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Backcountry Water Treatment Part 4: Methods for Making Water Safe

by Bill Straka
December 8, 2008

All drinking and cooking water should be treated, regardless of how clear and clean it looks.
(photo: A. MacLeay)

Short of transporting all your water from civilization and municipal water supplies, there are three general methods for treating your water for drinking and cooking, and two more recent additions:




Although sterilization by heating is referred to as “boiling,” it is not necessary to actually bring water to a full rolling boil or to boil for 5 to 10 minutes, as is often stated. Raising the temperature of water to 155°F (70°C) is sufficient to kill most biota encountered by backpackers. This is also the temperature required for pasteurizing milk.

The temperature at which water boils decreases with altitude. However, the boiling temperature is higher than 155°F (70°C) at all altitudes that you would camp at (even on Mt. Everest with a boiling point of approximately 167°F / 75°C). Since few people carry a thermometer capable of measuring water temperature on a backpack, the most practical approach is to heat water until it starts bubbling.

Note: the Environmental Protection Agency (EPA) and the Centers for Disease Control and Prevention (CDC) both recommend boiling water for one minute. If you are above 6,562 feet (2,000 meters), the recommended boiling time increases to three minutes.

Advantages of Boiling:

  • Boiling kills most microorganisms.

    Raising your water’s temperature to boiling is an easy way to treat it for cooking and drinking.
    (photo: D. MacLeay)
  • Boiling is the most practical and simplest approach to sterilizing water for meals, since you have to raise the water to boiling for most backpacking foods anyway.
  • In winter, when you often must melt snow or ice, you usually will raise the water to boiling, as well.

Disadvantages of Boiling:

  • Boiling does not remove silt, glacial flour, or volcanic ash, though it will sometimes reduce turbidity from plant material in the water.
  • Boiling does not remove chemical contamination, though it can drive off some of the sulfur compounds.
  • Boiling requires burning fuel. Since you generally will be carrying your stove fuel with you, this means carrying extra weight.




Filters depend primarily on forcing water through a medium with tiny holes to physically remove microbes and matter. Anything larger than the hole size is blocked, so a filter’s effectiveness depends on its “pore” size. Most filters available for backpackers remove all parasites and bacteria, plus some viruses. They remove most of the particles that cause turbidity. However, few filters for backpackers remove the smallest viruses.

The Katadyn Exstream XR bottle purifier uses a straw.

Filters sold as “purifiers” (First Need, for example) use a chemical purifier to kill viruses, usually an iodine resin. Purifiers must prove inactivation of all three biological contaminants (protozoa, bacteria, and viruses) to meet the EPA Guide Standard for Testing Microbiological Purifiers.

There are filters, known as “reverse-osmosis filters,” available for desalination. However, their size makes them impractical for backpacking, though usable for ocean-going sailboats and other larger vessels.

Backpacking filters may work via a pump (a manual pump forces the water through the filter element), “straw” (your suction pulls the water through the filter), or gravity. Ease of use and the speed of filtering are considerations when selecting a filter.

 A pump filter, the MSR Miniworks EX.

Advantages of Filtering:

  • Most filters are effective against bacteria and parasites.
  • Some filters remove larger viruses.
  • Filters remove some turbidity.
  • Filters are usually convenient and fast.

Disadvantages of Filtering:

  • In areas where viral contamination is present (Third World countries, for example), additional purification by boiling or chemical treatment is still required.
  • Few filters remove chemical contamination; those that do remove only a limited range of chemical contaminants (usually with an activated carbon element).

    A gravity-fed filter, the Katadyn Base Camp.
  • Fine particles like silt, glacial flour, and volcanic ash will clog any filter, requiring field maintenance (backflushing, cleaning with a brush made for the purpose, or replacement of the filter element). Many filters have pre-filters available to reduce the amount of material that will cause clogging – use them. Settling can also help.
  • If the filter freezes with even a tiny amount of water in the element, the element can crack, rendering the filter useless.
  • The moving parts of pump filters can break, rendering the pump useless.
  • Pumping can be tiring.

Proper Use of Filters:

  • Always store the intake hose separately from the rest of the pump.
  • Use the covers for the inlet and outlet to prevent contaminating the filter element on the outlet side.
  • Protect the filter from freezing.
  • Clean the filter regularly according to the manufacturer’s specifications.
  • Use a bucket, pan, fold-up basin, or other container as the source of the water for the pump, rather than pumping directly out of the stream or lake (at least 100 feet from the water source). This is both for avoiding the watering area for local animals and to reduce the amount of suspended material entering the screen on the pickup and into the pre-filter.
  • If possible, let water stand overnight to allow suspended material to settle out.


Chemical Treatment: Iodine and Chlorine


The chemical treatment approach uses one of two halogens (chlorine or iodine) to kill biota. This is the method most used by municipal water supplies around the world. The backpacker’s approach is to use a small amount of the halogen, rather than the massive treatment plant used by your local water company. The halogen may be applied in one of two forms – a tablet that is dissolved in the water to be treated or in a solution of the halogen in water. Both iodine and chlorine are very effective against the most common pathogens, but they require sufficient time to act. Like all chemical reactions, the time required increases as the temperature decreases.


Potable Aqua tablets use iodine.

Iodine is most commonly used in the form of tablets, such as Potable Aqua water purification tablets. One or two tablets are added to each liter of water, with the time required being about 30 minutes for “room temperature” water (always follow the manufacturer’s instructions).

Alternatively, a small amount of a saturated solution made from iodine crystals is added to the water to be treated. The most common form of this treatment is Polar Pure. With Polar Pure, the crystals are in a bottle that has a “trap” in the mouth of the bottle to ensure that only the saturated solution of iodine in water and no crystals gets into your drinking water. One capful of the solution is measured out and poured into your one-liter water bottle. Then the Polar Pure bottle is refilled to allow a saturated solution to be formed for the next treatment.

Advantages of Iodine

  • Chemical treatments, like iodine, are very effective against most common pathogens.
  • Iodine is faster than chlorine, approximately 30 minutes versus up to 4 hours.

Disadvantages of Iodine

  • Does not kill the parasite cryptosporidium.
  • Chemical treatments do not remove chemical contaminants or turbidity.
  • Requires time to work (approximately 30 minutes); Temperatures below 70°F (20°C) and turbid water require increasingly longer times.
  • Some people react badly to iodine, particularly people with thyroid problems.
  • The medical community recommends that continuous usage of iodine-treated water last no more than 3 to 4 weeks.
  • Many people find the iodine taste objectionable (mixing in some citric acid in the form of lemon juice or tablets will kill the taste, but be sure to wait until sterilization has had plenty of time to take place).


Chlorine is most commonly used in the form of chlorine dioxide tablets or drops, such as in Aquamira, Katadyn Micropur, and Potable Aqua Chlorine Dioxide. It can also be used in the form of laundry bleach (be sure it is pure hypochlorous acid and not mixed with perfumes or other chemicals) or a solution generated from rock salt, such as done by MSR’s MIOX purifier.

While chlorine will kill most bacteria and parasites in a few minutes, a much longer time is required to kill the cysts of giardia and cryptosporidium, as well as many viruses. Typically 4 hours or more is the recommended time for room temperature water (70°F/20°C), with an increase for colder water, such as from mountain streams (always follow the manufacturer’s instructions).

Chlorine can be used indefinitely, unlike iodine. It’s what you drink in most city water systems. On the other hand, one of the joys of fresh stream water is its clean taste with no chemical smell or taste. The much longer time required for chlorine treatment is also a problem. The solution is to carry two water bottles, which are alternately refilled and the chlorine added at each water source. That way, you have a full bottle available when you empty the other.

Chemical treatments do not remove chemical contamination, and in fact can react with certain agricultural runoff. This is unlikely to be a significant problem for the backcountry traveler, however. Chemical treatment also does not remove turbidity. Turbid water requires longer treatment times or an increased amount of the halogen.

Advantages of Chlorine:

  • Chemical treatments are very effective against most common pathogens, and chlorine kills most bacteria and parasites in minutes.
  • Chlorine can be used indefinitely.

Disadvantages of Chlorine:

  • Chemical treatments do not remove chemical contaminants or turbidity.
  • Requires time to work, up to 4 hours, for full treatment; Temperatures below 70°F (20°C) and turbid water require increasingly longer times.


Ultraviolet Radiation

The SteriPen is one fast, convenient method of treating drinking water. (photo: A. MacLeay)


A recent addition to the quiver of techniques for purifying water in the field is ultraviolet (UV) radiation. The most widely available device for doing this is the SteriPen. This is a battery-operated device that is stirred in the water (preferably a one-liter water bottle with a wide mouth) for a short period of time (couple of minutes).

To prepare for purification, fill a bottle with water, press a button on the SteriPen according to the directions (single push for one liter), then insert the pen into the water. When the contacts on the side of the pen are immersed, the light will light up. Stir the water until the light extinguishes, and the water is sterilized. The UV radiation is extremely effective against biological contaminants.

However, like the other treatments mentioned above, UV radiation does not remove chemical contaminants or turbidity. The water should be fairly clear (it need not be crystal clear), so the water should be allowed to settle or a filter used to remove most of the sediment (a coffee filter is sufficient, according to the last word I had from the SteriPen people). If the water appears slightly milky from glacial flour or volcanic ash, you may want to run a second treatment. We used this approach on Kilimanjaro with the water taken from streams on the mountain.

The Adventurer, one of several models from SteriPen.

A disadvantage of the SteriPen is that it requires batteries. However, one of the optional packages includes a carrying case with a solar charger in the lid. This requires 8 to 10 hours to recharge a pair of batteries. Since a charge will suffice for 10 liters or so of water (a couple day’s worth), this is adequate. You can carry two or three sets of the batteries as backups, although the batteries (both primary and rechargeable) are lithium-based, hence subject to the TSA restrictions on carrying no more than two spare batteries in your carry-on baggage and none in your checked baggage.

Advantages of UV Radiation:

  • Effective against biological contaminants—parasites, bacteria, and viruses.
  • Fast and convenient.

Disadvantages of UV Radiation:

  • Does not remove chemical contamination nor reduce turbidity.
  • Water should be fairly clear, though it does not have to be perfectly clear.
  • Requires batteries, but a solar charger is available.


PUR Water Treatment Kit

Open the PUR packet.


PUR, a Swiss division of Proctor and Gamble, developed a water treatment kit for use in Third World countries a couple of years ago. They have now made the PUR Clean Drinking Water Kit available in North America. This kit will purify water, removing biological and many chemical contaminants, along with suspended particulates.

The complete kit includes two 10-liter containers (one for the untreated water to be treated, the other for the treated water to be decanted into), a stirring tool, a cotton cloth filter to remove the flocculus with the trapped contaminants, packets of the treatment chemical, and a packet opening tool. The first container is filled with the water to be treated. The premeasured chemical packet, containing iron sulphate and calcium hypochlorite, is poured into the water and the stirring tool is used to stir the water to mix the chemical thoroughly. The container is then closed and let stand for 10 minutes.

Stir with the tool.

The iron sulphate forms a flocculus (similar in appearance to fluffy cotton), which settles to the bottom, carrying most of the biological contaminants, silt and other suspended particulates, and most chemical contaminants (including heavy metal compounds) to the bottom. After 10 minutes, the water is carefully decanted through the cloth filter into the second container, which is allowed to sit for an additional 10 minutes, during which the remaining pathogens (viruses) are killed by the calcium hypochlorite acting as the purifying agent. At this point the water is pure enough to pass international standards for drinking water.

The two major problems with the PUR kit are that it is currently somewhat difficult to obtain, and that the packets are pre-measured for 10 liters. You cannot use a partial packet for a smaller amount of water (the powder does not necessarily have the components uniformly mixed throughout the packet, so you must empty the complete packet into the water). While this is ok for a group (or, in Third World communities, for a family group), this is inconvenient for the individual backpacker or a small group of backpackers. However, the cost is very small, less than the cost per liter of most of the other methods described.

The resulting flocculus.

The big advantage is that this is the only method available for water contaminated heavily with suspended particulates, many chemical contaminants, and virtually all biological contaminants. It would work well for larger backcountry groups, as well as in case of a natural or man-made disaster that interrupts municipal water supplies. It will not desalinate water.


Advantages of PUR Clean Drinking Water Kit:

  • Removes biological, particulate, and some chemical contamination.
  • Particularly good for Third World and highly contaminated water sources, and during natural or man-made disasters.

    Decant. (photos: B. Straka)
  • Inexpensive.
  • Simple procedure.
  • Treats 10 liters at a time, so is best for large groups.


Disadvantages of PUR Clean Drinking Water Kit:

  • Treats 10 liters at a time—no smaller, no larger—so inconvenient for most backpackers.
  • Takes 20 minutes total time for treatment.


Chart: Which Treatment Methods Remove What


Bacteria Viruses



Yes Yes Yes No No
Filters Yes Yes

Remove some larger viruses, not smallest

A few remove a limited range of chemical contaminants Removes some turbidity
Purifiers Yes Yes Yes No No
Chemical: Iodine
Not cryptosporidium Yes Yes No No
Chemical: Chlorine
Yes Yes  Yes No No
UV Radiation (SteriPen) Yes Yes Yes No No
PUR Water Treatment Kit  Yes Yes Yes Some Yes


The information above is for general interest only. Always consult each manufacturer's specific product information for effectiveness of your treatment method against specific pathogens and contaminants.


The Bottom Line


As noted previously in “Part 3: What is in Backcountry Water,” the strong evidence is that for backcountry travelers in the United States and Canada, the major source of digestive tract illness is not waterborne pathogens or other contamination, but poor sanitation. The best prevention is paying diligent attention to simple sanitation measures:

  1. Wash your hands frequently, particularly after relieving yourself. Use soap and water and/or Purell or another alcohol-based purifier.
  2. Wash and sterilize your hands before eating, particularly finger foods (sandwiches, bars, trail mix, etc.)
  3. All individuals preparing or serving food for the group must wash and sterilize their hands, and should consider using gloves of the type now mandated for restaurant workers.
  4. All dishes and utensils should be washed before meal preparation and serving. Sterilizing by dunking in boiling water is one of the easiest and quickest ways to assure sterile utensils.

It is all too easy to be careless and neglect simple sanitation while hiking, backpacking, and climbing. But, by treating your backcountry water, staying hydrated, and practicing proper hygiene, you'll help ensure that you, and everyone in your group, has a great trip and will be back on the trail again soon.


 Read “Backcountry Water Treatment Part 1: Proper Hydration” »

 Read Water Treatment and Storage Reviews »