My daughter bought me a "LifeStraw" last Christmas. I typically don't filter but it is kinda fun to dip the thing in the lake from my kayak and drink away. I'm amazed how well it works and keep it in my day hike stuff. It filters to .02 microns but has almost no resistance when you drink through it. 2 ozs.
http://www.vestergaard.com/our-products/lifestraw
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paul, texas KD5IVP