Micropur? That's what I use, and I learned about it here.

There was some discussion about the "4 hours" part of the instructions. Apparently, it's 15 minutes for most things, half hour for most things in very cold and/or murky water, and the only thing you need to leave it for 4 hours before drinking is with cryptosporidium in cold and/or murky water. I also understand that there is some debate about whether you really even need to worry about crypto, unless you're immunocompromised. Dig through the articles in the main page of the site, Doug wrote about it.