When I'm travelling in the backcountry, and I'm in a fire-dominated ecosystem, I prefer boiling over an open fire as the most sustainable way to purify water. Otherwise, I use PolarPure, which is iodine crystals in a little bottle that I keep topping off with water. I add that solution to my 'wild water' and let it sit. The nice thing about iodine is that it's effectiveness depends on Time, Temperature, and Concentration. So I can make tradeoffs. I've used PolarPure for days, weeks, and months at a time on different trips. It's the most cost-effective, lightest, and most reliable thing I've found, besides boiling water with local material.