Water treatment devices, which include filters, purifiers, halogens, ultraviolet light, and stoves, are necessary for cleaning water of health hazards. It is widely accepted that most high-altitude, low-traffic, areas have little to no waterborne threats. However, for most, the peace of mind in clean water is well worth the extra room required to carry these small gadgets.
Water treatments endeavor to eliminate or neutralize potentially harmful things in water, including protozoa, bacteria, and viruses. Protozoan cysts, such as Cryptosporidium parvum and Giardia lamblia, are large parasites ranging from 1 to 350 microns. Bacteria are smaller at less than 10 microns. Viruses are incredibly tiny; less than 0.1 microns. Size is important because various treatment options can "catch" different sizes.
Most commonly these protozoa, bacteria, and viruses are introduced through human or animal feces near the water. Giardia lamblia (protozoa) and E. coli (bacteria) are the primary concerns in North America, where viruses are rarely found. Make sure to check up on local water health before committing to a less thorough method.
Water filters (pump action, gravity bags, or housed in bottles) are used to eliminate all protozoa and bacteria. Filters will not remove viruses because the pore size is too large. Water filters are fast to use, don't change the quality of taste, and are fairly easy to clean in the field. Drawbacks include more weight, slightly more bulk, and the labor for pump action filtration devices.
Filters clean somewhere between 250 - 800 gallons of water with proper care. The actual volume largely depends on the clarity of water being filtered. Murky water will clog a filter quickly, whereas clearer water will help a pump work longer.
Filters are an excellent choice for backcountry use where viruses are not a problem (such as in North America).
