How to purify water in the wild


THERE ARE FEW places in North America where you can safely drink water from a lake or stream. So unless you camp where safe drinking water is readily available, you’d best bring your own. But at about 8 pounds per gallon, water can be a real drag if you have to carry it far. The alternative is to go light and purify what you need.

Examine the methods described here and determine the best option for the type of trip, the geographic area you will be visiting and the number of people in your group.

The good news is that gastrointestinal pathogens are very sensitive to hot water. Boiling water from an in-the-wild water source will kill almost everything, even parasites like giardia and cryptosporidium that can survive long exposure to iodine and chlorine. (Technically, a water temperature of 150 F will kill all GI pathogens, but it’s easier and simpler to just bring it to a boil.)

Chemical Treatment
Chlorine bleach or iodine will kill most waterborne pathogens, but if you get the dosage wrong, they can be dangerous. Water-treatment tablets are safer and easier to use. Chlorine dioxide and sodium dichloroisocyanurate tablets are more effective against giardia than chlorine or iodine tabs. However, water-treatment tablets will not reliably kill cryptosporidium, which, fortunately, is uncommon in surface waters.

Some negatives about using water-treatment tablets: They require at least 30 minutes to work if the water temperature is above 68 degrees. Much more time is needed for cold or cloudy water. Heat speeds chemical reactions, so you’ll shorten the purification time if you set your water in the sun while the tablets work. Tablets lose potency over time; fresh ones work faster. And to reduce the bitter taste after treatment, aerate your water by pouring it back and forth between two canteens.

DrinkingWaterA Filter or Purifier?
Filters strain out microorganisms, but they don’t kill them. If hiking in the U.S., filtration is typically considered to be sufficient.

A filter should remove the smallest infectious agents. For instance, a filter with a 1-micron absolute pore size will trap giardia and cryptosporidium. Viruses fly through the smallest pores, but they can be killed with water-treatment tablets after filtering. Fortunately, harmful viruses are extremely rare in surface waters.

Because the “bugs” become trapped inside the filter, this can eventually become clogged and must be cleaned or replaced.

Filters may have pumps or be gravity-fed. Gravity-fed units are bulky, but they’re as effective as pumps and are easier to use — particularly with groups of Scouts. They’re great if you need to filter a lot of water fast and don’t want to labor with a pump.

Purifiers, on the other hand, typically use microfiltration and either chemical treatment or ultraviolet light to kill microorganisms, including viruses. This higher level of water purification is necessary when traveling outside of the U.S. in regions where viral contaminants are a risk.

Ultraviolet Purifiers
People who live near bodies of freshwater have long relied on ultraviolet light to kill microorganisms. The popular SteriPen was among the first to harness this technology for use in the wilderness. Just insert the UV bulb into a water bottle and activate the light. When the light turns off (after about a minute) the water is safe to drink. UV purifiers will destroy giardia, cryptosporidium and viruses. But they won’t remove sediment or work in cloudy water. And they can purify only small amounts (usually one liter or less) of water at a time.

Before You Go
Don’t buy more protection than you need. Talk to park rangers or other hikers or campers who are experienced in the area where you’re traveling. Once you find out more about the types of water sources available, you’ll be able to choose the best system for your needs.

What’s best for you depends on the microbes you expect to encounter, how light you want to travel and the volume of water you will treat.

Tips for Treating H2O

  • Use a cotton cloth or coffee filter to strain out sediment before you filter/purify.
  • Greenish water contains algae that harbor microorganisms. Algae can clog water filters and reduce the effectiveness of chemicals.
  • Brownish water contains organic tree tannins that are typically safe to drink.
  • Beavers are the favored host of giardia. Don’t take water from beaver streams or near beaver lodges.
  • Don’t draw water from shallows or near campsites.
  • Avoid clear water that’s tumbling over sunny rocks. While UV light does kill microbes, moving water mixes them and increases the chance they’ll get into your water sample.
  • Take water from near the surface of a calm, sunlit pool. Harmful organisms tend to lurk just out of reach of sunlight.



  1. My understanding is Ultraviolet Purifiers do not use ultraviolet light to “kill” microorganisms, instead the UV light actually sterilizes them so they cannot reproduce in your body. I’ve used this technology backpacking in
    New Mexico.

  2. chemical tabs should only be for emergencies, not part of regular treatment,
    the chemicals wreak havoc on the kidneys, if used on regular basis
    I oppose recommending tabs for youth outside of emergency kits

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