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Let’s face it: Getting sick sucks. And getting sick in the backcountry miles from the trailhead really sucks. There are few things less pleasant than having to hike for hours or days while nauseous, vomiting, and contending with explosive diarrhea. Yeah, it’s gross and a terrible way to get a trail nickname (e.g. Captain Mudslide or Princess Poopy Pants), but the good news is that you can prevent getting sick by practicing good hand washing and purifying or filtering your water. There are lots of creepy crawlies in springs, streams, and lakes like protozoa, viruses, and bacteria, and you definitely want to make sure to avoid consuming them — especially Giardia.

When it comes to purifying water, there are many helpful methods: boiling, chemical treatments like iodine tabs and chlorine drops, and ultraviolet light. Each has pros and cons. Boiling is safest, but takes a lot of fuel and time, which is especially inconvenient mid-day and makes hot water unpleasant to drink during warm weather. Iodine and chlorine dioxide drops are lightweight and kill many harmful bacteria, protozoa, and viruses, but require 20-30 minutes before drinking and frequently impart an unpleasant aftertaste. Ultraviolet light is effective in killing viruses, protozoa, and bacteria, but sterilizers require batteries, which are added weight and require periodic replenishment. Also, all of the aforementioned methods don’t address solid particles in the water like silt, which is unpleasant to drink. I prefer to find springs or swiftly moving creeks (well away from human agriculture) and filter my water.

Knowing whether to purify or filter depends on knowing what you are trying to avoid ingesting. Water purifiers work by using heat, chemicals, or light to kill microorganisms and neutralize viruses. Water filters work by passing water through an internal element with microscopic pores that filter out very small particles like protozoa and bacteria. The drawback to water filters is that the microscopic holes in their filter media are still too large to catch viruses. The advantage is that they remove most of the creepy crawlies, they’re fast to use, and remove unpleasant tastes and smells. 

For many years, I relied on a pump-style ceramic filter. It was reliable, but a bit of a pain in the ass to have to pump the device repeatedly to slowly produce clean water. Over many pumped liters over many years, over myriad bodies of water large and small, I often dreamed of a better, less labor-intensive way to get potable water in the backcountry. The good news is that there is — enter the Platypus GravityWorks water filter system.

Capacity: 4 ltrrnrnDimensions: 13.2 x 2.8 x 5.8 inrnrnWeight: 11.64 oz


I bought the Platypus GravityWorks water filter system in 2015 at Eastern Mountain Sports right before I took a trip to Wyoming to climb the Grand Teton. Out of the box, the Platypus GravityWorks almost looks like a medical device. It consists of two heavy-duty IV-like plastic bags connected with rubber hoses and a filtration unit. Specifically, the system consists of a 4-liter dirty water reservoir bag, a 12-inch dirty water connecting hose, the GravityWorks filter cartridge, a 48-inch clean water hose with integrated shut-off clamp, a 4-liter clean reservoir, and a mesh carrying bag.

In order to help prevent cross-contamination, Platypus clearly marked DIRTY and CLEAN on the appropriate reservoirs and further differentiated the dirty bag with a large ziplock top opening to make filling easy. Additionally, the filter cartridge is marked with a large blue flow arrow and color-coded connectors to designate dirty and clean ends. Both the clean and dirty water reservoirs have nylon carrying straps with lightweight plastic buckles. 

How we tested the Platypus GravityWorks water filter system

Reviews photo
The Platypus GravityWorks water filter system in action (Joe Plenzler)

My wife and I have exclusively used the Platypus GravityWorks water filter system over the past five years on our section hikes along the Appalachian Trail. We’ve used this system to filter more than 400 liters of water over 53 days in the backcountry. 

Setting up the Platypus GravityWorks is fairly intuitive. The dirty water hose snaps into the dirty water reservoir with a satisfying click. The clean water hose screws into the base of the clean water reservoir. Both hose ends attach to the GravityWorks filter, which is clearly marked with a large flow arrow from the dirty intake end to the clean output end.

Filling the dirty water reservoir is made easy by the large ziplock closure at the top of the bag. The wide mouth design allows the user to catch flowing spring water or scoop creek water to fill the bag. Once filled and zipped, the bag can be hung from a tree branch or placed on a rock, bush, or incline that’s higher than the clean reservoir. The higher the distance between the dirty bag and the clean bag, the faster the water will flow. Once in place, all the user has to do is wait as gravity pulls the water through the filter and into the clean reservoir. There’s no pumping required.

Platypus advertises a 1.75 liter per minute flow rate. On our last trip, our GravityWorks filter was averaging about three minutes to transfer the water from the dirty to the clean reservoir, or a rate of about 1.3 liters per minute. Under optimal conditions, the filter can process 4 liters of water in about 2.5 minutes. The transfer rate depends on the amount of silt and suspended solids in the dirty water, the presence of air bubbles in the filter, the height of the dirty bag over the clean bag, user competence or incompetence, and other physical factors. We’ve filled the dirty bag with some pretty murky water over the past few years, and it’s done a great job producing clear water and getting rid of smells. 

It’s best to start by filling the dirty bag with the clearest water available at the water source. If the flow rate bogs down, it can be markedly improved by backflushing the filter by raising the clean bag over the dirty bag and reversing the flow of the water for a few seconds. The backflushing allows any air bubbles in the filter canister to escape into the dirty bag and also cleans the filter by expelling any dirt that is in the filter tubes. 

Once filled, the clean reservoir can be detached from the filter and hung and used to fill smaller water bottles or serve as a Camelbak-like drinking bladder.

Reviews photo
The Platypus GravityWorks water filter system in action (Joe Plenzler)

What we like about the Platypus GravityWorks water filter system

There’s a lot to like about the Platypus GravityWorks water filter system. Here are ten reasons you should consider it:

  1. The system is lightweight. At 328 grams (11.57 ounces), it won’t weigh you down. Platypus offers a 2-liter system, which weighs in at 309 grams (10.9 ounces) for $99.95, but I strongly recommend going with the larger 4-liter unit for parties of two or more. The doubled capacity (from 2 to 4 liters) is well worth carrying the extra 19 grams (.67 ounce). For larger backpacking groups, Platypus also offers a 6-liter system. For two people, the 6-liter system will be overkill. 
  2. The system is fast, generating up to 1.75 liters of water per minute. I like to take breaks while backpacking, but each minute spent hanging out in one spot detracts from total miles hiked per day. The GravityWorks will get you re-watered quickly and get you on your way.  
  3. The Platypus GravityWorks is a large-volume system. Between the dirty and clean bags, it offers 8 liters of available water. This is especially handy for establishing camp at the end of a long day and reduces the number of trips one has to make between camp and water source. 
  4. The system is time- and energy-efficient. Once set up, gravity does the work — no mechanical pumping required — so you can spend time doing other, more important things like sitting on your ass, relaxing with your feet up, and eating a well-earned snack. 
  5. The system is simple and intuitive, essentially Marine-proof! If you can read, you can set it up. The clear DIRTY and CLEAN markings on the reservoirs and big blue arrow on the filter almost ensure correct assembly and usage. The large ziplock opening on the dirty bag makes it easy to fill it with water, and the small opening on the clean bag makes it hard to fill with dirty water, a morphological reminder of which bag is which to prevent cross-contamination. 
  6. The system is well-made from durable Polyethylene that is 100 percent BPA-, BPS-, and phthalate-free. It is rugged and resists punctures well. 
  7. The system is versatile. The clean reservoir can be used as an in-pack drinking system or hung from a tree branch to serve as a water and handwashing station, or even a shower. 
  8. The system is compact. Emptied of water, the system can be rolled up and takes no more space in a pack than a 1-liter Nalgene bottle. 
  9. The filter is replaceable. Platypus recommends changing filters every 1,500 liters. The replacement filters cost $54.95 MSRP through Platypus, but can be found elsewhere for around $40. 
  10. Most importantly, the filter produces great-tasting, clean water free of most water-borne pathogens. The hollow-fiber technology in the filter screens out any particle larger than 0.2 microns, which means it keeps bacteria and protozoa out but may let some viruses through.    

What we don’t like about the Platypus GravityWorks water filter system

My main complaint about the Platypus GravityWorks system is that it will clog (as will all water filter systems) and require periodic backflushing if using water with a lot of sediment in it. This can be avoided by selecting less turbid water sources or allowing the water to settle before filtering. Also, the filter requires special attention as you need to protect it from falls/impact and from freezing, both of which will damage the filter tubes inside. In freezing weather, care needs to be taken to prevent the tubes from icing up and preventing the flow of water through the system. Also, the zip lock at the top of the dirty bag requires a bit of special attention to ensure it is debris-free prior to sealing.

Lastly, it’s important to note that the GravityWorks filter doesn’t filter out viruses, heavy metals, or chemicals. I’m not as worried about viruses in backcountry water sources as I am about bacteria and protozoa, so this isn’t a major issue for me. Also, water source selection is important and I don’t recommend you drink the stuff from the cooling pond at the Three Mile Island nuclear reactor as the GravityWorks filter isn’t designed for that. However, I’ve used the Platypus GravityWorks on over 16 trips on the Appalachian Trail and have maintained digestive harmony for over 600 miles.    


The Platypus GravityWorks water filtration system is mature technology that reliably provides a high volume of great tasting potable water in minutes. The durable construction, field-cleanability, and weight-to-capacity ratio make this an excellent choice for water filtering in the backcountry.

Reviews photo
The Platypus GravityWorks water filter system (Joe Plenzler)

FAQs about the Platypus GravityWorks water filter system

More questions? Here’s Task & Purpose’s additional brief. 

Q. How much does the Platypus GravityWorks water filter system cost? 

A: The MSRP is $109.95 for the 4-liter unit from Platypus, which also offers a 6-liter option for $119.95 and a 2-liter option for $99.95. Cheaper versions are available on Amazon.

Q. How does the Platypus GravityWorks filter work? 

A. The filter is essentially a plastic tube filled with hollow fiber filtering media that allows water molecules to pass through, but stops anything larger than 0.2 microns from passing through. Most of the creepy crawlies in water are larger than 0.2 microns.

Q. Is the Platypus GravityWorks system easy to clean? 

A: Yes. After a trip, I disassemble the bags and hoses and clean them with mild soapy water and then rinse until clear. I’ll also backflush the filter with clean tap water and set the entire system to dry on a bottle drying rack. 

Q. How do you protect the Platypus GravityWorks filter from damage? 

A: In above-freezing weather, I just roll the clean bag around the filter and put it in the top of my pack. In below-freezing weather, I carry the filter in my zip-up breast pocket to keep it from freezing. I also sleep with the filter in my sleeping bag during below-freezing weather.

Q. How often should I backflush the Platypus GravityWorks filter? 

A: I backflush the filter every time I use it. Doing so at the start of the filtration process speeds up the flow. I also backflush it when I get home to prepare it for storage between uses.

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Joe Plenzler is a Marine Corps veteran who served from 1995 to 2015. He is a backcountry expert, long-distance backpacker, rock climber, kayaker, cyclist, wannabe mountaineer, and the world’s OK-est guitar player. He supports his outdoor addiction by working as a human communication consultant, teaching at the College of Southern Maryland, and helping start-up companies with their public relations and marketing efforts.