Filter, Treat or Boil: Picking a Water Method

There are three categories of thing you are trying to remove: protozoa such as giardia and cryptosporidium, bacteria, and viruses. In most of the backcountry United States and Canada, protozoa and bacteria are the realistic concern and viruses are not, which is why a hollow fiber filter is the usual answer. Travel outside that context and the calculation changes.

How to do it

1. Match the method to the threat

   A hollow fiber filter handles protozoa and bacteria and does not remove viruses. Chlorine dioxide handles all three but needs four hours of contact time for cryptosporidium. Boiling handles everything. Ultraviolet works but needs clear water and batteries.

2. Collect from the best available source

   Moving water above any trail crossing, campsite or grazing, and upstream of any drainage from a use area. A clear spring beats a clear lake beats a stagnant pool, every time.

3. Pre-filter silt

   Pour through a bandana first. Silt clogs hollow fiber filters fast and a clogged filter is a slow filter you will be tempted not to use.

4. Respect the contact time on tablets

   Thirty minutes for most things, four hours for cryptosporidium in cold water. Dose it at the last water source before camp and it is done by the time you want to drink.

5. Boil correctly

   A rolling boil is enough at any elevation you will hike at. The idea that you need a long boil at altitude is a myth, though it costs fuel to run one anyway.

6. Keep clean and dirty separate

   Dedicate one bottle to untreated water and never let the clean threads touch it. Most backcountry stomach illness is actually contamination at this handoff, or from unwashed hands, rather than the water itself.

Where people go wrong

• Letting a filter freeze. Ice cracks the fibers invisibly and the filter still appears to work while passing everything through. Sleep with it in cold weather.
• Cutting the tablet contact time short. Cold water needs the full time and cryptosporidium needs four hours.
• Cross contaminating bottles. The single most common source of illness, and it has nothing to do with the water source.
• Assuming clear alpine water is safe. Clarity says nothing about what is in it.