"I used this filter for an extended expedition in Central Asia. Due to the design you are able to pump contaminated water through the system if the O-ring fails (or falls out), which it did. The O-ring is located on the actual filter element, which you have to remove and wash repeatedly every few liters or so.
Essentially, without the O-ring, the water will circulate around the filter element. Since this happens internally, there is no way to catch this defect.
And personally, this particular design defect resulted in a severe case of Cholera, loss of 30 pounds in a week, and a near death situation in an already unforgiving locale.
This is not an isolated case, as I have talked to other individuals who have experienced the same malfunction. I'm sure the filter works well when it wants to, but in expedition situations - I personally demand more.
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