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How to Purify Water

Nothing’s more important in the backcountry than staying hydrated, but the same water that quenches your thirst and keeps you alive can also sicken you if you don’t take proper precautions. Out on a multiday backpacking or paddling trek, you should play it safe and purify any water you’re using for drinking or cooking. How? In this blog post, we’ll review the basic water purification methods and when they should be employed.

But ... Do I Need to Purify Water in the Backcountry in the First Place?

We already hear some of you questioning the whole premise. Can’t you drink straight from that icefield trickle, that mossy seep, that babbling high-country brook? What purer water could there be than that in the deep wilderness? There are a lot of myths and misleading advice circulating around wilderness water treatment. Some folks claim they’ve never purified water in the backcountry, drinking freely from mountain streams and glacial lakes without ever suffering negative consequences. It’s certainly true you could go a lifetime of backpacking happily free of gastrointestinal miseries, but you’re playing a game of chance. Those people who sup directly from wilderness lakes and waterways and don’t contract waterborne illnesses have had luck, an especially hardy immune system, or both on their side. You’ll also hear it said that one used to be able to slurp merrily from wilderness water sources, but that the pollutions and contaminations of our modern world now preclude that option. Certainly we’ve fouled watersheds all over with our activities, but many waterborne pathogens exist naturally in even the most “pristine” basins. After all—and not to get too graphic, but such is the nature of our subject—animals have been answering Nature’s call, not to mention expiring, in lake shallows and forest streams for eons. (They do call giardiasis “beaver fever,” after all.) Water-related ailments have been a reality for humankind forever, and countless ancestors of ours wandering much less trammeled and altered landscapes died from waterborne germs. Now, it is true that some recent research suggests wilderness water sources may be less biologically contaminated than previously thought. But it’s impossible to judge by the naked eye, so why tempt fate? It’s easy to be blasé about waterborne illnesses until you’re hit with one. If it strikes when you’re back home in civilization, it can be a miserable few days. If you’re still in the backcountry when you fall ill, it’s that much more miserable—not to mention more dangerous, because you’re less able to combat the dehydration that arises from all that vomiting and diarrhea.

What is Purified Water?

For our purposes, purified water is that from which pathogens (“germs”) have been removed or destroyed. And when we’re talking about pathogens a backpacker or paddler is facing in the North American wilderness, we’re talking about the following:

  • Bacteria:nasty (to us, anyway) microbes such as Escherichia coli (“E. coli”)and Salmonella found in many waterways.
  • Parasites:such delightful critters as flatworms, tapeworms, and—most commonly afflicting North American outdoorspeople—protozoa such as Cryptosporidium parvum and Giardia lambia.
  • Viruses: generally less of an issue in the U.S. and Canada, but Hepatitis A and others can be a problem overseas.

The typical vehicle by which a stream, lake, snowfield, or other water source becomes contaminated with such pathogens is animal waste (including that of humans). As we alluded to above, carrion can also be the source. In North America, backcountry pathogens are more likely to sicken you than kill you (those with compromised immune systems are more at risk of serious effects). The diseases they cause—giardiasis, for example, or cryptosporidiosis—can certainly be highly debilitating, though, involving an awful lot of stomach cramps, diarrhea, nausea, vomiting, fever, and other torments.

How to Purify Water

There are three primary water purification methods: boiling, chemical treatment, and filtration. Each has its benefits and drawbacks, and most experienced backpackers don’t rely on any one alone; rather, they’ll use one or another or a combination depending on the situation. Having backup water-purification tools on hand is always a smart idea in case your primary method fails or can’t be used.

Boiling Water for Drinking

Does boiling water purify it? Absolutely. In fact, boiling is the most effective method of water purification: It kills all pathogens—viruses, bacteria, and protozoa. The only drawback is the hassle, time, and fuel required to bring water to a boil (and the time necessary for the boiled water to cool to a safe drinking temperature).

How Long Do You Boil Water to Sterilize It?

Yes, there’s a time expense involved when getting water to boil and cooling it down again, but the actual interval required to sterilize by this surefire method is negligible. When you’re boiling water for drinking purposes, heat to a rolling boil and keep it there for a full minute. As REI notes, even the highly heat-resistant Hepatitis A perishes in 208°Fahrenheit water in less than a minute. Now, what about the elevation factor? A bit of a gray area. The Centers for Disease Control and Prevention suggests you should extend the rolling boil to three minutes at elevations above about 6,500 feet. Other authorities claim that a minute’s sufficient regardless of elevation.

Chemical Treatment

Chemical treatment is used to kill or destroy waterborne pathogens. It’s a straightforward process, if often a time-consuming one: You just plunk a tablet, sprinkle powder, or squirt a droplet or two in the water you want to treat and wait. Iodine is the best-known chemical treatment: It’s effective in killing bacteria and viruses, less so protozoa. Iodine can be used to treat for Giardia, but you typically have to soak water for an hour to do so. Iodine doesn’t disinfect against Cryptosporidium. Chlorine is an alternative chemical treatment with similar properties as iodine: good against bacteria and viruses, so-so against protozoa. It may be somewhat less effective than iodine in destroying the cysts of Giardia, and like iodine doesn’t treat for Cryptosporidium. Chlorine dioxide is a chemical that can, given enough time (up to four hours), kill Cryptosporidium, though it’s not foolproof. Chemical treatments such as iodine and chlorine tend to give treated water an off-putting taste. Some tablets or powders come mixed with other agents to counter that flavor, or you can add, say, Vitamin C before drinking. Overall, chemical tablets or powder are lightweight and space-efficient; you might as well have them handy as a backup means of water-treatment.

Survival Water Filtration

Unlike boiling or chemical treatments, water filtration doesn’t kill pathogens but rather strains them out. As they’re typically 0.1 micron in size or smaller, viruses are usually too miniscule to be filtered out. Some filters include an iodine component to kill viruses (in which case they’re more properly called water purifiers); otherwise, where viruses are a concern you should disinfect the water with iodine before filtering or else simply boil it. Pay attention to the pore measurement of a filter to determine which kinds of pathogens it wards against. Choose filters with a pore size of 0.4 microns or smaller. There’s quite the dizzying array of water filters and purifiers on the market, from pump and gravity models to squeeze-action ones. Again, pore size should be the deciding factor, but personal preference and budget play a role, too. A benefit of the squeeze-action water filters is they don’t have as many working parts to break. Because sediment and other debris will gunk up a filter, they need to be cleaned at varying intervals depending on design and degree of use. Many come with a prefilter that strains out larger particulate matter before water passes through the filter proper.

Where Should I Gather Water for Purification?

Where and how should you gather the water you’ll boil, treat, or filter/purify for drinking and cooking purposes? Although, again, your naked eye isn't an effective judge of the purity of water source, it’s certainly better to avoid visually fouled ones: for example, those crossed by human or animal trails, edged by heavy animal sign, or mucked over with algae, which isn’t generally harmful in and of itself but which indicates biologically fertile conditions that may breed pathogens. Flowing water is preferable to stagnant water, clear water to murky water. If you can, fetch lakewater as far offshore as you can. You can “prefilter” water of silt, plant bits, and other debris using a bandanna spread over the mouth of your collecting vessel. You can also cut down on the detritus in your water by facing your vessel downstream.

Thanks to Mountain House Blogs for the above information.

LDP Camping Foods
109A Gill Dr., Lafayette, Louisiana, 70507


800-826-5767 or  337-235-4695

This page was last modified on February 22, 2023.