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What methods can I use to purify water? And what's the difference between those several methods (safety, duration needed, ...)?

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possible duplicate: outdoors.stackexchange.com/questions/7/… –  Ben Crowell Apr 30 '13 at 22:34
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2 Answers

up vote 23 down vote accepted

Boiling- 185° water will become sterile in minutes. Bringing the water up to boiling point will typically sterilize it. At high altitude you may need to boil for a minute.

Chemical- Iodine, chlorine, and other chemicals can sterilize water. Follow directions on products specifically designed for this purpose. Typically you'll need to mix it and then let it sit for some number of minutes. Although chemicals are required, this method is often the most convenient as carrying a few pills is simple. If chlorine scares you, consider that you have been drinking it your whole life in tap water. Some battery operated systems work with salt and electrolysis, producing chlorine. This is convenient as well due to the availability of salt.

Distillation- This method works by distilling the water, usually in a solar still, to remove almost all impurities.

UV Light- Battery powered SteriPens produce UV light and kill everything including viruses. This is similar to some of the best pool water treatment systems, is very convenient and may work in less than a minute, although pens are costly and you'll need fresh batteries.

Filtration- As simple and quick as pumping the water through a filter. Each filter is different, but typically they remove 99%+ of bacteria, but not viruses unless a chemical is contained in the filter. You'll need to replace parts periodically and keep the system clean, but overall its pretty convenient.

Sunlight Pasteurization- Depending on the sunlight strength and outside temperature, you can leave a plastic bottle of water in sunlight for 6 hours+.  The heat and UV light will make water safe. The downside is that you need to look up or guess how long based on the conditions.

Backwoods tricks- Survival techniques include many ways of obtaining water that is probably pure. They include distillation techniques (covered pits with pots, etc), and careful rainwater and dew collection or extraction and filtration by plants or clothing. None of these methods are convenient for daily use, and some are risky.

None of these methods necessarily protect against chemical contaminants, although some water filters are specifically designed to "filter" chlorine or other common chemicals. I have heard skepticism regarding some of those claims.

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A few years ago, we purchased a couple of hand-pumped ceramic-based filters which were rated to remove metals like arsenic and so on - iodine/chlorine was still needed for sterilisation though. –  HorusKol Jan 24 '12 at 22:45
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Added distillation –  Russell Steen Oct 1 '12 at 14:30
    
+1, but the information on boiling is not correct. 161 F (72 C) is sufficient to kill all bacteria within 15 seconds. See National Advisory Committee on Microbiological Criteria for Foods: Requisite Scientific Parameters for Establishing the Equivalence of Alternative Methods of Pasteurization, USDA , 2004. Folklore about boiling has simply become more and more exaggerated over time, like the folklore about how many words Eskimos have for snow. –  Ben Crowell Apr 30 '13 at 22:38
    
Great resource. We were always told to boil for 10 minutes when I was a young backpacker, and that seemed ridiculous. –  Peter DeWeese May 1 '13 at 13:36
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THE official information can be found on the CDC webpage.

The CDC defines the problem in terms of what you're trying to kill: Protozoa, Bacteria, or Viruses. They address each of those in terms of water treatment methods: Boiling, Filtering, Chemical Disinfection, (or now-a-days UV Treatment).

Please read what they have to say before you head into the wilderness. It is short and authoritative.

What does each method kill?

  1. Boiling: kills everything. Period.
  2. A good filter (here and here): will clean your water of everything except (usually) viruses.
  3. Iodine and Chlorine Dioxide: mostly great, but don't work well against Giardia.
  4. UV Treatment: kills everything.

Here is my personal experience with each method:

  1. Boiling: This is great, but it takes a long time and so almost no one does this exclusively. If you wanted to do this exclusively, you would want to get a Jetboil. (These are also a great way to cook while backpacking.)
  2. Filtering: This is a great option. But I do my backpacking with a doctor and she thinks you should get a pump because the other filter styles are subject to operator errors.
  3. Iodine and Chlorine Dioxide: This is the cheapest and lightest solution. But (as the CDC page points out) most people don't know this is not highly effective against Giardia. And Giardia is usually your biggest concern.
  4. UV Treatment: Yeah, SteriPens are great. But I warn you, they are just another electronic toy. I have seen them fail because: the batteries ran out, they were dropped and broke, they were dropped in water and died, and the batteries ran out. Would you trust your life to your cell phone?
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