MSR Guardian Purifier

Let’s say you treat 100 liters of water a year in the backcountry. That’s a lot—at least 25 days of backpacking, in most conditions. Now let’s say you have a partner, and you filter water for him or her, too. So 200 liters a year. At that rate, the Guardian will last 50 years with zero cleaning.

We were skeptical about MSR’s claims for the Guardian. Normally we try to avoid clogging water filters at all costs. When gunky water gums up some filters, they’re done, and even models that can be cleaned are a hassle to maintain when water is really bad. But during trips on which we pumped some 1,000 liters, from the silty Colorado River to Turkish ponds filled with moss, mud, and goat poop, we couldn’t get the Guardian to clog. How’s it work? With every stroke of the pump, 10 percent of the water backflushes the filter, so it’s self-cleaning. We never detected any slowing of the flow (it’s fast, about 2 liters per minute). “The lever handle is way easier to use than traditional pump style handles,” a tester says.

The hollow-fiber filter is also unusual in two other ways: It eliminates viruses in addition to other pathogens (rare for filters); and, according to MSR, it can withstand freezing temperatures (down to 0°F) without damage. Credit that coldproofing to the filter’s unique physical and chemical makeup (in other words, it’s proprietary). There are certainly lighter, more compact, and cheaper options available, but if you want to clean bottle after bottle of the world’s gunkiest water, this is your filter. $350; 1 lb. 1 oz.; msrgear.com