Kosttab: Good question. The real issue with water tends to be weight. There is a very real limit to the amount of water you can carry. You can comfortably carry a quart to two for an afternoon hike. You simply cannot carry enough for an extended 3 o4 4 day hike, nor are you likely to have 8 or 10 gallons on hand when an emergency arises.

Boiling is an option. However, you have two choices. You can carry fuel or you can find fuel. Carrying fuel isn't very practical. Carrying a small amount for cooking is okay, but the fuel required for boiling all drinking water as well as cooking is not practical. Nobody wants to carry a ton of naptha, keorsene, whatever. As for using natural fuels, a lot of wet weather hiking has taught be it isn;t alwasy practical. If I'm hiking, I want to hike, not spend all day finding fuel to boil water. If I'm surving, I do not want to spend all my time trying to find fuel to boil water. Finding fuel, making a fire, keeping the fire going, boiling the water, letting it cool and pouring it into a container is a 45 minute job. I can unpack,filter and be gone in about 3 minutes.

Good filters aren't cheap, but they offer significant advantages. I own a Katadyn ceramic filter. Weighs something like 16 ounces. In the North Eastern US, where adirondack streams are plentiful, this 16 ounces of katadyn means I have nearly unlimited drinking water. And my water load is about 2 pounds (1# of filter and a small amount of filtered water).

The cheaper chemical/carbon filters work good, but have limited lives. They can filter a set amount of water, but once used, even if only for a quart, they have a life limited to a few months. The cermics are expensive (and do not remove chemicals) but are practically unlimited in life. Dry it out, pack it away, and its ready for next year.

There are several types of water filtering 'devices'.

Actual "filters" force water through porous materials, removing or trapping particles of dirt, bacteria, etc. They work on a physical removal prinicple. Cheap Filters remove big chunks of junk (2 microns). Really good filters remove much much smaller chunks in addition to the big ones (0.2 microns).

"Purifiers" often work by chemical means. In addition to a certain amount for filtering, they also introduce some form of chemical agent that kills assorted nastys.

There are pros and conc to each side. I like the extended life of a good ceramic. It does not "kill" the little creepies, but simply strains them out. Main systems are now hybrids, using both filtering and purification.

Whether you filter or purify, these units beat boiling hands down.