I have done the AT twice. I carry four 12 oz soda bottles. One I am drinking from, and one that is waiting out the purification time. The other two are usually empty until it's time to make camp. One of them is marked for unpurified water that I will boil for dinner (sometimes I use the fourth one for unpurified breakfast water as well). I've never run out (though there are a few places where I'm cutting it pretty tight and twice (I think) I've been dry for a while while I purify. Remember also that if you are experienced, it's usually fairly easy to tell when you can probably safely use the water unpurified. I have done so fairly frequently with no ill effects. BrianLe may come on here as well and he always has good, sensible ideas. Have a good trip. best, jcp