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I very rarely need to purify water when /out/ but if I have to, its all about the Condy's Crystals (potassium permanganate). If water looks really suspect, I'll filter through cotton, boil and add Condy's, but I've only had to that twice in 15+ years.
I always carry a small vial in my kit, its so small that you'd be an idiot not to carry it. 1 vial will last you a lifetime as you only need a few grains to purify water and it doesn't affect the taste of the water like other purification methods. Also you can use more grains to create anti-septic and disinfectant, so its multi-purpose.
(pic related - my vial for reference)
