What you are trying to remove
Backcountry water can carry bacteria, protozoa and viruses. Bacteria and protozoa are the common problems in North America; viruses are a bigger concern in areas with human waste upstream.
A hollow-fiber filter removes bacteria and protozoa but not viruses. A purifier adds a virus-killing step, usually chemical or UV. Tablets kill most things but take 30 minutes to 4 hours depending on the product and water temperature.
- Filter: removes bacteria and protozoa. Fast, no taste.
- Purifier: adds virus removal. Best for international travel.
- Tablets: light and cheap, slow, leave a chlorine taste.
- Boiling: kills bacteria, protozoa and viruses, but not chemicals or sediment. Bring water to a rolling boil for 1 minute; at altitudes above about 2,000 meters, boil for 3 minutes.
Choosing a water source
Take water from a moving section, above any campsite or trail crossing, and below a spring rather than a pond. Clear water is not sterile, but it clogs a filter less than silty water.
Avoid water with a strong smell or a bright green scum. If the source is a puddle, look for another one. Filtering muddy water wears out a cartridge in days instead of months.
Speed and the pre-filter
A clean filter moves about 1 liter per minute. When it slows to a drip, the cartridge is clogged. Most models have a backflush syringe or a gravity bag you can squeeze; do it in the field rather than waiting until you get home.
Use a pre-filter cloth or a coffee filter over the intake when the water is silty. It costs nothing and doubles the life of the cartridge. Pre-filtering also makes the water taste better.
Field hygiene
Keep the clean end clean. Do not set the mouthpiece on the ground, and do not let a dirty bottle touch the filter outlet. If you are filtering for a group, fill bottles one at a time rather than passing a wet mouthpiece around.
Wash your hands or use sanitizer before you eat, especially after handling a filter that has been in a stream. The filter protects you from the water, not from your own hands.
Do not let it freeze
Water expands when it freezes and cracks the hollow fibers inside a filter. A filter that has frozen may still pass water but no longer remove anything. Treat a frozen filter as broken and switch to tablets.
In cold weather, sleep with the filter in your bag and carry it inside your jacket on the trail. If you are not sure whether it froze, do not trust it for the rest of the trip.
Storage between trips
Backflush with clean water, shake out the excess, and let the cartridge air-dry before you store it. A filter sealed wet for months grows things you do not want to drink.
Note the cartridge life in your calendar and replace it at the manufacturer interval or when the flow drops and will not recover. A cheap spare filter weighs less than a wasted trip.
