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It's sometimes easy to fool ourselves into believing that water is clean enough to drink as we're out paddling. As we sit in the middle of a remote wilderness area, far away from factories, cities and other sources of pollution it seems that we should be able to dip our cup into the river and have a long, cool drink.
Unfortunately, there is no area of the country that can claim to have water this clean. Even if drinking from a clear mountain stream seems safe, we should keep in mind that there may be a rotting deer carcass 50 ft. upstream of our location.
Any water can contain pathogens which can cause illness. Boiling is effective, but time consuming. Most backcountry travellers now use water filters or water purifiers.
Most filters use the same principle - they have a ceramic filter insert, and water is pumped under pressure through this ceramic material. The pores in the ceramic filter are smaller than the parasites in the water, and they are filtered out. The result is clear, immediately-drinkable water.
It's important to review the technical specifications before you buy a filter or a purifier. A filter with a 0.2 micron filter will effectively remove giardia cysts, which are much larger than that size; and also bacteria like E Coli. However, the smallest parasites (viruses) will travel through a 0.2 micron filter with ease. See the "Problems" section of this site for an explanation of Giardia and all the other nasties that can be found in our water.
Know what you're trying to remove and obtain a filter that meets this requirement.
For the most part, in North America we're mainly concerned with the large parasites like Giardia, and suspended solids like silt, algae and organic material. Nearly any filter will effectively treat water like this. It's a good idea to have some sort of pre-filter to remove most of the silt, or your filter will clog up much sooner than it should. If that happens, you're stuck with disassembling it for cleaning, or in the case of a filter with throwaway cartridges, buying a new filter cartridge. Really murky water can be pre-filtered through a clean cloth or a coffee filter before putting it through the water filter.