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The Limitations of Water Filters and Water Purification

Most of the water filters carried by backpackers and long distance hikers cannot remove organisms smaller than 3 microns in size. While protozoa such as Giardia (shown below) and Cryptosporidium are about 5 microns in size, water filters miss bacteria such as Cholera, E. Coli, and Salmonella (0.2 – 0.5 microns) and viruses such as Hepatitis A, rotovirus and Norwalk virus (.004 microns).

The safe solution is to always use a water purifier such as Chlorine Dioxide after you filter your water. However, like filters these chemical additives have limitations that you need to know about in order to prevent getting sick. All chemical additives have difficulty purifying water that contains sediment because the particles suspended in the water interfere with the chemical reaction. Therefore, filtering to remove the sediment or letting the water sit overnight so the particles can settle before purification is recommended. In addition, cold temperature slows the purification process if your water is below 60 degrees F. As a general rule of thumb, you should double the recommended contact time for every 20 degrees F below 60 degrees F.

P.S. I went whitewater kayaking in October, 2007 with an old friend and kayak mentor who is in charge of the chemical engineering unit responsible for certifying water purification technologies for the entire U.S. Army. He was adamant that the combination of water filtration and purification are needed for backcountry travel in the eastern US, for the reasons I cite above. He’s also a sectionhiker.


  1. What about boiling the water instead of a two part process like filtering and chemical? My biggest fear out on the trail is water. I think that is why I day hike more then backpack.

  2. I felt the same way before I made the switch to a water filter. Once you make the switch, an entire new world will open up for you. I recommend that you get the best filter on the market – the First Need water purifier to start. It's about $100 at REI and removes all giardia, crypto, viruses, and chemicals (fertilizers, etc), and organics (tannins) down to 1 micron from your water without the need for an extra chemical purification step. It's a fantastic product. I've tried others but they all fall short. If you want to eliminate any possibility of bacterial infection, you can drop some chlorine dioxide tablets into your water (3 for 3 Liters) for 15 minutes, but I don't normally do this myself on the AT or in Vermont.

    The problem with boiling water is fuel and time. I drink 5-7 liters of water a day in summer. To boil this much water, I'd need to carry several more pounds of fuel per day and spend several hours boiling that much water.

    Trust me on this – get a first need and bring it on some day hikes. Buy a second bladder and fill it before you run out of water in your first bladder. Chances are that the water the you filter will taste better than what you carry in. I have never gotten sick using a water filter, and I doubt you will either.

  3. do you have a link to that filter your talking about? What about sterilizers?

  4. First Need Filter from General Ecology

  5. I've (so far) not seen you mention UV systems such as "steripen"… Is this because you find they don't work as well as a pump? I've used a steripen on my last few outings and found it to be a spectacular low-weight option for making water safe to drink. They also sell small sediment filters as a pre-filter to get crap out… Am I missing something and as a result, not totally "covered"? There is surprisingly little written on the subject, except of course on sites selling the product…

  6. Ryan Jordan also likes the Steripen. It wouldn't be my first pick because it requires a battery. It also won't filter out harmful chemicals, like fertilizer or other industrial waste, which admittedly you may not encounter.

  7. You have an excellent point about the wayward chemicals… I had never thought of that… On the battery, you shouldn't let that stop you from experimenting if you ever have the inclination… It should last through several months of hiking on one battery (lithium)… There's also a new model with a solar charge attachment.

    Great, now I'm going to be paranoid about chems in every stream! ;-)

  8. You make a damned excellent point there about wayward chemicals… Stupid humans. Now I'm going to be paranoid about every stream…

    By the way, steripen makes a 4oz model now that also accepts a solar charge attachment if batteries are a worry.

  9. I've read that viruses aren't really a big deal in usa/european water sources. My filter goes to .2microns so I've never worried, as long as the water source is running.

  10. The Sawyer 3-way filter is another great option. No, it won’t get all viruses but it does guarantee 0.1 micron absolute and a 1,000,000 gallon lifespan. At $60 it goes great with any dromedary bag.

    • Christopher Moore

      And they have a new realy cool (mini) size option as well I’m sure it cuts down on life expectancy. But bang for the buck and weight wise it was my first choice, as I like to move fast and packing light helps.

  11. Great read. Nice to be reminded about filter limitation. But I may not need to worry because my filter works excellently.

  12. What’s wrong with you idiots, didn’t you read the article you are commenting on. He states ” water filters miss bacteria such as Cholera, E. Coli, and Salmonella (0.2 – 0.5 microns) and viruses such as Hepatitis A, rotovirus and Norwalk virus (.004 microns).” At least some organisms are likely to get thru most filters, that’s why he recommends filtration plus chemical treatment.

  13. Nothing’s perfect, including city water. That’s why God gave us immune systems. Awesome electron micrograph of Giardia lamblia, by the way.

  14. I prefer using a two step process when filtering water that I believe is contaminated. I filter it with the Survivor Filter Pro (0.01 microns) first and then use the Steripen Ultra afterwards.

    • After a lot of research on CDC’s website I concur with the writer of this article: filter, then sterilize. The problem with viruses is that they are very small. Some filtration systems such as the Survivor Filter Pro claim to filter down to .05 microns. However, some viruses are even smaller than that. Plus, who knows what crazy, elusive virus pops up next? How small will it be?

      TRUE PURIFICATION: This goes beyond filtering. The CDC and WHO have already done the research and have provided recommendations such as boiling water, reverse osmosis, filtering & chemically treating, etc. You may be reasonably safe by filtering only (with a good filter system) but you are not going to eliminate all viruses. They recommend using household bleach, chlorine dioxide and other chemicals to treat water as well. There are some issues with UV, iodine and several other methods listed there too.

      BTW: you may reevaluate buying certain types of bottled water after reading the CDC report. They listed only 4 acceptable processes and about a dozen unacceptable ones. No more “spring” water for me :)

  15. Just out of curiosity, what’s your source for .004 microns? Most of what I’m seeing from the CDC suggests .04 microns for the smallest waterborne viruses.

    I completely agree with everything else you’re saying though–physical filters < chemical treatment if you're concerned about viruses.

  16. Very informative debate. I work with camp setup in disaster hit zones. What we use is lifesaver systems, bluebox and hydroways waters. Lifesaver doesn’t see much use as the flow rate is too low but the concept is very neat and smart. Bluebox is a good system but very sensitive and requires monitoring amd weighs too much (450lbs). Lately we got this hydroways waters system. It combines the manual spec of the good old lifesaver jerry can with high flow of bluebox. I think they use another technology than the classic membrane UF/RO. They have excellent result in cyst, parasite, virus and bacteria reduction with no aftertreatment at all. But them again how good is good enough? IIRC some engineers once talked about that some types of bacteria can even survive boiling. And that they need to be boiled twice before to kill them. The boiling is energy inefficient and the twice thing going on just pushed the inefficient to another level. Stay safe and never drink yellow water Lol.

  17. Thanks again for definitive info.
    Prefiltering with a 75micron cold brew bag (or similar) helps get much if not most of that large (microscopically speaking) sediment and flotsam out. These are easily attainable and throwing it over a collection container add about 10 seconds to the process. Then filter and add Micropur tablets (or similar) as usual.
    Easy peezy.
    Then f

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