How to Choose a Water Filter or Purifier, Part 1

By REI staff

In This Article

• The Murky Truth About Clear Water
• Explaining Water Filters and Purifiers
• What's the Difference?

Is it possible to drink straight from backcountry streams and never become ill? Yes.

Is it possible to drive down a large city's main boulevard, ignore a few red lights and never have a fender-bender? Yes.

Is either practice worth the accompanying risks? In our opinion, no.

Here's a brief overview to help clean up the confusion surrounding which type of water-treatment system is right for you.

1. Remember the difference between a filter and a purifier. While both remove bacteria from water particles using a mechanical process of pumping then forcing water through a filtering device, only purifiers can render viruses inactive using either an additional chemical or electrostatic process.
2. Waterborne viruses are believed to be less common in North American wilderness waters. But if you're traveling outside of the United States or Canada, you may want to opt for the more involved, and hence perhaps more expensive, purifying system.
3. When comparing filters and purifiers, look for an absolute—not nomimal—pore size of 0.2 microns. This industry wide benchmark indicates the system's smallest filtering capability. No bacteria larger than 0.2 microns can get through. Though a larger pore size of 0.3 or 0.4 may suffice in most situations, why take chances with your health?
4. Depending on its function, a water-treatment system can have numerous parts, some of which will need to be maintained or replaced on a regular basis. If your travel plans include many nights away from a reliable water source, you will not only have to pack the filter but the back-up and replacement parts that go with it.
5. And finally, to get the cleanest water possible be sure to clean and dry your filtering system regularly. When in the outdoors, try to collect still, clear water. Whenever possible, boil the water before treating it. Then, just as if you were seasoning a skillet, let the first few streams of water pass before you begin collecting your treated water.

The Murky Truth About Clear Water

Free-flowing mountain streams, for all their beauty and clarity, are not always the fountains of purity we imagine them to be. Backcountry water sources — crystal-clear rivers, lakes and streams — sometimes harbor microscopic pathogens (disease-causing agents) that are tough to pronounce, difficult to spell and, for many people, awful to ingest.

Giardia lamblia. Cryptosporidium. Campylobacter jejuni. Hepatitis A. All are members of an invisible fluvial zoo that may be present in pristine-looking backcountry water.

How do they get there? When water becomes tainted by animal or human feces. What impact could such microbes have? They can leave you reeling with diarrhea, abdominal cramps, nausea, weight loss and fatigue. How long might these symptoms last? Between 4 and 6 weeks. Maybe longer. Ugh.

More details on waterborne pests, and techniques you can use to defeat them, are explained in our Water Treatment clinic. In this presentation our goal is to provide guidance on the water-treatment strategy favored by most wilderness travelers — using a water filter or purifier.

Explaining Water Filters and Purifiers

Portable water filters and purifiers both operate on the same mechanical principle. Using a hand pump and intake hose, both slurp up "raw" water from a lake or stream and force it through an internal element (a filtering "medium"). This medium traps suspended elements — from fine sediment to invisible microorganisms — before dispensing clean water into a container of your choice.

What's the Difference?

Some definitions:

• Water filter— A microbiological device that removes bacteria (e.g., Campylobacter jejuni) and protozoan cysts (Giardia lamblia, cryptosporidium) from contaminated water.
• Water purifier— A microbiological device that removes bacteria, protozoan cysts and viruses (e.g., hepatitis A) from contaminated water.

Viruses are infinitesimal organisms too tiny to be trapped by a filter. Devices identified as "purifiers" usually cause water to interact with iodine (often in the form of iodine resins), which can render viruses inactive. Another purifier uses a positive electrostatic charge in its filter medium to capture viruses.

Viruses:

• may exist in water wherever there is a reasonable chance of human fecal contamination;
• are believed to be less prevalent in North American wilderness water sources than protozoan cysts or bacteria, but may be a greater threat in less developed countries.

Over time, filters have proven that they reliably protect wilderness travelers from the most common waterborne pathogens found in the North American backcountry: giardia and cryptosporidium. Still, purifiers and their antiviral feature offer an elevated level of security.

To fully disinfect suspect water using a water filter, the Centers for Disease Control and Prevention recommends 1) mechanically filtering the water, 2) treating it with a halogen (chlorine or an iodine solution), 3) letting it sit 15 to 60 minutes, 4) then drinking. For more details on this process, and a discussion of what pathogens may be found in backcountry water, refer to our Water Treatment clinic.

The difference between water filters and purifiers can seem arcane. In a separate discussion, we examine some of the more technical points.

How to Choose Water Filters and Purifiers Part 2.

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