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MSR Guardian Water Purifier Review

The MSR Guardian is a first class water purifier that removes viruses, bacteria, protozoa, dirt and silt from backcountry and international  water sources. Weighing just 17.3 ounces, the Guardian has a self-cleaning filter which constantly purges itself while you filter water so you never have to backflush or scrub the filter element. It also has an exceptionally high flow rate of 2.5L per minute, making it ideal for individual or small group use. You can even perform a field integrity test with the Guardian purifier to make sure it’s still working correctly after it’s been accidentally frozen or dropped. That’s unique! Let’s take a closer look.

Specs at a Glance

  • Removes:
    • 99.9999% Bacteria (E.coli, Salmonella, etc.)
    • 99.9% Protozoa (Giardia, Cryptosporidium, Amoeba, etc)
    • 99.99% Viruses (Hepatitis A, Norwalk)
  • Type: Pump Water Filter
  • Flow Rate: 2.5 L per minute
  • Bottle/Bladder Compatibility:
    • 1L Wide-mouth Nalgene Bottles
    • Wide-mouth Nalgene Soft Canteens
    • Wide-mouth MSR Dromedary and Dromlite Bladders
  • Filter Lifetime: 10,000 liters, replacement filters sold separately
  • Field Weight: 17.3 oz

Water Purifiers vs Water Filters

Th  Guardian is a water purifier which is different from being a water filter, because it can remove viruses in addition to bacteria and protozoa. Viruses are infectious microbes that are smaller than bacteria or protozoa and the pores in regular water filters aren’t fine enough to trap them. The Guardian, for instance, can remove pathogens down t0 .02 microns in size, or about is 10 times smaller than a comparable water filter like the MSR Miniworks EX water filter, which can only remove organisms down to 0.2 microns in size.

While Steripen UV lights can neutralize viruses, bacteria and protozoa, they can’t remove suspended solids or dirt from water like a filter or purifier. Chemical purification in the form of chlorine dioxide or iodine tablets can also purify water, but the treatment times can take up to 4 hours and isn’t instantaneous or as taste-free as the water processed by a purifier.

MSR Guardian Components
MSR Guardian Components

Pump Style Water Purifiers

Pump-style water purifiers are better than UV or chemical purifiers for processing from turbid and cloudy water sources like snowmelt, beaver ponds, and cattle troughs because they can filter out particulates in addition to pathogens. Pressure provided by a pumping action is required to force clean water through the walls of a filter element to remove foreign matter.

The Guardian has two hoses, an intaked hose, and a dirty water hose that expels what that's been used to backflush the purifier element
The Guardian has two hoses, an intake hose, and a dirty water hose that expels water that’s been used to backflush the purifier element.

Hose and Pre-filter

The MSR Guardian come with two hoses, a pre-filter, and a float. The first hose is an intake hose that pulls water from the pre-filter into the purifier. The second hose is expels dirty water and backflushes the purifier filter every time you pump water through the Guardian, cleaning and removing trapped impurities in order to maintain a high flow rate. Having an intake hose also lets you obtain water from hard-to-reach water sources such as shallow seeps, boulder-covered springs, water flowing underneath obstructions like snow and ice, or down steep stream and river banks. It also lets you sit in a more comfortable position while you filter water, which can be a godsend if you have to do it for a group.

The pre-filter is designed to block or remove as much muck (soil, sand, and suspended solids) from the water as possible before it reaches the purifier, which is responsible for removing microorganisms. Having a pre-filter is extremely useful if you need to get water from sources that are cloudy or have a lot of suspended solids in them like puddles, cattle troughs, beaver ponds, creeks, and snow melt because it increases filter life and decreases filter maintenance.

The pre-filter is suspended under a foam float so it pulls in water from below the surface
The pre-filter is suspended under a foam float so it pulls in water from below the surface.

The Guardian pre-filter is suspended from a foam float and oriented so that it pulls water from just below the surface of a water source, where it’s usually the cleanest. The foam float has two hoses attached to it, the intake and outlet hoses, in order to keep both hoses under control and manageable. The pre-filter hangs below the float at a right angle so it can reach water under the surface. The float and pre-filter work best when they’re used to extract water from a still pool, but have a tendency to flip over in moving water, which can reduce the Guardian’s flow rate because less water is reaching the filter element.

The Guardian screws directly onto wide mouth water bottles and reservoirs and is compatible, out of the box, with plastic Nalgene style bottles, Nalgene soft canteens, MSR Dromedary, or MSR DromLite water bladders. The base of the purifier also has a nipple that you can attach a hose to if you want to output water to a small mouth bottle or reservoir. Unfortunately, an extra hose is not provided with the unit for this purpose.

The prefilter requires minimal water depth to work although you may need to position it by hand
The prefilter requires minimal water depth to work although you may need to position it by hand

The Guardian is super easy to use. You just screw it onto the bottle or reservoir you want to fill, drop the float and pre-filter in the water and start pumping. It doesn’t take much pressure to pump and fills your bottles quite quickly. You just have to be careful to keep the bottom section which screws onto your bottles clean and out of contact with un-purified water. The Guardian comes with a screw-on clean side cap for this purpose, but you have to be careful to keep it clean as well, between uses.

When you’re finished processing water, remove the float and pre-filter and pump the filter a few more times to empty any residual water trapped inside. Then screw on the clean side cap, wrap the hose around the pump, and store it in a dry bag or external pack pocket where it can drip dry.

Potential Issues

Fine Silt

The Guardian’s greatest strength, its .02 micron pore size, is sometimes its greatest weakness if you need to filter water that has very fine suspended silt in it, because it can clog the filter element and make pumping water through it impossible. I haven’t experienced this myself because we have very clean water where I hike, but I’ve read about it occurring elsewhere. If silt is an issue in your water supply, the best solution is to collect water in a separate container and let it sit for a while so that the silt settles out. You would need to do this regardless of the purification method you use because silt makes it harder to purify water with UV or chemicals. Another option is to filter the water using a water filter that has a larger pore size like the MSR MiniWorks EX, and then use UV or chemical purification to kill off the virus pathogens in the water in a second purification step.


Most hollow fiber tube water filters (like the Guardian) are damaged if they freeze and should be replaced. The Guardian can be frozen, but must be completely re-thawed before use. If you do accidentally freeze it, it’s best to perform the simple purifier integrity test specified in the Guardian manual, before you resume using the filter. This test can be performed in the field, provided you have a clear wide-mouth bottle with you.

The base of the Guardian screws on to wide mouth bottles, but you can also run a hose to a small neck bottle if that's what you use
The base of the Guardian screws on to wide mouth bottles, but you can also run a hose to a small neck bottle if that’s what you use

Comparable Pump Water Filters and Purifiers


The MSR Guardian is the most sophisticated and effective water purifier available today. Self-cleaning, it has a high flow rate, and generates remarkably good tasting water. While it is an excellent tool for backcountry use and international travel to countries with suspect water, it’s also quite a nice product to have on hand for emergency preparedness at home, especially if you live an area prone to flooding or other natural disasters. If you like great tasting water and don’t want to wait for it, the Guardian is a safe way to get it.


  • Self-cleaning
  • Removes viruses as well as bacteria and protozoa
  • Compatible with wide-mouth Nalgene-style bottles and reservoirs
  • Pre-filter helps minimize silt and suspended solids before they reach the filter
  • Replaceable Filter
  • Field Maintainable
  • Easy to maintain and store when not in the field


  • Heavier than other alternatives
  • Still have to carry a wet hose after filtering
  • No adaptors/hose provided for narrow-necked bottles or reservoirs

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Last updated: 2018-08-24 08:54:09

Disclosure: MSR provided the author with a Guardian for this review. receives affiliate compensation from retailers that we link to if you make a purchase through them, at no additional cost to you. This helps to keep our content free and pays for our website hosting costs. Thank you for your support.

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