One thing to watch for is that the location of the bottle in the sun can be a concern. You can actually have a prism effect that can catch the grass behind it on fire. At least one forest fire has been attributed to a prism effect from a discarded water bottle. Highly unlikely to happen, true, but worth keeping in mind.
Gunco Member #10
The "original" Boltcutter Rivet Squeezers:
Project Pink - the Pink and Blue AK-74:
I'm not sure why anyone would prefer taking the time to wait on UV rather than just boiling the water for 5 minutes. Hate to say it, but my survival equipment includes a metal Sierra Cup, at a minimum.
When in danger or in doubt, run in circles, scream and shout... R.A. Heinlein
There are times that you do not have fire, or do not want to risk exposure with a fire. SODIS works with or without fire.