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There are a number of ways to purify water, off the top of my head we have filters, iodine, and boiling. Clearly each has some advantage/disadvantage (iodine tastes terrible, boiling takes time). Which of these is the safest? Is there another method that's safer?

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possible duplicate: outdoors.stackexchange.com/questions/29/… –  Ben Crowell Apr 30 '13 at 22:34
    
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Does it really need to be the safest way? Make it "safe enough," striking a balance between the risk of getting sick and purifying it to ridiculousness. Don't get carried away by thinking it has to be perfectly pure. It never will be, and that's okay, except for people with unusual medical conditions. –  Don Branson Aug 6 '13 at 22:40

8 Answers 8

up vote 25 down vote accepted

When you're asking for the safest way to purify water, you're asking for the method that removes the most harmful stuff from the water, like bacteria, viruses and larger impurities like mud or sand.

No one method is really perfect at removing everything, so I usually use a two-stage approach:

  • Filter: If the water source is cloudy, your first step should be to remove the larger impurities. You can buy expensive pump filters for this, or you can just pour the water through a sock, bandanna, or any other tightly-woven fabric to remove the larger stuff.

    Note that you can probably skip this step if the water you're collecting is clear and fast-moving.

  • Boil: Heat sterilization is really the only sure-fire way to kill bacteria, viruses and other undesirable microbial life in your water. It's a commonly used technique to clean medical instruments since it's so effective.

    It does require a heat source, and it'll take some time to get a full rolling boil. But if you're at all worried about the quality of your water, it's your safest bet.

Edit: As commenters noted, if you're in an area with potential chemical pollutants or metals in the water, you should filter even if the water appears clear. But if you're really in a situation where mother nature's water has been tainted by man-made waste, you should probably pack in all of your own water anyways.

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I wouldn't rely on whether the water is clear and flowing. A carcass up river could be rotting away without discolouring the water. I'd always boil water I collect (if possible). –  Bernhard Hofmann Jan 24 '12 at 20:49
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Also - seemngly clear water can still have a high metal content - drinking arsenic and lead laden water is not particularly advisable. –  HorusKol Jan 24 '12 at 22:46
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NOTE: An alternative to boiling which works very well is Iodine treatment and bleach treatment. Both work very well for field sterilization on the go, or where you cannot, or do not want to expend the fuel to boil the water. Keep in mind that both require the water to be clean (ie. No particulates) and not be diluted with sugar (deactivate the disinfecting agents). If you use this method, also make sure that the disinfectant gets into the threads of the container, as pathogens can hide there. –  Dangeranger Jan 26 '12 at 18:36
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"It's a commonly used technique to clean medical instruments" Just wanted to mention that this method also requires pressure and steam. Don't expect medical sterilization just by boiling. Water can't get past 100 Celsius at standard pressure. en.wikipedia.org/wiki/Autoclave –  Tanner Aug 27 '12 at 20:31
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It's not true that heat sterilization is the "only sure-fire way" to kill all disease-causing microorganisms. UV treatment (steripen) also kills all types of microorganisms (bacteria, viruses, and protozoan cysts). Unlike boiling, UV treatment is practical for water collected during the day while hiking. –  Ben Crowell Apr 30 '13 at 22:32

I have no science to back me up, but the SAFEST way would be boiling. Since water all boils at the same point; you know as soon as it starts rolling that the nasties are all being killed. Hold at a rolling boil for 2 minutes (as I was taught), and it is safe to drink.

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I recently read it should boil for at least 10 minutes to be sure all the "nasties" are dead. –  Noam Gal Jan 24 '12 at 20:25
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Wow. I wasn't sure if it should be 1 minute or 2 and had to run, so I picked the longer. If something can survive 2 minutes at a boil, I guess I'm ok with it killing me, it's earned the right. –  Pulsehead Jan 24 '12 at 21:08
    
mm - You were probably right, I just looked for a reference for that 10 minutes claim, and found this - jolly-green-giant.blogspot.com/2009/03/… - it says 1 minute is enough in most places, up to 3 over 6500 feet altitude. Sorry for the wrong info. –  Noam Gal Jan 24 '12 at 21:13
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Boiling wouldn't remove metals –  HorusKol Jan 24 '12 at 22:55
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Bringing it to a boil is sufficient at all altitudes. It is not necessary to boil for any amount of time. See my comment on berry120's answer and the longer article linked to from my own answer. –  Ben Crowell Apr 30 '13 at 22:22

Boiling is the best thing to do, as stated already - I was always taught to boil for 2 minutes and then it's safe. (Far from an accepted time though. For debate on how long you should boil, see here: How long does water need to be boiled for to kill all bacteria / viruses?)

As pointed out below though it may be wise to use a filter to get rid of any toxic metals prior to boiling. I've personally not had an issue with them but it depends heavily on the area, and better safe than sorry!

In situations where you can't boil it then chlorine tablets can be an alternative; while I don't have a source to hand I'm sure that I read they're generally recommended above iodine tablets because the latter if used extensively over time can damage the thyroid.

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Boiling wouldn't remove metals –  HorusKol Jan 24 '12 at 22:55
    
@HorusKol Good point - an oversight on my behalf. I've updated the answer accordingly. –  berry120 Jan 24 '12 at 23:01
    
It's not necessary to boil for two minutes. Even raw milk (which can be swarming with microbes compared to backcountry water) is normally pasteurized for only 15 seconds at temperatures of no more than 72 C (161 F), based on standards designed to kill the most heat-resistant disease-causing bacteria. See National Advisory Committee on Microbiological Criteria for Foods: Requisite Scientific Parameters for Establishing the Equivalence of Alternative Methods of Pasteurization, USDA , 2004 –  Ben Crowell Apr 30 '13 at 21:40
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@BenCrowell A hotly debated point - I've seen people argue for it both ways. Personally I prefer to boil for a bit longer because I've never had any trouble doing that, and don't really see the need to cut down with most of the hiking I do. I agree it may be safe to boil for less time though. –  berry120 Apr 30 '13 at 22:43
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@BenCrowell Sure, but almost everyone who has an opinion on the matter will throw some form of "evidence" to back themselves up. I'd call your source into question on a number of points, for instance is raw milk often swarming with the same type of microbes that can be in backcountry water, or is it a more restricted subset that can't survive over 72C? Official guidelines also disagree with your assessment (water.epa.gov/drink/emerprep/emergencydisinfection.cfm#method), they recommend boiling for one minute. Of course, some microbes can even survive boiling indefinitely... –  berry120 Apr 30 '13 at 23:22

I would get a ceramic filter that is rated to remove metals like arsenic (this is a particular issue in the UK where most surface water in the wild country can harbour arsenic - but I'd rather assume all ground water to be 'tainted' than drink it and become poisoned). I've used some hand-pumped filters which can draw water from a lake or river, and filter at the rate of about a litre a minute.

Then, drop an chlorine tab into the water - this usually requires 30 minutes to completely kill off biologic contamination. You can then get neutralisers to remove the taste of the chlorine from the water.

Boiling shouldn't be necessary after this treatment - unless you want a cuppa.

One thing - do not rely on the apparent clarity of the water - bacteria/viruses and dissolved metals will almost always be invisible to the naked eye.

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If boiling isn't an option, aqua mira is usually a good choice. It's a doddle to use, and is very effective. It doesn't make the water taste like soap either, like some of the tablets do.

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Halogens such as aqua mira aren't very effective against protozoan cysts such as Giardia and Cryptosporidium. The real question is what disease-causing organisms you want to protect against. In backcountry areas in the US that have been well studied, the answer is that there simply aren't any organisms, of any type whatsoever, in concentrations capable of causing disease. (For scientific references, see my answer and the longer article linked to from it.) –  Ben Crowell Apr 30 '13 at 22:29
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@BenCrowell what about those backcountry areas that haven't been well studied? Do you have a map of the well studied areas? –  HorusKol Jun 6 '13 at 14:14
    
@HorusKol: My own answer gives a link to a longer article I wrote. The article gives all the scientific references I was able to find. A lot of the water-analysis results refer to the Sierra, and most of the epidemiology was done for the US. It would be interesting, for example, to see specific studies for the Alps, which have more permanent human settlement and more cattle farming. –  Ben Crowell Aug 20 '13 at 17:48
    
true. i'm in NZ and i suspect a lot of the back country water away from animal farming is drinkable straight out the river, but i still use mira as a backup –  Sirex Aug 20 '13 at 19:45

There is typically no need to purify water collected from natural sources in the wilderness. For example, in a survey of 69 sites in the Sierra, every site had concentrations of Giardia cysts much too low to make anyone sick.[Rockwell 2002] The perception that backcountry water is unsafe to drink without treatment is folk wisdom that is controverted by the available scientific evidence, at least in the backcountry areas in the US such as the Sierra that have been extensively studied. Of course none of this applies outside of the backcountry, e.g., you certainly don't want to drink water in Arkansas that comes from runoff from a poultry farm.

When people do actually contract backpacker's diarrhea from exposure during a hiking trip, by far the most common reason is hand-to-mouth contamination.[Welch 1995] The most effective disease prevention measures are to wash your hands after pooping, refrain from sharing pots and pans, and use freezer-bag cooking so that food never goes in your pots.

If you do want to purify your water, there's a lot to be said for UV (steripen), which is one of the few methods that will kill all three types of disease-causing organisms: viruses, bacteria, and protozoan cysts. Unlike boiling and halogens, it gives you water that you can drink immediately while hiking during the day.

For more detail, I have a long article on this topic here, with lots of citations to reliable scientific and medical sources: http://lightandmatter.com/article/hiking_water.html . I realize that my answer goes against the conventional wisdom, and that's why I've provided references to scientific papers in this answer, as well as much more extensive references in the longer article.

Rockwell 2002 - Robert L. Rockwell, Sierra Nature Notes, Volume 2, January 2002, http://web.archive.org/web/20051026030831/www.yosemite.org/naturenotes/Giardia.htm

Welch 1995 - Thomas R. Welch and Timothy P. Welch, "Giardiasis as a threat to backpackers in the United States: a survey of state health departments," Wilderness and Environmental Medicine, 6 (1995) 162, http://www.wemjournal.org/article/S1080-6032%2895%2971046-8/abstract

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Perhaps what you say is true for the Sierras, but is certainly NOT true of other areas. Various places in the White Mountain National Forest of NH were tested, and enough giardia found that there is a overall recommendation not to drink the water directly. If giardia is the only issue, then even filters work since these buggers are rather large. I am surprised you were the only one to mention germicidal UV. It seems like a really good idea to me. 90 seconds for a liter and you're ready to drink. By the way, I wrote the firmware for the latest SteriPen, the Ultra. –  Olin Lathrop May 1 '13 at 21:50
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I looked around a bit but couldn't find a definative source. Back in the early 1980s everyone pretty much used backcountry water in the WMNF without thinking much about it. By the late '80s that was not done anymore. Giardia had been found in several fairly deep backcountry sources, so one had to assume it could be in any water, even if it actually wasn't in most. The one I remember specifically was a spring on Signal Ridge, on the south shoulder of Mt Carrigain, but there were others. Since giardia can pop up quickly, it's smart to assume any water could be infested. –  Olin Lathrop May 4 '13 at 22:00
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@OlinLathrop: The time period you're describing is a period during which there was a public-health scare about giardia in backcountry water, which was never based on solid scientific evidence. It later became clear that the whole thing was bogus. Basically it's all about potty hygiene. People get giardia from their hiking partners, not from backcountry water. –  Ben Crowell May 5 '13 at 14:36
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I wish I could give you more than one upvote, Ben. I'm not sure I would personally skip treating water when I'm not sure of the wilderness upstream from where I'm sourcing, but otherwise... There's nothing more satisfying than dipping water out of an alpine stream and drinking deep :) –  Ryley Jun 5 '13 at 19:32
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@HorusKol: I see. I hadn't realized that arsenic could be removed by ceramic filters. The big issue IMO is simply that people are so hung up on treating their water, which is typically pointless, that they completely fail to pay attention to the real issue, which is hand-to-mouth contamination. –  Ben Crowell Aug 21 '13 at 5:05

If safety is your primary concern, then the only two methods can guarantee safety from major contaminants (microbial or otherwise) these are Distillation and Reverse Osmosis

Distillation

Boiling alone will leave most contaminants in the water, such as heavy metals, hydrocarbons, etc... multi-stage distillation will both kill pathogens and leave most contaminants in the residue.

Distillation can be carried out using a solar still if a fire and appropriate equipment isn't available. One stage distillation is usually enough unless the water source is heavily contaminated.

Reverse Osmosis

Is usually combined with another method of filtration/purification, but has very high water purity if used properly, although the energy required and the efficiency when using low pressures (like a hand pump) make this a poor choice when hiking.

Ensuring safety

In both cases, the water really should be filtered until clear beforehand for the best results and ease of use, although this is not strictly necessary.

For the best results, purifications/filtration methods should be used together, for example, activated carbon filtration of distilled water (to remove volatile organic copounds) or chlorination of water recovered by reverse osmosis.

Distillation and reverse osmosis are also the only two methods mentioned so far able to desalinate seawater and recycle urine.

For example, The ISS uses a low pressure distilation system to recover drinking water.

You will need to re-mineralise the water in both cases, both for taste and electrolytes.

Both methods require a lot of energy, but will guarantee exceptionally safe water.

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Fecal contamination should not really be an issue unless someone is already sick. Recent research has suggested that even eating ones own poop should not make you sick. Fecal transplants is a new way of treating some issues that fomr from a lack of flora in your gut. http://gawker.com/5985723/can-you-eat-your-own-poop. I for one would use something to treat the water. My cousin hikesd the Sierra MTS a few years ago and contracted giardia so just from that knowledge I would def treat the water with something.

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Interesting link, however they don't mention the self-infection from various parasites such as ascaris. –  РСТȢѸФХѾЦЧШЩЪЫЬѢѤЮѦѪѨѬѠѺѮѰѲѴ Dec 1 '13 at 16:49
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It's very unlikely that your cousin got giardia from backcountry water in the Sierra. See web.archive.org/web/20051026030831/www.yosemite.org/naturenotes/… . It's much more likely that she got giardia from hand-to mouth transmission. People are tolerant of their own gut flora, but not necessarily other people's. wemjournal.org/article/S1080-6032%2895%2971046-8/abstract Fecal transplants are a desperate measure for people who are really, really sick. –  Ben Crowell Dec 3 '13 at 0:17

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