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MSR Guardian Water Purifier Review

MSR Guardian Water Purifier ReviewThe 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 that 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.

MSR Guardian Water Purifier

Treatment Capacity
Ease of Use

Best Water Purifier Available Today

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.

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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

The  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. The 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 comes 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 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 snowmelt 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

Make / ModelWeightPore SizeFilter MediaFilter Life
MSR MiniWorks EX17 oz0.2 micronCeramic2000L
MSR Guardian17.3 oz0.02 micronsHollow fiber10000L
MSR HyperFlow7.8 oz0.2 micronHollow fiber1000L
Katadyn Hiker11 oz0.2 micronGlass Fiber750L
Katadyn Hiker Pro11 oz0.2 micronGlass Fiber750L
Katadyn Vario16 oz0.2 micronCeramic, Glass Fiber2000L
Katadyn Pocket19 oz0.2 micronCeramic50000L
General Ecology First Need16 oz0.4 micronGlass fiber680L


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

Disclosure: MSR provided the author with a Guardian for this review.

SectionHiker is reader-supported. We only make money if you purchase a product through our affiliate links. Help us continue to test and write unsponsored and independent gear reviews, beginner FAQs, and free hiking guides.


  1. The MSR Guardian seems like a well designed piece of equipment that offers features few if any other treatment options can match. However under dislikes, I would add that it is quite expensive. If you need to purify could carry a Sawyer Squeeze and Steripen both and still save half a pound and a few hundred dollars.

  2. I’ve been using the First Need Deluxe for about 25 years or longer…. It was the ONLY PURIFIER on the Market until MSR arrived a year or so ago after a Government Contract win…. The First Need Deluxe, a simple hand Pump Operation, via a Canister and was the original Unit to fit on the Nalegene Bottle that many years ago and way ahead of MSR. Now called the First Need XLE Elite it still costs near $100…. saving you on average some $250.. Sorry but MSR is ripping you off in my opinion….

    • The First Need was the first water purifer I used and I still have it. But the Guardian is a better product. It’s self cleaning for one, which means the filter will last a long long time.

      • The First Need was amazing for its time, but I found it to choke on southern clay silt and western mountain silt. I have used the Guardian in the South, but not yet in the West. I will do so before long and then I will be able to say whether I find it easier in that environment.

    • It only filters down to 0.4 vs 0.2 for other filters and the Guardian is even better than that: 0.02. On top of that, the filter life is 10,000 L vs 680 L for the First Need. Guardian wins hands down!

  3. subalpine bliss

    I’ve used the MSR Guardian exclusively on all my backpacking trips since 2016.

    One of the points not mentioned in the review is that the intake can rapidly filter water in as little as an inch of water. I carry a small piece of fine mesh from Sherwin Williams paint store to throw around the intake and I’d even trust this thing to filter a mud puddle. That absolute security/adaptability plus the low-maintenance convenience plus the speed are the top three reasons I can’t bring myself to replace it with a Sawyer or the like. I don’t know how the Guardian can suck up water almost as fast even when only a small portion of the intake is submerged, but it does. The design is simply amazing. On a circuit around Mt Rainier last summer, my bottles were empty on an 85° day with 4,400 ft of elevation gain and I faced a long uphill climb with no signs of streams crossing the trail on my map/GPS. I came across a trickle of water an inch deep at best that briefly crossed a corner of one stone step before vanishing again between rocks, and I filled a liter bottle almost as quickly as if I dropped that intake in a creek. On two trips to Peru’s Cordillera Blanca and Peru’s Huayhuash, I routinely plugged a hostel sink drain with a trail bar wrapper and used it to rapidly fill all my bottles. Or in a pinch this intake could even filter either a small cascade of water or the water coming out of the tap without being submerged at all.

    It was designed for the U.S. Special Forces and it’s supposed to be rated to drop five feet onto rock/concrete without damage (not that I’ve tested that). You have to hold this in your hand and see the build quality to understand.

    Do I sometimes think I’d rather carry something 6 or 10 oz instead of 17oz? Sure, sometimes when I’m going up a lot of really steep grades. Everybody on the JMT had Sawyers, and I mean everybody. Then when my group camped next to a stagnant pool on the JMT everybody was delighted when I offered to filll all the water bottles for four people in ten minutes, none of them wanted to use their Sawyers in those ponds. With my previous Katadyn ceramic filter I had to remember to occasionally clean with a scratchy pad when green stuff clogged the pores and it got harder and harder to pump. The Guardian automatically backflushes itself with every stroke and it never slows down. It doesn’t get stiffer and stiffer if I neglect maintenance. (what maintenance?) I never wonder if I have to bring a spare filter element. You absolutely must never let the hollow fiber element dry out – it has a cap you leave on any time you’re not pumping whether the pump is in the field or in storage. No cleaning a Sawyer before and after every trip. Just screw the cap on and forget it. I can remember to do that.

    I drink more water than most follks, regardless of whether I’m in the wilderness or at work, and I may drink three or four liters just on the trail some days plus another 3 liters in camp for supper and breakfast. So this pump does definitely save me more time than it will save most folks. I never hesitate to stop and fill up my bottle because it takes so little time. Filtering water when I arrive at camp never feels like a chore anymore.

    Every year I ask people I meet on the trail to show me their Sawyers, ask them about their setup, consider what it would take to save myself at most ten oz. I weigh it against the time and convenience of the Guardian, and my decision is always the same. It saves me so much time on the trail and in camp and is so headache-free even under the most extreme and unusual circumstances, I’ve never been able to bring myself to do it.

  4. subalpine bliss

    fwiw – I have a First Need narrow mouth adapter which screws on tightly to the Guardian so it can attach directly to the small mouth Platypus bottles and the like, although I don’t currently use small-mouth bottles or carry the adapter (my current go-to bottle is a Nalgene ultra-light wide mouth bottle with CapCap lid). I got the First Need adapter long before the Guardian. I just checked and this narrow mouth adapter is still available as a set online direct from General Ecology Inc – and perhaps one or two other places online.

    Also I’ve never actually used this purifier in mud/silt and would caution use against this use just as Philip (and others) warn. I do carry enough of the Sherwin Williams fine mesh (for straining paint) to make a triple layer (not a single layer) of fine mesh to protect the intake – and would only do this in a near emergency. I don’t want to sound like it’s a good idea to use this in mud and fine silt on a regular basis – after going back I found an old 2016/2017 review where someone returned the hollow fiber element to MSR because the end was covered with silt, and the manufacturer advised exactly what Philip recommends: to allow fine silt to settle out of water first.

  5. What’s the maximum number of liters you’d want to purify at a time with the pump-style MSR Guardian Purifier after which pumping would become arduous and you’d prefer to be using a gravity system instead?

    • Have you every tried to filter water from a canal, a river , or stream with very high banks. A gravity system does not help with it, but it’s very easy if your pump filter has a long hose.

  6. That’s for sure a situation in which a pump system is preferable to a gravity system. I’m trying to decide between a pump or gravity system for use at a base camp at which I may want to filter between 10 and 30 liters of water at a time. I love the idea of the MSR Guardian Purifier for the reasons you mention in your reply and in your review above. I’m just wondering if for that much water you’d choose a gravity system or if you’d stick with a pump system.

  7. What do you think of this filter for mountaineering applications? I’m wary of the weight, but I also know from experience that a group of 3-6 people doing a big glacier hike drink a ton of water and it seems like the high flow rate and bombproofness of this filter might make up for it (e.g a group of 3-6 need only bring this filter instead of 2 or 3 filters and its will work very fast and reliably).

    I know some would say this is way too much filter for backpacking use, but my experiences with lesser pumps quickly clogging from filtering melted snow suggests otherwise. Tentmate was using an MSR Hyperflow this weekend for snow camping and it started clogging after only a gallon or so of filtering.

    • I would definitely recommend this water purifier for that application, both for a group, and because it backflushes itself constantly which you want in snowmelt, which potentially has a lot of suspended matter.

  8. Jo Anne Reinhard

    I’ve been looking at the Guardian Purifier for a couple of years and finally bit the bullet and bought one today from Backcountry (currently 25% off). Subalpine Bliss says to never let the filter dry out. When the purifier is in storage, would you recommend storing it in a Ziplock bag (in addition to the cap) and in a closet? It will probably be stored for a couple of months between trips. Mt current filter is ceramic so it is thoroughly dried and stored in a Ziplock in the freezer between trips. Thanks for any additional info. I like your website, you are straight-forward and matter-of-fact. Much appreciated in the age of TikTokers, etc.

  9. Jo Anne Reinhard

    Thanks for the link! Happy hiking!!

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