Modern hollow fiber water filters are true technical wonders with their ability to get rid of bacteria, protozoa, and even viruses from unbelievably large amount of contaminated water. After reading the descriptions of some of the best filtering devices in the market, one easily gets impression that problems with dirty water are ancient history, at least for the wealthy westerners. However, somewhere from the manufacturer’s website you may find a following kind of warning: “After iniatial wetting, do not expose your water filter to temperatures below freezing. There is no definitive way to tell if a filter has been damaged due to freezing.”
I have heard of several occasions where a hiker has ran into trouble with a clogged water filter due to cold weather, “but after I kept the filter inside my jacket for awhile, everything was OK, and the filter worked again.”
No, everything was not OK! After the thawing, the water filter did let the water flow again OK, but most probably, it did not any more do its job as it should have, as the freezing process itself had destroyed the filtering structures.
And here is what happened. (Warning: light science content!)
As you may remember from high school science class, water has some interesting properties.
Water has the highest density, pretty close to 1g per cubic cm (0,999973 g/cm3, to be exact), i.e. it is heaviest, at +4 C temperature. When water freezes, its density decrease to about 0.917 g/cm3, i.e. it becomes lighter than liquid water. One can easily test that by dropping an ice cube into a glass of water, and see whether the cube floats. The reason why the ice cube floats is in the formation of ice chrystals, which need more space than free water molecules, and therefore when a gram of liquid water happily fits into a single cubic centimeter of space, a gram of ice needs about 1.09 cubic centimeters. And the ice truly takes its space. There are some variables in the behavior of the ice, but it can be pretty safely said that in standard natural situation, freezing water applies some 100 Mpa pressure against anyone or anything trying to resist it (that is about 1000 kg/cm2, and about 14500 psi). So, no wonder that boulders crack and water pipes burst during cold spells, and it is not difficult to imagine what happens to the tiny filtering pores and the tubes in a water filter filled, or even partially filled with freezing water. Yes, they crack, and even if the water flows beautifully through the filter after thawing, the filter is no longer filter, but a sieve, no longer capable of filtering harmful bugs.
The internet is full of “expert” advices of how to test whether your water filter is OK after freezing, and each advice is, if possible, even more stupid than the previous one. For example, one rather common “expert” way to test the filter is to try to blow air through the filter against the direction of the water flow, and according to this wonderful advice, if you cannot blow the air through the filter, your filter is in perfect working condition. OMG! The only thing the blow test is good for, is to check whether the filter is still capable of filtering bugs such as ants or cockroaches. Size of bacteria is typically about 0.5-5 micrometers, and there is no way that someone could detect some 10-100 micrometer cracks by blowing air to the filter, where as our friend, the bacteria, which used to stay behind the filter’s 0.1 micrometer pores, now happily swims through the giant cracks.
So, please, if you even suspect that your wet water filter could have been exposed to freezing temperature, get rid of it, buy a new one and be 100% sure that you know where your water filter has been at all times!
In my honest opinion, a warning about the dangers of freezing the water filter should not be hidden somewhere in back pages of companies web sites, but written with flaming letters (with proper explanation of hows and whys) at the home/front pages of all of the water filter manufacturers!
FYI, here is a screenshot from Sawyer.com FAQ: