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Thursday, November 20, 2014

Sawyer Point One Filter Test

This link was sent to us by a friend, so I thought I'd go through the information presented and give my opinion and a review.

The link leads to a PDF file that contains what appears to be 21 slides used in a PowerPoint presentation at Tufts University School of Engineering. In late 2011, “Pure Water for the World” (PWW) distributed 200 SawyerPoint One water filters to households in Honduras for use in cleaning water for drinking and cooking. Since this is a common brand of filter (I use the Sawyer Mini myself), I thought the information that they gathered from a field test might be useful.

According to the slides, the recipients were given training on the use and maintenance of the water filters, including how to back-wash them when flow or quality decreased. Initial laboratory tests confirmed that the Sawyer filters achieved 5-log (99.999%) removal of protozoa and 6-log (99.9999%) removal of bacteria when new. The filters were then used on a daily basis for two months and a few were tested. They were still averaging 99.6% bacteria removal. After almost two years (23 months to be exact), six filters -- 3% of the total shipped out -- were brought back to the lab for testing against a brand new filter. The results were not good.

Of the six filters brought back for testing:
  • One was completely plugged, allowing no flow.
  • Four failed testing for bacteria removal (54% removal).
  • One had ruptured internally, allowing water to pass through unfiltered.
  • All six showed no external damage.
  • Recipients showed proper back-washing procedures when asked.

About half of the slides go into the very boring details of the testing procedures that they used, all standard microbiology and drinking water tests for bacteria and “turbidity” (a fancy way to measure how cloudy or murky the water is) along with electron microscope imaging of the fiber from inside the filters.

There is then a section of slides that explains the mechanics of the Sawyer filters and the types of fouling that they can encounter. Not bad information to know when you're looking for a filter for your own use.

The “Summation” slides at the end point out the limits of the test and suggest further areas of study. I agree with all of the limitations that they listed, but had a few others to add. The limitations they listed were:
  1. Few filters were brought back for testing
  2. Source water quality was largely unknown
  3. Reported use and maintenance could not be verified.

To which I would add the following limitations:
  1. Quantity of source water was not addressed in the study. With a household of 4 people, using a minimum of two gallons of water a day each, you're dealing with about 3,000 gallons a year.
  2. The laboratory tests were sufficient, but the presentation of the results was poor. A good example is the electron microscope imaging of the new filter fiber and the used filter fiber: they chose three images of each, and all six were at different resolutions. This is like comparing a 1:25,000 scale map of California to a 1:1,000,000 map of Kentucky.
  3. No reason was given for the 3% sample rate. Why were only 6 out of 200 filters brought back for testing? Were the other 194 totally inoperative and discarded by the recipients, or did someone just pick the worst ones they could find?
  4. A 3% sample is not normally considered a statistically valid sample (unless you're running a political poll). The results are interesting, but largely meaningless.
  5. Why did they choose a portable filter for use by a household for two years? Despite the marketing claims, small portable filters are made for individual use for a limited time.
All in all, what they have given us is the results of tests on a small number of filters from a large batch that they dropped off in a third-world country with limited training and no observation. From the data presented, I have no idea of how this test is relevant to much of anything other than how not to conduct a field study. My professors would have given this report a "C" at best, but I went to a small college a long time ago.

Even under the conditions they selected, the Sawyer held up quite well for the first two months (still averaging 99.6 % bacteria removal). I don't expect to be using a portable filter for my daily water needs for anything beyond that, so I am still comfortable with my choice of filters. Proper maintenance and pretreatment of your source water is vital to getting the most out of any filtration system, and they are something that you need to take into consideration when selecting a filter.

The Fine Print


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