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


4 Answers 4


Boiling- 185°F (85°C) 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.

  • 3
    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
    Commented Jan 24, 2012 at 22:45
  • 1
    +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.
    – user2169
    Commented Apr 30, 2013 at 22:38
  • Great resource. We were always told to boil for 10 minutes when I was a young backpacker, and that seemed ridiculous. Commented May 1, 2013 at 13:36
  • 90 seconds is the generally agreed upon number. Commented Jan 2, 2015 at 19:10
  • By the way, I wrote the firmware for the SteriPen Ultra. It has a display that counts down the seconds so you know how long to keep it sterilizing for. If I remember right, it takes 90 seconds to serilize 1 liter of water. Commented Jan 2, 2015 at 23:38

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 nearly everything.
  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. I do my backpacking with a doctor and she thinks pump filters are by far the best 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 mysteriously just stopped working, and the batteries ran out. Would you trust your life to your cell phone?
  • "kills everything. Period." Just for paranoids - Siberian plague can endure up to 1 hour of boiling.
    – Les
    Commented Nov 11, 2016 at 5:26
  • @Les Okay, I just checked. You're right. There are a few thermophilic things that will survive boiling. I want to add something quick to the answer here to make this accurate, while still being quick and helpful. I will do a little more research and try to improve the answer. Thanks! Commented Nov 14, 2016 at 15:52

Be aware of the difference between removal of harmful pathogens and removal of chemical contamination.

Harmful pathogens include amoeba, bacteria, viruses, and worms, and these are effectively destroyed by the following methods:

  • boiling water for 90 seconds kills all the above
  • iodine and chlorine kills bacteria, worms and amoeba but not viruses
  • filters remove everything except viruses

So you can use any of the above as a "blanket" one-size-fits-all method for killing pathogens.

The methods used for killing pathogens do not remove chemical contamination. The removal of chemical contamination is a different matter. First you must identify the contaminant. Depending on what it is, there may or may not be a method for leaching it out, it varies from one contaminant to another.

  • "boiling water for 90 seconds." Just for paranoids - Siberian plague (not only) can endure up to 1 hour of boiling.
    – Les
    Commented Nov 11, 2016 at 5:27

The cheap & tight way? 1 box of black trash bags. 24 rubber bands. 12 1/4 inch plastic tubes. 4 foot long. catch container's. fill bags with water. Pull tight at top, put in tubes. secure with rubber bands. run tubes down hill. catch evaporated water in pans. This is drinkable water. I keep a kit like this in the nose of the boat in the S Pacific. + have trash bags when I come into shore. You never know when out in the big lonely. Set bags in sun. Keep tubes in shade & covered with a damp rag or such if have. This the bags of water set in the sun will give you hot wash water in the bush when you come in also. Reuse bags for trash when breaking camp. I have only expermented with this in the boat. It worked. Have used to heat water in the bush to wash with.

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