have very little wood to use for fuel to boil water.
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