Introduction

In this article, we review the 6-Liter model of the Platypus GravityWorks Water Filter, intended to provide water for groups and for basecamp scenarios. The potential advantages of this gravity-powered system are the saving of time, effort, and weight in purifying water for groups.

schematic of platypus gravity filter system
Schematic of parts and assembly. From the owners’ manual.

The GravityWorks system has been around since 2008 and is a mature product. We reviewed the 4.0 L incarnation of this filter in 2016. That review found the GravityWorks system to be robust and easy to use, offering good value for weight. According to the design team at Cascade Designs (parent of Platypus), the current version of the filter is little changed from previous models. The most significant change is that the cartridge walls are now thinner. The team found that they could save a bit of weight there without sacrificing robustness or reliability.

Our previous review focused on qualitative factors: ease of use, quality of build, and design. Our current review expands on those observations with a quantitative analysis of key performance factors. Together these reviews should give you a good sense of what to expect from the GravityWorks filter, and whether it is a good fit for your filtration needs.

About This Review

This Limited Review examines some of the key performance parameters of the system: rate of filtration, how this rate is affected by water quality and hydrostatic head, and how this performance declines with use. I also make a qualitative assessment of ease of use based on my experience with similar previous models and compare the GravityWorks filter with its nearest competitors.

platypus gravity filter hanging from a tree
The Platypus GravityWorks 6.0L filter in its basic use configuration: dirty water reservoir hanging from a tree limb, feeding water through the filter to the clean water bladder on the ground. No animals were harmed in the production of this review.

Features and Specifications

  • Gravity driven
  • Effective against bacteria and protozoa, but not viruses
    • Meets EPA and NSF standards for removal of 99.9999% of bacteria and 99.9% of protozoa
    • Pore size: 0.2 µm
  • Field-cleanable by backflush
  • Hollow-fiber filtration media
  • Wide-mouth collection reservoir
  • Simple operation controllable by in-line clamp
  • 6.0L capacity
  • Filtration rate
    • Claimed: up to 1.75 L/min
    • Measured: see text
  • Weight:
    • Claimed: 12.3 oz (350 g)
    • Measured: 12.7 oz (360 g)
  • Packed size: 12 x 6 x 3 inch (30 x 16 x 7 cm)
  • Replaceable cartridge treats up to 1500 L
  • Polyethylene reservoir, silicon tubing
platypus gravity filter, packed in mesh bag
The GravityWorks filter, packed.
platypus gravity filter, disassembled components
The GravityWorks filter components unpacked

Performance Analysis

I tested the GravityWorks filter for several parameters:

  • flow rate with both clean and dirty water sources,
  • dependence of flow rate on hydrostatic head, and
  • loss of flow rate as a function of cumulative liters filtered.

Those are a fair number of variables to examine, and they are variables that I expected to show some dependence on each other. Rather than examine each in a traditional one-factor-at-a-time experimental design, I used statistical design of experiments methods to minimize the number of runs needed to extract the desired information.

To measure the effects of hydrostatic head, I hung the collecting reservoir 3, 5, and 7 feet (the maximum extension of the tubing) above the receiving reservoir. I used tap water for clean water. Per the manufacturer’s recommendation, I backflushed the filter with 0.5 – 1.0L of clean water before every run. To generate dirty water, I stirred up the algae from a downspout rain barrel in my garden and withdrew water from that barrel.

testing the setup of the platypus gravity filter
Testing setup: the dirty water reservoir was hung from a post at 3, 5, or 7 feet (max extension) above the receiving clean reservoir. The rain gauge and bee nesting block were not part of the test protocol.
platypus gravity filter, dirty water
To generate dirty water, I stirred up the algae from a downspout rain barrel in my garden.
platypus gravity filter, clean water
For comparison, here’s what the filtered (clean) water looked like.

Experimental Results

Here’s my raw data:

hydrostatic head (ft)water qualityfiltration time time (hh:mm:ss)filtration rate (L/min)cumulative water filtered (L)ambient temperature (F)
7clean0:03:471.59050
7clean0:04:021.491250
7clean0:04:271.351850
7clean0:03:561.532450
7clean0:03:501.573050
5dirty0:05:531.023658
7clean0:04:391.294258
3clean0:09:390.624858
3dirty0:16:490.365458
7dirty0:06:350.916040
3clean0:11:140.536640
3dirty0:21:100.287240
5clean0:08:150.737840
7clean0:05:561.018447
3dirty0:22:540.269047
7dirty0:16:380.369647
5clean0:11:580.510247
3clean0:17:140.3510847
5dirty0:15:390.3811449
3clean0:17:200.3512049
7clean0:06:340.9112649
5clean0:08:300.7113250

Note: Data from the second run (the 6L run) excluded due to a kink in the tubing.

I used these data to create a multivariate model of test performance using a response surface design. A Bonferroni correction was applied to reduce the false discovery rate of significant factors. Factors that were thus excluded were: temperature, non-linearity of hydrostatic head, and interaction of water quality on hydrostatic head.

Significant factors and their Bonferroni-corrected P values are:

Significant factors and their Bonferroni-corrected P values

The derived multivariate model fits well to the observed data (R2=0.94, RMSE=0.13 L/min, Pvalue < 0.0001).

Here is a graphical representation of the model, showing the relative effects of each of these factors:

prediction profile, head, filtered, and water quality vs filtration rate
Clicking on the graph will take you to an interactive file that will open in a new browser tab.

Conclusions from this analysis:

  1. Dependence on the hydrostatic head is about 0.2 L/min/ft. Not surprisingly, the higher you hang the filter, the faster filtration rates will be. This dependence is linear over the range tested.
  2. The maximum filtration rate is about 1.5 L/min, not 1.75 L/min as claimed by the manufacturer.
  3. Dirty water is filtered about 17% more slowly than clean water. This is less of an effect than I expected. Obviously this depends on just how dirty the water is. A limitation of this result is that I tested scummy (algal) water, but not silty water, which might behave differently.
  4. Filtration rate drops with use. The initial rate of 1.5 L/min dropped to 0.9 L/min after 100 L. That’s a lot. To be fair, most users won’t be filtering as much dirty water as I did in this testing and should see a lower loss of filtration rate. Unless you are hiking the Arizona Trail, I can easily imagine that the drop in rate would be much slower than what I measured.
  5. The loss of filtration rate is non-linear and appears to slow down after an initial rapid drop. This effect is of borderline statistical significance (p = 0.06) but still more likely than not to be real. My physical interpretation of these results is that there are a finite number of sites on the filter to which scum will stick irreversibly.

Qualitative Observations

My testing plan was to use this filter on a Sierra Club ICO trip with middle-schoolers to Canyonlands National Park in April 2020. The coronavirus outbreak led to the cancellation of all our outings for the time being. However, I have used earlier versions of this filter on previous outings. As the basic design and form factors appear little changed, I think it is valid to apply those experiences to the current product.

The operation of this filter system is very simple. Both elementary and middle-school kids have no trouble in both setting it up and getting water from it. Collecting water from still sources using soft-sided containers is always harder than using rigid containers. However, the wide mouth and stiff zip-seal at the opening make collection from still water relatively easy.

Backpacking Light publisher Ryan Jordan has used large-volume Platypus GravityWorks filters for several years with Scouting groups in mountainous areas of the Northern Rockies, including Glacier National Park, Yellowstone National Park, the Bob Marshall Wilderness, and the Absaroka-Beartooth Wilderness and notes that as long as they aren’t filtering extremely silty water, the efficiency of using a system like this as a group filter far outweighs the filtration rate slowdowns that come with filtering clean water.

Earlier versions of this system featured an adapter that allowed filtration directly into a variety of bottles, rather than the supplied Platypus bladder. Although this was convenient, it was not unusual for kids to fill their bottles and forget to clamp the line shut, draining filtered water out onto the ground. Filtration into a closed reservoir prevents this waste.

The narrow mouth of the clean reservoir also makes it difficult to use for water collection, thus minimizing the risk of cross-contamination.

Performance Summary

criterionobservations and issues
Filtration rate1.75 L/min claimed, but 1.5 L/min max rate for a full 6 L batch observed. Not surprisingly, this rate decreases with reduced hydrostatic head, and for dirtier water.
Ease of useSimple to assemble. Wide, rigid mouth facilitates water collection. Walkaway operation. Easy to backflush.
Robust designNo moving parts or power sources required. Sturdy construction. Very few failure modes.
Product lifetimeSignificant drop in filtration rate with use.
backflushing the playtpus gravity filter
Backflush setup: hang the clean reservoir above the dirty reservoir for a minute.

Commentary

Gravity filtration technology is mature. The first version of the Platypus filter was launched in 2008. With several well-established competitors in the field, there are no striking innovations that distinguish one product from another. Instead, these products are differentiated on the basis of execution. I reached out to the design team at Cascade Designs (parent of Platypus) for their view of what distinguishes their product from the competition:

  1. Clean side/dirty side complete system. Makes it easy to backflush and purge air bubbles, reduces the risk of contamination, provides storage capacity so that clean water is always available. Comment: These claims are fair. Several competitors do not include a clean water reservoir. Backflushing only requires placing the clean reservoir above the dirty for about a minute.
  2. Internal testing. Platypus does actual microbiological testing rather than relying on dry particles to verify manufacturing quality. Comment: I can’t vouch for competitors’ methods, but microbiological testing is the gold standard. Bacteria (especially Gram-positive cocci, which can be < 0.5 µm) are flexible and can pass through filters whose pore size is smaller than they are. Also, nominal pore sizes are an average over a distribution. The width of that distribution matters greatly to actual filter performance.
  3. Taste-free. The proprietary film of Platypus products has a thermoplastic polyurethane outer layer that provides strength and durability and is lined with an inner-water contact polyethylene layer. This construction keeps water from tasting like polyurethane.  Comment:  I did not detect any plastic flavor from the clean reservoir. I have had other products, notably an MSR Dromedary bag, that had a strong plastic taste (incubating several hours with a warm, dilute solution of Dr. Bronner’s helped fix this).

The Cascade Designs team also noted that they test 100% of filters inline on their production lines. They also believe that their lifetime claims are more realistic than those of their competitors, and plan to update this spec to best-case/worst-case (i.e., clean vs. dirty) scenarios. Comment: I noted a significant decrease in the filtration rate in less than 100 L use. However, the rate seemed to be stabilizing at about 1 L/min, which is still comparable to rates for mechanical filters.

Compared To

I’ve used the comparable Sawyer product on group outings. In my experience, it is a distinctly inferior product. Flow rates under the best of circumstances were significantly slower than the Gravityworks filter and it clogged readily. Repeated backflushing was required to get it to work at all.

The MSR Trail Base Water Filter System is more expensive, heavier, treats a smaller volume, and has a slower filtration rate. On paper, it loses out to the Gravityworks filter.

The Katadyn Gravity Camp 6.0L appears much more competitive:

criteriacommentsedge
MSRPPlatypus $120, Katadyn $90. Katadyn does not include a clean reservoir.Katadyn
weightPlatypus 12 oz (350 g), Katadyn 10 oz (283 g). The Platypus clean reservoir weighs 3.2 oz (90 g) and can be left at home, making it lighter in a comparable configuration.Platypus
packabilityClean reservoir accounts for most of the small difference.Katadyn
capacity6 L bothtie
pore size0.2 µm bothtie
backflushablePlatypus easy to backflush, Katadyn cartridge must be replacedPlatypus
filtration rateNominal rates 1.75 L/min Platypus, 1.9 L/min Katdyn. Measured rate 1.5 L/min Platypus. Katadyn claim not verifiedKatadyn
rated cartridge life1500 L bothtie

Strengths and Limitations

Strengths

  • Walkaway operation, low hands-on time
  • High capacity
  • Robust design unlikely to fail in the field
  • Field cleanable
  • Good weight to capacity (volume) ratio for groups
  • Easy to collect and transport water

Limitations

  • Does not remove viruses or chemicals
  • No adapter for wide-mouth bottles
  • Requires trees or other support, preferably > 5 feet height
  • Decreased filtration rate over time, even with backflushing

Closing Comments

The Platypus Gravityworks 6.0L Filter is a mature product that addresses the use cases of high-volume water treatment for groups and basecamp scenarios. It is simple to operate and is robust and reliable. It provides clean water at a higher rate and with less effort than mechanical pumps, and at a comparable weight. Its main limitations are that it will not remove viruses or chemicals and that it is more difficult to use in non-forest environments such as above treeline and deserts. I have used this filter for years on group outings and will continue to do so.

Where to Buy

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