Alright, now we're getting into a good discussion! <img src="/forums/images/graemlins/smile.gif" alt="" />
I'm certainly familiar with using sunlight to disinfect water. I was just under the very strong impression that UV rays are blocked by most plastic. Mostly because of this off the Steripen site and from a little bit of UV experiments on the lab:
"UV-C will not pass through most materials. Drinking containers made from glass, ceramic, metal, and nearly all plastics block UV-C transmission. Also, the underside of the air/water interface in a water container acts as a very effective reflector for UV-C. As a result, when SteriPEN™’s lamp is immersed in virtually any drinking vessel, the UV-C is well contained. Note that SteriPEN™’s water sensors prevent it from operating unless the lamp is completely immersed.
While very few materials are transparent to UV-C, there are a small number of uncommon materials that are. These include optical grade quartz (the SteriPEN™ lamp material) and a few fluoropolymers in the Teflon family – both unlikely materials to be used for drinking containers." http://www.steripen.com/faq.html
-- I am discussing specifically UV-C, apparently. So my source tells me at least <img src="/forums/images/graemlins/smile.gif" alt="" />
Your source speaks specifically of UV-A. This is an interesting divide.
Also, your source states the heating effects of the sun would be enough to deactivate 99.9% of organisms, leaving the effect of the UV (in my mind) somewhat up in the air. The UV has a contributory effect but is not the main sterilizing action from what I read, as long as the water peaked over 50 degrees C.
As far as botulism - it's only a concern with stagnant water. I don't drink from stagnant water. However, I realize that I'm in the minority when I tap from clear-running, non-stagnant, highly oxygenated mountain stream water <img src="/forums/images/graemlins/grin.gif" alt="" /> If I had to drink from jungle pools, aie!