Using a Millbank bag and a Ghillie Adventurer is useful for long term field use. Sterilised water will probably used for making tea/coffee and freeze dried food anyway.

Filters such as Ceramic filters produced by Katadyn are also very useful as well. I keep a Katadyn Syphon filter inside a Ortlieb 10 litre water storage bag with shower attachment next to an S10 respirator and a Geiger Counter in a PLCE respirator bag.

http://www.amazon.com/Katadyn-1120070-Siphon-Filter/dp/B0007U0102