A hollow-fibre squeeze filter is the single best value item in a British water kit. It is light, it needs no power or chemicals, it removes bacteria and protozoa reliably, and a good one filters an enormous volume before it wears out.
It also has one failure mode that is genuinely dangerous, because it is completely invisible.
The freezing problem
Inside the filter are thousands of hollow fibres with microscopic pores along their walls. Water is forced through the pore walls, and organisms too large to fit are left behind.
This is the most important thing on this page and the thing most owners have never been told.
How to avoid it:
- Once a filter has been wet, it stays somewhere that will not freeze. Indoors, in a heated part of the house.
- In cold weather in the field, sleep with it. Inside your jacket during the day, inside the sleeping bag at night. This is standard practice among people who use them in winter and it is not fussy, it is the only reliable protection.
- Blow it out thoroughly after use to remove as much water as possible. This reduces the risk substantially but does not eliminate it, because the fibres retain moisture.
- Keep one filter sealed and never used, stored dry, as the reserve. A filter that has never been wet cannot be damaged by freezing.
If you are unsure whether a filter has frozen, retire it. It costs a few pounds to replace and the alternative is drinking unfiltered water while believing you are not.
Backflushing
Flow slowing down is normal. It means the filter is doing its job and the pores are loading up with the material it removed.
Backflushing pushes clean water backwards through the fibres and clears them.
- Fill the syringe or squeeze bag that came with the filter with clean, already-filtered water.
- Push it firmly backwards through the outlet, the end you drink from.
- Expect discoloured water out of the inlet. That is the point.
- Repeat until it runs clear and flow is restored.
Do this whenever flow becomes annoying, and always before putting the filter away for storage. A filter stored dirty is harder to recover later.
Day to day
- Pre-filter cloudy water. A coffee filter, a clean cloth, or simply letting the water stand and settle before you draw off the top, all dramatically extend the life of the filter. A filter clogged by silt has been wasted on mud rather than organisms.
- Never let it dry out completely between uses if you can help it. Some membranes lose flow permanently once fully dried. Blowing it clear is fine; baking it on a radiator is not.
- Keep the clean side clean. Do not let the outlet touch raw water, dirty hands, the ground, or the inside of a bag that has held dirty gear. Cap it.
- It does not remove viruses, salt, or chemicals. Hollow fibre is sized for bacteria and protozoa. For viruses, boil or chlorinate as well. For salt or chemicals, only distillation helps, covered on the solar still page.
- It does nothing for blue-green algae toxins. See UK water risks, which is the page that explains why boiling that water makes it worse.
What the pore size actually means
The page has said twice that this filter does not remove viruses. Here is why, because the number makes it obvious and it is the number that tells you what else the filter cannot do.
A hollow-fibre membrane of the kind used in portable filters is typically rated at about 0.1 micron. It is a physical sieve: anything larger than the pore is retained, anything smaller goes through.
So the filter is excellent at exactly the things that are largest, which in British surface water is most of the biological risk, and it is blind to everything smaller.
The practical consequences:
- Filter, then boil or dose the drinking portion. The filter takes out the cysts that chlorine handles poorly, and the boiling or the chlorine takes out the viruses the filter misses. The two methods cover each other's gaps, which is why the pairing is the standard advice rather than either alone.
- Filtered-only water is fine for washing, for cooking water that will boil anyway, and for cleaning.
- Nothing here touches salt, fuel, solvent or cyanotoxin. Those need distillation or avoidance. See read UK water risks and improvise a distillation setup.
Ceramic elements are usually a little coarser, commonly around 0.2 to 0.5 micron, which still retains bacteria and protozoa. Some carry a silver treatment or a carbon core, which addresses taste and some chemicals rather than viruses. Check what your specific element is rated for and write it on the filter, because the range on the market is wide and the assumption is always generous.
When to retire it
Retire the filter if:
- It has frozen, or might have frozen, while wet. No exceptions.
- Backflushing no longer restores useful flow.
- It has been dropped hard onto a solid surface. The internal fibres are fragile and impact can break them, with the same invisible result as freezing.
- The manufacturer's rated volume has been reached, if you have any idea what you have put through it.
Write the date of first use on the body in permanent marker. Without it you will have no idea how old the filter is when it matters, and the honest answer to "how much has gone through this" is otherwise always a guess.