NSF P231 water filter test

[ncwild]

Has anyone found any water filters that have been through the NSF P231 test? They all make claims about protecting against Crypto.. etc. but fail to back up these claims with third party testing. Even if they claim they used this test, have they done the more thorough and accurate 10 day testing using this protocol?

[slowdive]

This might answer your question. It is interesting, and I have never given it much thought about the testing and standards until I saw your post.

https://www.fs.fed.us/t-d/pubs/pdf/h...07231304hi.pdf

Also here is what MSR has to say on the subject.

https://www.msrgear.com/blog/water-1...esting-claims/

[nsherry61]

As microbiologist, I find this an interesting question.
1st: MSR claims ALL their filters meet those test standards and being as conservative as MSR is, I suspect that claim is true.
2nd: Crypto & Giardia are easy for filters. They're large and easily removed "bugs" even in spore form. For the same reason, Crypto and Giardia are hard for chemicals to kill as they are large and take lots of time and concentration of poison to finish off those buggers.
3rd: We've been "filter sterilizing" stuff in microbial labs for decades with 0.2 micron membrane filters. With the exception of dissolved toxins and viruses (which pass through most backcountry filters with ease), any filter with a pore size of 0.2 micron or less is removing all the bacteria or protozoa (like crypto or giardia). Filters like the BeFree & Sawyer Squeeze are types of membrane filters.
4th: Probably most applicable to the OP or in need of verification tests as noted above are depth filters (as apposed to membrane filters). Depth filters remove particles by forcing them through what is essentially a big sticky maze of material that adheres to and thus removes the particles (bacteria, viruses, dirt, protozoa, etc) from the water. Things are not removed based so much on their size as they are on the relative "stickiness" within the filter material. Depth filters can let larger particles through that aren't "sticky" and remove even the smallest particles (like viruses) if they are appropriately "sticky" and the filter creates a pathway long enough to capture enough of them to meet the specific needs of the filter.

- Depth filters need more testing because they are more of a statistics game. They need to be sticky enough and big enough to remove enough of the particles of concern.
- In contrast, Membrane filters simply remove anything bigger than their pore size, so as long as the filter is not compromised, they work.

The NSF P231 testing is probably critical for depth type filters used in most of the older backpacking pump type filters. The protocol appears rather silly and pointless to me for membrane type filters such as the BeFree or Sawyer Squeeze filters.

[Odd Man Out]

As microbiologist, I find this an interesting question.
...3rd: We've been "filter sterilizing" stuff in microbial labs for decades with 0.2 micron membrane filters. With the exception of dissolved toxins and viruses (which pass through most backcountry filters with ease), any filter with a pore size of 0.2 micron or less is removing all the bacteria or protozoa (like crypto or giardia). Filters like the BeFree & Sawyer Squeeze are types of membrane filters.... Membrane filters simply remove anything bigger than their pore size, so as long as the filter is not compromised, they work.

Which brings up a common question. How do you know if they have not been compromised? I too have filter-sterilized solution with 0.2 micron filters in the lab (biochemistry) and we could always tell if our filter disk was intact because water would go through it easily but air would not. (I don't have a good explanation for why this should be true, but it is). Anyway, I have observed the same with my hollow fiber filters. In squeeze mode, I would leave an air bubble in the bladder with the dirty water. When the water was gone I keep squeezing. If I can't force the air through, I take this as a good sign that my filter and gaskets are intact. For the same reason I find filter rates are inhibited if you get too much air in the filter. I sometimes wonder if this isn't the cause of poor performance reported by some people when using hollow fiber filters.

[HooKooDooKu]

Which brings up a common question. How do you know if they have not been compromised? I too have filter-sterilized solution with 0.2 micron filters in the lab (biochemistry) and we could always tell if our filter disk was intact because water would go through it easily but air would not. (I don't have a good explanation for why this should be true, but it is). Anyway, I have observed the same with my hollow fiber filters. In squeeze mode, I would leave an air bubble in the bladder with the dirty water. When the water was gone I keep squeezing. If I can't force the air through, I take this as a good sign that my filter and gaskets are intact. For the same reason I find filter rates are inhibited if you get too much air in the filter. I sometimes wonder if this isn't the cause of poor performance reported by some people when using hollow fiber filters.

I'll take a stab at why air can't be forced through these filters. Keep in mind this is just an educated guess... but I believe it has to do with air being compressible.
First of all, let's talk about the dimensions involved here.... microns.

A water molecule is about 0.00027 microns in diameter
An oxygen molecule is about 0.000299 microns in diameter
A typical virus is about 0.02 to 0.05 microns in diameter
Most bacteria is about 0.4 microns in diameter

These filters are basically barriers filled with a bunch of holes 0.2 microns in diameter.
So anything larger than that (bacteria) can not go thru, but anything smaller than that (virus or water molecule) can.

But wait... aren't water and air/oxygen molecules about the same size?
But the thing that is different is the space between the molecules.
Water is essentially an incompressible fluid, and it's because all the molecules are basically rubbing elbow to elbow. So when a drop of water is at one of those 0.2 micro holes, there's thousands of water molecules at the opening, and so a little pressure is all that is needed to push those water molecules thru the hole.
But air... it's a gas, and gas is mostly empty space, where the average distance between molecules is about 1,000 times bigger than the molecules themselves. This is why air can relatively easily be compressed... we're simply pushing the molecules closer together.

So when a filter is filled with air, there is maybe one air molecule at each hole in the filter, and so applying pressure only slightly increases the odds that one molecule will fall thru the hole rather than traveling in a different direction.
So if you try to force air thru the filter, it will go, but it will take thousands time longer for the same number of air molecules to get thru as water molecules.

And that is why the "air" test is NOT a valid field test for the filters. If freezing water has created microscopic hole that are say 2 to 100 microns in size, that doesn't change much on how fast air can pass thru the filter, but it definitely allows bacteria thru the filter now.

[nsherry61]

I believe that the reason air doesn't go through wet material as readily as it does dry material is due to the surface tension of water essentially holding the water molecules together and blocking the holes. This doesn't require a sub-micron membrane filter to be observed. You can also take a fabric bag (like an uncoated stuff sack or the leg of a pair of pants) and roll it up forcing air through it. Then get it soaking wet and it becomes really hard to force air through. In contrast, a small slit (or a small tear) in the porous material will readily let air through. To HooKooDooKu's point, as such, letting air through suggests damage, but not letting air through doesn't suggest a lack of damage.

Finally, in the end, the likelihood of drinking contaminated water is relatively small in most backcountry areas of the US. So, while filtering water may reduce the risk of getting infected to almost zero. And, using a damaged filter may be a waste of time. Accidently drinking water a few times through a damaged filter will rarely lead to any harm being done.

So, I wouldn't worry too much about it. I would recommend taking reasonable precautions and replacing your filter after any trips you think may have compromised it. And, it is probably a good idea to replace those hollow fiber filters (like BeFree & Sawyer Squeeze) annually anyway.