Thank you for saying about the alum method - that's a really good way to start before the purification process and it's dirt cheap if you just pack some with you. It will bind to many of the nasty gunk in water, drag it down and you can separate it. Then from there do your normal distillation/boiling + filtering and that will give you about as good as you're going to get unless you have a reverse osmosis system to use in addition/aside from that.

Excellent article on it:
http://www.wikihow.com/Purify-Water

And that ladies and gents, is (IMHO) the best process we have available.