Water treatment in the backcountry

The methods for ensuring you have clean water to drink in the backcountry keep getting lighter and easier. In addition, there are more treatment options than ever, from gravity filters to UV light.

Which one is right for you? Or do you even need to treat water at all? This article discusses your choices.

Shop REI's selection of water filters and purifiers.

Water Treatment Options

Your choices include:

  • Pump-style filters and purifiers
  • Gravity-fed filters and purifiers
  • Ultraviolet light pen purifiers
  • Sip/squeeze bottles with in-line filtering straws
  • Chemical tablets
  • Boiling

First, it's good to know the difference between a water filter and a water purifier.

Water filters use microfiltration to rid water of harmful bacteria and protozoa. This level of protection is considered quite sufficient for backcountry use in the U.S. and Canada. Water purifiers typically use microfiltration plus a chemical treatment (UV light is one approach) to meet an Environmental Protection Agency standard for eliminating viruses as well as bacteria and protozoa. Purifiers are best for international travel where the risk of viral contamination is greater. For details, see the REI Expert Advice article, Water Treatment for International Travel.

Your decision ultimately comes down to personal preference, but this chart presents the criteria that can help you decide:

Gravity filters Sip/squeeze bottles UV light Halogens
Speed 3-4 3-4 2 4 1-2
Weight 2-3 2-3 2 4 5
Size 2-3 2-3 2 3 5
Pore size 5 5 4 NA NA
Convenience 3 4 5 4 5
Ease of use 2-3 5 3 5 5
Maintenance 3* 4 2 5 NA
Longevity 4 4 2 4 3
Durability 3 4 3 3 3
Quality of taste 5 (Filters), 3 (Purifiers) 5 5 (Filters), 3 (Purifiers) 5 1-2
Cost $75+ $50+ $50+ $70+ $6.75+

5 = Excellent, 4 = Very good, 3 = Good, 2 = Fair, 1 = Poor; NA = Not applicable.

* Some filters are not cleanable.

Shop REI's selection of water filters, water purifiers and chemical treatments.

Is Water Treatment Even Necessary?

Q: Why do I need to treat water in wilderness areas?

A: Regardless of how pure water may look, any water source on the planet could be tainted with microscopic waterborne pathogens—disease-causing pests that, if ingested, could cause severe diarrhea, cramps, vomiting and fever.

Recent research, though, suggests that wilderness water at higher elevations is much cleaner than previously believed. Some experts argue that the blame for intestinal infections is more often traceable to preexisting conditions and lax sanitation, particularly unwashed hands.

Q: What's in wilderness water that can affect me?

A: Three groups of waterborne critters are most commonly linked to water-related illness:

  • Protozoan cysts (Cryptosporidium parvum, Giardia lamblia). Tiny (1 to 300 microns; 1 micron = one-millionth of a meter).
  • Bacteria (Escherichia coli, or E. coli, Salmonella, Campylobacter jejuni, Yersinia entercolitica, Leptospira interrogans and many others). Very tiny (0.1 to 10 microns).
  • Viruses (hepatitis A, rotavirus, enterovirus, norovirus, Norwalk virus). Exceptionally tiny (0.005 to 0.1 micron). Viruses are rarely found in North American wilderness waters. Only purifiers (not filters) eliminate viruses.

For a more detailed look at these pathogens, please view our companion article "Water: What Are the Risks?"

Q: How do they get in the water?

A: Protozoa (parasites) and viruses are present in surface water that has become contaminated by animal or human waste, principally feces. Bacteria, including many beneficial bacteria, naturally occur in water. Water contaminated by fecal material often results in a population of harmful bacteria such as E. coli. When soap is introduced to water, it creates a supply of nutrients of that can sustain pathogens or cause algae blooms. Soap residue is a growing problem in backcountry water.

Q: How can a person know if a water source is tainted?

A: Unless you are a scientist with testing materials, it's guesswork. While experienced wilderness travelers can learn to make reasonably educated guesses, everyone should carry some method of water treatment for the inevitable situations where water is viewed with suspicion.

Comparing Water Treatment Options

In the sections below, we evaluate how various treatment methods perform point by point.


Method Protozoa Bacteria Viruses Process
Boiling Water is boiled for at least 1 minute.
Filter __ Water is strained through an internal element by pumping, gravity or sip/squeeze bottle.
Purifer Same as a filter but may include a chemical component.
Ultraviolet light Water is exposed to UV light; takes about 90 seconds per 32 fl. oz.
Chlorine dioxide
Tablets are dissolved in water; requires 15 minute wait time.
√* Tablets, crystals or tincture are dissolved in water; requires 15 minute wait time. (Important: not for pregnant women or people with a thyroid condition).

* Not effective against Cryptosporidium.

To compare most of the specs listed above, click the Specs tab on product pages at REI.com or look for Product Information Guides at REI stores.

Speed (Output or Flow Rate)

Speed (Output or Flow Rate)

Boiling: Poor

Requires setup time, more time to bring water to a boil, then more time for the water to cool. As such, boiling is usually best reserved for cooking, emergencies or a last resort.

Filters/purifiers: Good to very good

Filters or purifiers that promise an output (or flow rate) of greater than 1 liter per minute are pretty speedy. Some new models can exceed 2 liters per minute and are the fastest pump-style devices now available. In general, the larger the surface area of the filtering medium, the faster the flow.

Gravity filters: Good to very good

Flow rate depends on the type of filtering media being used. Estimates range from 0.5 to 1.5 liters (or more) per minute.

Sip/squeeze bottles: Fair

Equipped with filtration or purification straws, they offer dip-and-drink swiftness but deliver water in thin streams. You get fast action, but a small quantity.

Electronic devices: Poor to fair

Iodine, chlorine dioxide and some purifiers require some wait time before the water is drinkable. The time ranges from 15 minutes to 4 hours, depending on the temperature and/or murkiness of the water. The colder the water, the longer the wait. Cryptosporidium, likely due to its hard shell, has shown a high resistance to halogens. If rangers or land managers indicate that Cryptosporidium is a major concern in the area where you plan to explore, consider choosing a different treatment method.

Note: Output figures for filters and purifiers (liters per minute) are estimates provided by manufacturers. One liter (33.8 fluid ounces) per minute is a commonly cited flow rate, though sometimes people find the 1 liter/1 minute ratio to be an optimistic calculation. Our online specs also include a "strokes per liter" estimate, indicating the kind of pumping action you can anticipate with individual pump devices. Realize that your results may vary.

Tip: With a pump device, when the flow becomes slow, clean or backwash your device to restore its output.

Weight and Size

Filters/purifiers: Fair to good

Most range between 6 and 20 inches in length, 2 to 4 inches in diameter and 8 to 16 ounces in weight.

Gravity filters: Fair to good

When dry, they weigh between 11 and 17 ounces. The water bags lie flat when empty; with hoses and filtering media, they occupy about the same amount of space as a standard filter.

Sip/squeeze bottles: Fair

Bottles are around 12" high x 4" diameter and weigh less than 9 ounces when empty.

Electronic devices: Good to very good

The SteriPEN (using an ultraviolet lamp) is about 2 inches wide, 6 to 8 inches high (depending on the model) and between 9 and 16 ounces in weight.

Halogens: Excellent

Carried in small bottles or paper sleeves, chemicals such as iodine or chlorine dioxide weigh just ounces and occupy minimal space.

Pore Size

Filters/purifiers: Excellent

Pump devices use filtering media, such as a ceramic cartridge, to physically strain disease-causing agents from the water.

The "absolute" pore size of the filtering media used in filters carried at REI ranges between 0.2 and 0.4 microns—considered quite adequate for use in North American wilderness areas. Protozoa and most bacteria are 0.5 microns or larger.

Filters cannot strain away viruses, however. Viruses are exceptionally small (0.005 to 0.1 microns). A device (or system) designated as a purifier is needed if viruses are a concern.

To be marketed as a purifier, any device or chemical treatment must meet or exceed the U.S. Environmental Protection Agency (EPA) Guide Standard and Protocol for Testing Microbiological Water Purifiers. EPA-registered products must destroy 99.9999% of bacteria, 99.99% or viruses and 99.9% of protozoa.

Independent labs, not the EPA, conduct such testing. The EPA does not approve or endorse filtration devices or purifiers; it simply provides registration numbers.

Gravity filters: Excellent

They use the same filtering media as select pump filters.

Sip/squeeze bottles: Very good to excellent

Same as filters/purifiers.

Electronic devices: Not applicable

Halogens: Not applicable

Tip: Don't sweat pore size unless you are traveling internationally or expecting to treat water sources near urbanized areas, farmlands or grazing areas. For such areas, consider a purifier, chemical treatments or plan to boil.

Convenience/Ease of Use

Filters/purifiers: Fair to good

Some physical effort is required, and if the device is pumping murky or silt-heavy water, the pumping action may feel laborious. Generally, though, the effort needed for most pump devices is modest.

Gravity filters: Very good

Sometimes you might be challenged to find a place to hang the bag or get water into the bag (if your only source is a thin trickle or a hard-to-reach pool). Otherwise, just hang it and wait.

Sip/squeeze bottles: Excellent

One potential downside—sip tubes do not accommodate big gulps of water.

Electronic devices: Very good

SteriPEN recommends that you stir the water while the UV light is on to expose all water inside a bottle. It's best to use the clearest possible water. If the water is cloudy, consider doubling its exposure.

Halogens: Excellent

Drop in tablets and wait. Easy.

Maintenance (Field Cleanable)

Filters/purifiers: Good (some models are not cleanable)

Many devices use filtering media that can be cleaned in the field. That's a big help if the water being treated contains grit or silt. When device run slow or become hard to pump, cleanable models can disassembled so the cartridges can be wiped or brushed clean. On some models, hoses can be rerouted so the unit can be backflushed.

Filters using an unprotected glassfiber cartridge cannot be cleaned in the field. They must be replaced.

Gravity filters: Very good

At the time this article was written, all gravity filters offered at REI use cleanable filtering media.

Sip/squeeze bottles: Fair

Backwashing a sip/squeeze bottle is possible, though it is not always effective. Usually a replacement insert is the preferred option when the internal unit becomes clogged. Attempt to use only clear, sediment-free water.

Electronic devices: Excellent

Batteries are the only maintenance concern. It's wise to carry extras.

Halogens: Not applicable

Just make sure to carry an adequate supply.

To optimize your use of a water filter, it's important to read the instructions that accompany any device, mechanical or electronic.

Tip: Make sure that your device is allowed to dry completely between trips. Disassemble all parts that you can to speed the process.


Filters/purifiers: Very good

Manufacturers estimate that their units can handle between 200 and 1,000 gallons. (The venerable Katadyn Pocket Filter, with its silver-impregnated ceramic cartridge, promises to deliver 13,000 gallons.) Using the clearest, grit-free water possible will boost a filter's longevity. Keeping a device clean, lubricated and well-maintained will also extend its life.

Handle a filter's internal media with care, since these cartridges can break. In subfreezing conditions, purge water from the internal media, particularly overnight. Water left inside could freeze and crack the cartridge.

Gravity filters: Very good

Estimates range from 750 to 1,500 liters (200 to 400 gallons). As with traditional filters, try to use the cleanest, particulate-free water possible. Avoid letting water inside the filtering media freeze.

Sip/squeeze bottles: Fair to good

To optimize longevity, use only clear, sediment-free water in a sip/squeeze bottle. For areas where the water carries a high level of grit or silt, carry a replacement filtering (or purifying) cartridge. When the filter clogs, it must be replaced.

Electronic devices: Good to very good

Ultraviolet light and mixed-oxidant devices are not impacted by grit. They are, however, battery-dependent. For the SteriPEN Classic, estimates range from 15 liters (using alkaline batteries) to 100 liters (using NiHM batteries). Care must be taken when handling the glass element of the SteriPEN; avoid dropping and breaking it.

Halogens: Good

Be mindful of "use by" dates included on the packaging. Halogens, in general, are believed to maintain effectiveness between 1 and 3 years.

Tips: Grit is not good when filtering water. To screen it out, use a prefilter. This can be included with the unit or offered as an optional add-on.

Quality of Taste

Filters: Excellent

Water that passes through a filter is instantly drinkable and tastes natural. Some filters include (or offer as an optional add-on) a carbon component that can rid water of any unpleasant tastes, such as tannin from a leaf-filled pond.

Gravity filters: Excellent

Same as traditional filters, above.

Purifiers: Good to excellent

One line of purifiers, the First Need series, uses a proprietary internal matrix that captures even tiny viruses. Since no chemicals are used, the water's taste is unchanged.

The MSR Sweetwater Purifier System requires the user to treat filtered water with a chlorine-based solution. The resulting taste resembles chlorinated urban water.

Sip/squeeze bottles: Good to excellent

Purifier-grade bottles, which cause water to interact with iodine-based beads, result in some detectable chemical taste.

Electronic devices: Fair to excellent

With the SteriPEN, no change in the water's taste occurs.

Halogens: Poor to fair

Iodine, in any form, noticeably alters the taste of water. Consider the use of taste-neutralizing tablets as a method to make the taste more acceptable.

Chlorine dioxide tablets give water an urban-chlorine taste that most people find less objectionable than iodine.

Note: Some filters include (or offer as an optional add-on) a carbon component that can rid water of any inherent unpleasant tastes, such as tannin from a leaf-filled pond.


Less expensive: Halogens ($6.75 and up); sip/squeeze bottles ($50 and up).

More expensive: Filters/purifiers, gravity filters and electronic devices ($75 and up).

Other Water Treatment Considerations

Water filter/bottle adapters

Bottle or hydration reservoir adaptors: These items directly connect a filter's outlet to a bottle or reservoir, eliminating the risk of spills. They are included with some filters; with others, they're offered as an optional add-on. They're very handy, especially if you are using a pump filter on uneven ground or in a cramped space.

Intake hose prefilters: Also very handy. These items help keep grit and silt from clogging a filter's internal element. If your water has a persistently high sediment level, or you're on an extended trip, bring some coffee filters. By securing one around the prefilter with a rubber band, it further minimizes the grit that reaches your filter. This can affect the flow rate nominally but helps you get the maximum mileage out of your filter—particularly useful on a longer trip. In a pinch, a bandana can serve as a prefilter.

Carbon component: Some chemicals and unpleasant tastes (such as leaf tannins) are removed from water when it passes through some sort of carbon core or element. Some filters include a carbon component or offer it as an optional add-on. The Katadyn carbon cartridge, for example, is an impressive add-on for chemical based systems. It really helps remove the taste of dissolved chemicals (and to some degree, leaf tannins).

Take care when using carbon with chemical treatments. Carbon stages can deactivate active iodine in solution. Used too early in the treatment process, carbon may lessen the chance of the water becoming fully "purified" by halogens should the chemicals not receive their recommended contact time with the water. If you're purifying water in 2 stages, this is important to understand. The correct order: Halogens first, filter with a carbon component second, perhaps as long as 4 hours later. Some filters include a carbon stage, and running halogenated water through one too soon can reduce the effectiveness of purification.

Base camping: Gravity filters make an excellent choice for base camping in the field or in campgrounds where piped water is not provided. No pumping necessary, just patience. Fill it up at night, wake up to clean, filtered water.

A designated "dirty" water collector: REI carries a variety of lightweight, foldable, small-packing buckets or sinks that can be used for dipping water out of lakes and creeks. (Search REI.com for "sinks" or "buckets.") This offers a number of advantages:

  • Enhanced hygiene—potentially tainted water will contact only this designated container.
  • You can carry your water to a location away from the source, thus minimizing your impact. Plus you can tote water anywhere you like (like a location where it's easier to maneuver and less prone to accidents).
  • Water can be allowed to sit so any sediment sinks to the bottom and you can avoid pushing grit through your filter.

International travel: In almost every situation, come equipped with a purifier, which is effective against viruses as well as bacteria and protozoa. For more details, please see our companion article Water Treatment for International Travel.

Chemical contamination: Herbicides and pesticides usually can be absorbed by filters equipped with a carbon element or counteracted by some purifiers that employ a chemical component. With bioterrorism agents, it depends on the size of the organism. Anthrax, for example, is a bacterium that can range from 1 to 8 microns and thus is likely to be captured by all filters carried at REI. High concentrations of chemicals and heavy chemicals, though, most likely will not be removed by portable filters or purifiers. Always avoid collecting water from water sources connected to areas of intense agricultural activity, mines or mining tailing ponds.

Medication: If visiting an area where waterborne illness is commonplace, consult with a physician about obtaining a prescription to combat symptoms. For example, Metronidazole (Flagyl) is a frequently prescribed remedy for giardiasis and/or amoebic dysentery. Nitazoxanide (Alinia/Annita) is also effective against giardiasis and cryptosporidiosis and is commonly carried by travelers, even hikers.

Notes on product availability: REI's product selection varies from time to time. Any of the specific products mentioned in this article could drop out of our product assortment at any time. Sometimes a particular item REI stocks may be temporarily unavailable due to production issues at individual manufacturers. New products may be added before we have a chance to update this article. We apologize if any of these circumstances complicates your efforts to acquire the water-treatment system that you prefer.

Bottom line: Always carry some sort of treatment method. Experienced wilderness travelers can grow to discern drinkable backcountry water from water that needs treatment. Yet uncertainty inevitably arises at some point, so always come prepared to treat water.