In the really nasty refugee camps that the Red Cross and Peace Corp operate in they often use something akin the Katahdyn Gravidyn filter (or so I'm told). I would think that if it works there where the latrine is an open hole or less then it should work in most places. I agree with Don on the boiling. I would filter and then boil. The boiling will get rid of most if not all bio hazards but not do a thing against some of the nastier HazMat problems. A decent activated charcoal filter will rid your water of most HazMat problems but do a mediocre job on the bio-nasties. The Katahdyn Gravidyn comes with silver impregnated ceramic and an activated charcoal core. This combination will filter out almost everything. Anything that comes through that can be killed very effectively by boiling. By this process you should approximate distilled water nicely from almost any source. Of course if you are truely paranoid the reverse osmotic filters are the most expensive and they usually require power. <br><br>If you are preparing for being in a house or car for the duration of a 72hr interruption of normalcy then I would expect that you could store the required amounts of water with little difficulty. If you are on foot in refugee status then a pocket filter with iodine tablets is all you are likely to be able to carry aside from your canteen (whatever size it is). that should be good enough if you stay away from raw sewage or industrial waste.