We pumped hundreds of liters of dirty water to find the fastest and most reliable filter

Fastest flow in our test.

Fast filtration and effective against viruses.

Large and quick enough for big camping groups.

Imagine this: you’re in the backcountry, nearly out of liquids on a scorcher of a day, when you hit the first water source you’ve seen for hours. You sit down, pull out your bladder and your pump, and drop the intake tube into the water. But when you go to pump, the pressure goes slack. Or the nozzle jams, barely a drop coming out no matter how much you squeeze. Chances are, if you’ve spent enough time in the backcountry, you don’t have to imagine this scenario: It’s happened to you. Maybe more than once. 

It’s certainly happened to me. So for Outdoor Life’s test of the best backpacking water filters, we decided to see which models from the top brands—including MSR, Lifestraw, Katadyn, Grayl, Platypus, and RapidPure—could handle the gnarliest water conditions backpackers can expect to find: stagnant water with high levels of bacterial contamination, the dark-tea tannin-filled water of a wetland, and silt-filled glacial runoff. 

Before I even began testing, I interviewed the folks at MSR, LifeStraw, and Katadyn about how they guarantee that their filters are indeed removing bacteria and protozoa (and in the case of a few of them, viruses). All of them engage in rigorous testing, using EPA or NSF standards, both in their own facilities and through third parties, like BCS Laboratories out of Florida. I visited the MSR facility where they showed me the lab where they not only grow E. coli bacteria, but also a virus that attacks E. coli, to test the performance of their Guardian filter. The thoroughness of this testing is not something that can be easily replicated in the natural environment, where the pre-existing levels of bacteria, protozoa, and viruses are unknown. As such, my testing did not look at how well the filters dealt with bacteria, protozoa, and viruses, because all the filters in this review use EPA or NSF standards.

Something else that you’ll often see on the packaging of backpacking water filters is the number of liters per minute that they filter. This number, however, only reflects what the filter is capable of doing right out of the box—not after you’ve taken it out on a couple of backpacking trips—or 20. The brands of course are testing to ensure that the filters can withstand a certain level of grit, but it’s rare to learn what the flow rate of the filters is after they’ve been exposed to some less-than-ideal water sources, let alone how much grit and sediment and muck a filter can take before it craps out completely. So my testing looked at what conditions can the backpacking filters withstand and still return a reasonable (or any) flow rate.

To start, I called in or purchased the most popular backpacking water filters available, along with corresponding models from their competitors. Before I began testing the filters out in the field, I first timed how long it took each to filter one liter of water. Generally, these times corresponded to the advertised times of the manufacturers. During this process, I also noted how complicated the filter setup was out of the box. 

Then I took the filters to was a public beach in Bellevue, Washington, which had been closed due to high levels of fecal matter (three times more than what’s considered safe for swimming).

In addition to the recorded high levels of fecal contamination, the water was also fairly scummy.

The results of this first round of testing informed the below recommendations. In the coming weeks, I’ll continue to test the water filters in two other environments: a wetland, with the classic tannin-colored water, and in the streams coming off of Mount Rainier, which are full (especially in the warm days of summer) of glacial silt. I’ll update this story after I complete each test.

Why It Made the Cut

After the first stage of testing, the lightweight Katadyn BeFree Water Filtration System had the fastest flow of any in my test, while the Katadyn BeFree Gravity had the fastest flow of the gravity filters. 

At its core, all a water filtration system needs to do is move dirty water through a filter, without inadvertently contaminating the clean water vessel on the other side. One of the most popular styles of backpacking water filters leans into this simplicity: all a user has to do is fill up a plastic bag with dirty water, attach the filter, and then squeeze the water through into their drinking vessel.

In testing, the Katadyn BeFree had far and away the fastest flow rate of anything I tested—twice as fast as several other leading models. It was also impressively lightweight (although, like all water filters, expect the weight to go up a bit after the filter is saturated with water for the first time). Check and check. 

There are a few design details, however, that I wish were different about the BeFree. The first, and biggest one, is the choice to house the filter itself inside the water flask. This made it difficult to squeeze all of the water inside of the flask out, which may be frustrating for individuals backpacking in parts of the country with difficult to access water sources. The other is the design of the lid cap, which basically resembles what you would see on a sports drink at the convenience store. When I lightly twisted it to test the durability it easily snapped off. Of course, I can still use the filter as long as I hold onto the cap, but I would rather see a more durable cap in exchange for a few extra grams. 

Similarly, the Katadyn Gravity BeFree had the fastest flow of the three gravity filters in my test, both when initially unboxed and after the first stage of testing. It was also lighter than the other two models in my test by several ounces. 

One small, but potentially significant, difference between the Katadyn BeFree and the Platypus GravityWorks and LifeStraw Peak Series Gravity is that the hang strap is not attached to the water flask when the gravity is first unboxed. This strap, however, should not be considered optional. The bail handle at the top of the water flask is not intended to be slung over a branch, and is too small for the branch size that you would need for the filter to be safely suspended. 

Another difference between the Katadyn Gravity BeFree and the other two filters in my test is that it is not designed for backflushing (which could damage the fibers), but the filter can be cleaned by carefully swishing it in water. I’lll be watching carefully to see if this impacts performance as testing continues. 

Why It Made the Cut

The most affordable filter in my test had a completely respectable flow rate after the first round of testing.

The Sawyer Squeeze systems have been popular among serious backpackers for years for two simple reasons: they are inexpensive, and they are versatile. While other similar squeeze systems (like the LifeStraw Peak Series and Katadyn BeFree) must be used with the bags or flasks that they came with, the Sawyer Squeeze (as well as the more expensive Platypus QuickDraw) can be screwed onto a standard 28mm water bottle, such as a smartwater bottle. Some backpackers will even just carry a single smartwater bottle filled with dirty water, and then drink straight from the filter. 

I used the provided dirty bag during testing and was impressed with its durability. Even after squeezing the bag as hard as I could to push the water out over ten rounds of filling and filtering to reach the desired five liters, the bag still looked like new. I only wish that it was larger—the bag that comes with the mini is 16 ounces rather than the standard liter. Something else I noticed during testing was that the filter itself did not appear to screw onto the bag correctly. Each time I tried to fix it, the angle would still be lopsided. Fortunately, there were no leaks during testing so this appears to be mainly an aesthetic concern. 

While the flow rate for the Sawyer Mini was not especially impressive (about four times as long as the Katadyn BeFree), I did appreciate that it came with a lightweight syringe to backflush the filter when it starts to clog too much. 

Why It Made the Cut

The 8-liter capacity of the Platypus GravityWorks is one of the best backpacking water filters to ensure there is enough water at camp for larger groups.

Gravity filters have been growing in popularity due to the user’s ability to get up and walk away to handle other camp chores, or even just take a break on the trail, while gravity does the work of actually filtering your water. 

The idea behind a gravity filter is fairly straightforward, instead of squeezing or pumping dirty water into your drinking vessel, let gravity do the work for you. After filling up the dirty bag, you then elevate it by placing it on a large rock, hanging it in a tree, or even holding it yourself, whatever works. The higher the water is, the faster it will flow through the filter, but it will work as long as the bag itself is higher up than the end of the outlet hose. Then you can just walk away while gravity forces the water from the dirty water sources into your clean container. 

All the gravity filters in my test came with a flask of at least three liters, which is nice if you are traveling with multiple people, as it reduces the number of rounds of filling up your dirty flask that you need to do. The Platypus GravityWorks takes this a step further, with a dirty flask capacity of four liters. Even better, it comes with a second bladder to filter your clean water directly into it.  

In testing, the GravityWorks took about twice as long to filter a liter of water after the first stage of testing as the Katadyn BeFree. One other important difference with this filter, however, is that it can be backflushed, which will be completed after the second stage of testing to see what difference it makes in the flow rate. 

Something else I liked about the Platypus Gravityworks is that it was relatively easy to avoid cross-contamination while out on the trail, even with all of the tubes and flasks. The clean flask is clearly labeled as such, so there is no risk of accidentally filling it with unfiltered water, and the the two tubes connecting the flasks to the filter have different connection points to the flasks, so there is no risk of connecting the wrong tube to the wrong flask. 

Why It Made the Cut

Of the water filters that are effective against viruses, the MSR Guardian was by far the fastest.

For backpacking water filters, protection against viruses, including norovirus and rotovirus, comes with a tradeoff—if you want a filter with a small enough pore size to weed those suckers out, you better be prepared to put in some serious elbow grease. But the MSR Guardian manages the hat trick, filtering out viruses at a totally respectable flow rate with the same, or even less, elbow grease required from other filters.

Part of this due to a unique feature to the MSR Guardian: instead of directing all of the water out through the bottom of the filter with each pump, a portion of the water is directed back through the filter—essentially backflushing the filter with each pump. During testing, this meant that the filter had the same performance after the first round of testing as before. 

I was also impressed with how easy the MSR Guardian was to use straight out of the box: after it had been unmoored from its packaging, I simply unscrewed the cap on the bottom of the filter, plopped the intake filter into my pool of water and started pumping away. If you’re looking for a no-brainer filter that can remove the full spectrum of bacteria, protozoa, and viruses from your water with basically no maintenance required, then this is the filter for you. 

Backpacking water filters can cost as little as $25 to as much as $400. Much of the difference in cost reflects the long-term durability of the filter, as well as their ability to handle more complex water quality problems, including viruses and heavy metals. 

The question of whether you need a water filter or purifier for backpacking is, to an extent, about the level of risk you are willing to tolerate. In the high alpine of a national park, for instance your odds are low, while if you are sharing a water source with livestock in a national forest, the odds increase considerably. Because the consequences of contracting a waterborne illness like giardia or cryptosporidium can be so severe, and both the cost and weight of low-profile filters like the Sawyer Mini Squeeze, Platypus Quickdraw, and LifeStraw Peak Series is not a burden for most backpackers, I recommend always carrying one in your hiking backpack and treating all your water sources with it. If you are unsure as to whether you need a water purifier—which will also remove viruses like rotovirus and norovirus—contact the land manager where you plan to travel for more information about the waterborne contaminants typically found there. 

Even today, it is fairly common to see a blanket recommendation to always boil your water when out in the backcountry. The reason for this is simple: most backpacking water filters do not remove viruses, whereas chemical treatment methods are not effective at neutralizing protozoa like giardia and cryptosporidium. Boiling water, on the other hand, does it all. The trouble with this recommendation is that boiling water is not only time consuming, but can also require you to bring in an excess of fuel or a backpacking stove if where you are traveling doesn’t have options readily available. Even if you do plan to boil all your water, carrying a lightweight filter is an important backup to have in the event that your fuel sources run out. 

The difference between a water purifier and a water filter comes down to its filter pore size. Traditional water filters have pore sizes, typically .2 microns or 200 nanometers, that are small enough to weed out bacteria and protozoa, but not the much smaller viruses, such as rotovirus (70 nanometers) or norovirus (38 nanometers). Individuals traveling to backcountry environments known to be infected with viruses should take extra caution that their filter will protect them, but it is not common for water sources to be contaminated with viruses.

If you plan to put a ton of miles on your hiking shoes, then the lighter weight of a squeeze-bag style filter may be a higher priority for you. However, if you are only hiking a short distance to camp, then the convenience of a gravity filter or the lower-effort pump filters may be an easier choice.

If you expect the water sources on your next backpacking trip to be unusually dirty—full of swirling sediment or plant matter—then it’s best to choose a backpacking water filter with a prefilter. The prefilter helps to ensure that some of the muck in the water source doesn’t make its way into the more delicate fibers of your filter, which can ultimately clog it and render it inoperable.

The best backpacking water filters produced by reputable brands like MSR, LifeStraw, Katadyn, Platypus, RapidPure, and Grayl provide excellent protection against waterborne diseases in the backcountry. Which filter is right for you will depend in large part on whether you prioritize cost, weight, or long-term durability. While there are increasing numbers of low-profile filters on the market that are capable of removing viruses from water sources, during testing these were so arduous to use after even the first round of testing that I would recommend backpackers pair a chlorine-based disinfectant or even a UV-based purifier like a SteriPen with a water filter instead. 

I will continue testing the backpacking water filters in this review in “tea” colored water at a wetland, as well as in glacial runoff to see how they handle even more challenging conditions.

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