Hollow fiber filter at a glance
- Pore size
- 0.1 to 0.2 microns
- Removes
- Bacteria and protozoa
- Does not remove
- Viruses
- Killed by
- One freeze cycle
What is inside the filter
Inside a Sawyer Squeeze, a Katadyn BeFree or a Platypus QuickDraw is a bundle of hollow fibers, each one a tube with microscopically porous walls. Water is pushed through the wall from outside to inside, or the reverse depending on the design, and anything larger than the pore size stays behind. It's mechanical straining, with nothing to run out and no chemistry involved.
What the pores catch
The pores are typically 0.1 or 0.2 microns, which is small enough to remove bacteria and protozoa, including giardia and cryptosporidium. Viruses are smaller than that and pass through, which is why these are filters rather than purifiers and why they're recommended for North American backcountry water but not for travel where sewage contamination is likely.
Why freezing destroys one invisibly
The failure mode that catches people is freezing. Water left in the fibers expands as it freezes and splits them, and a split fiber leaves a channel wide enough for anything to pass. There's no visible damage, no change in flow, and no way to test it in the field. Once a filter has frozen with water in it, the only safe thing to do is retire it. In cold weather, that means sleeping with it in your bag.
What this means when you're choosing
- A backflush syringe or coupling included, which decides whether you maintain it
- The rated pore size, usually 0.1 or 0.2 microns
- Whether it has ever frozen, which is invisible and unrecoverable
The common mistake
Assuming a frozen filter is fine because it still flows
Split fibers pass water at a normal rate and pass everything else too. There's no field test and no visible sign, so a filter that has frozen wet has to be treated as compromised.
Common questions
How do I stop my water filter freezing?
Keep it in your sleeping bag overnight and inside a jacket pocket while hiking in sub-freezing conditions. Blowing it out after use helps but doesn't remove the water held in the fibers.
How long does a hollow fiber filter last?
Manufacturers quote very high volumes, sometimes in the hundreds or thousands of gallons, and in practice it's flow rate that ends its life. When backflushing no longer restores usable flow, replace it.
Where this is defined
- Water disinfection for travelers, US Centers for Disease Control and Prevention