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I have been using water filter from very known brand for the last year. I didn't use it a lot perhaps for 50 - 60 liter of water. I changed 1 pouch. Water was clean most of time and after each hike I used cleaning syringe. I hiked near freezing point, but filter was in backpack and tent during night and I don't think it froze. I can't blow air through it.

Each time I use it I get slight diarrhea or stomach cramps. I tried new pouches, ran chemically treated water through filter to clean or kill any bacteria in it. It doesn't help a lot I am afraid to use it, seems better without or I use chemical treatment of water.

What do you think is the issue?

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    What kind of water are you filtering? A water filter won't help against toxins.
    – PMF
    Jul 21, 2023 at 18:22
  • It is good water. From alpine, hilly streams, where there are no roads or buildings up. Cities are far away. that is why I don't understand it. Should be perfect good after ran through filter. Even without it should be good.
    – Alex J.
    Jul 21, 2023 at 18:29
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    Do others with you also get stomach problems from the filtered water? Do you (and/or others) get stomach problems from filtering bottled drinking water? If you are getting sick from filtering bottled drinking water then it's clearly the filter. If it is just you, and not others, perhaps you are allergic to something in the filter.
    – noah
    Jul 21, 2023 at 19:15
  • Only a reverse osmosis water filtration system will effectively remove harmful bacteria. See Australia depatment of health. Jul 21, 2023 at 19:47
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    @WeatherVane the question is about travel water filters, not domestic filters designed to filter out sediments. Travel filters can and do remove bacteria - most of them by using a 0.2 or 0.45 micrometre filter, which are the same diameter filters used in biology labs to sterilize solutions that can't be heated. Almost all pathogenic bacteria are in the 1-10 micrometre range, with the exceptions being things like mycoplasma, which are only commonly found on people and transmitted in warm, moist environments such as heated pools.
    – bob1
    Jul 22, 2023 at 1:38

2 Answers 2

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Based on what you said, I'm pretty sure you've ruined the water filter. If you're sick after using the filter, then it's not doing its job anymore. Now, let's look at some facts:

I hiked near freezing point, but filter was in backpack and tent during night and I don't think it froze.>

"I don't think it froze". It certainly did. Water conducts cold 25x faster than air. So the filter may have frozen even though you "think" it didn't. Freezing water expands, so it destroys the hollow fibers inside it. You must keep the filter inside your jacket at all times when it gets cold. The best is to use a ziplock bag so you can have it inside your jacket during the day, and inside your sleeping bag at times. "Inside the tent and backpack" doesn't cut it. The filter you're using is a Sawyer (The syringe). It's a very good brand, but like everything else it has to be taken care of. Those filters are made of hollow fibers and if frozen, they will not do the job anymore.

Also, you used the syringe, but did you use CLEAN water and did a REVERSE flush? (asking just to make sure". And if you used clean water, did you use tap water? If so, it's most likely treated water with all kinds of stuff that will clog any filter over time. The best is to use water you just filtered or bottled water (best is DE-mineralized water).

I'd suggest you buy a new Sawyer ASAP and make sure to keep it away from the cold next time.

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"@noah If you are getting sick from filtering bottled drinking water then it's clearly the filter"

I took that advice and it is the filter(0.1 micron). It is not the pouch, I used bottle. After I drank filtered bottled water I got slight diarrhea even if filter was cleaned according to manual(push water with syringe in opposite direction).

It is terrible, because I got problems at multiple hikes, filter did the opposite. At least now I know for sure what is the problem.

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    If the filter is contaminated you should be able to clean it with bleach (don't use a scented one, preferably pure bleach, not NaOH as well) or a water sterilizing tablet (once tablet is just dissolved, flush the water into the filter and allow to sit as long as recommended by the tablet instructions). Pushing water in reverse might dislodge some bacteria on the filter, but biofilm forming ones won't be removed easily, so you need a sterilizing step. Reverse also won't work for contamination on the clean side...
    – bob1
    Jul 25, 2023 at 20:19
  • I already soaked the filter in chlorinated water(NaDCC tablets) for a day some time ago and flushed it with that water, perhaps it helped but I still got diarrhea. I will ask for warranty to cover that. I can't trust that filter again.
    – Alex J.
    Jul 25, 2023 at 20:41
  • If the NaDCC tablet was added to the water at the same time as the filter then that should have worked, however if you dissolved the tablet, let the water sit for the recommended time and then soaked the filter, it might not have worked as the effective concentration of chloride falls quite rapidly post-treatment.
    – bob1
    Jul 26, 2023 at 1:28
  • I don't know. They won't recognize warranty. It is hard to trust a filter, which made me ill a few times and even had to abort hike, because fever. After cleaning the story repeats itself. Now I will clean it again this time with Sodium hypochlorite and with strong concentrations for extended period few days with flushing. If I use the filter again and still have any problems after a few usages then I will do legal action. The point of filter is that if I use it a few times, I don't need to dezinfect it after every usage. The filter should keep the bad out.
    – Alex J.
    Jul 26, 2023 at 15:21

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