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One thing I’ve yet to figure out… Intuitively, bigger things are easier to block than smaller things, right? In terms of size:
o bacteria – 0.2 to 3.0 microns
o cysts – 5.0 to 15 microns
So, how come filter stats all tend to show higher block rates of smaller bacteria than bigger protozoa cysts? For example:
First Need – Bacteria – 99.9999% / Crysts – 99.99%
LifeStraw – Bacteria – 99.9999% / Cysts 99.9%
‘Enquiring’ minds want to know.
To call itself effective against bacteria, a filter has to remove 99.9999%;to call itself effective against cysts, only 99.9%.
So while some filters (e.g., Sawyer) might be much more effective, they might hesitate to,say so because those efficacy claims can be very expensive to substantiate.
That's my story, and I'm sticking to it. Still, I'd spend a little extra for the filter that does make and substantiate the greater assertion. Just my 2¢ worth
Hi Rick — so it's basically claiming just enough to pass the legal bar. Never thought of it that way, but makes sense. Do you have a ready link by any chance?
Hi Ben
What they are claiming is that they meet the EPA requirements for 'purification', which are different for viruses, bacteria and protozoa.
Except that many filters skip the virus bit, so they are not 'purifiers', just filters.
See
A Survey of Water Hazards and Water Treatment Methods – Part 1 (Overview of bugs and wogs)
http://www.backpackinglight.com/cgi-bin/backpackinglight/water_treatment_methods_part1.html
A Survey of Water Hazards and Water Treatment Methods – Part 2 (Remove, kill or deactivate)
http://www.backpackinglight.com/cgi-bin/backpackinglight/water_treatment_methods_part2.html
A Survey of Water Hazards and Water Treatment Methods – Part 3 (Summary of methods)
http://www.backpackinglight.com/cgi-bin/backpackinglight/water_treatment_methods_part3.html
A Survey of Water Hazards and Water Treatment Methods – Part 4 (Chemical methods)
http://www.backpackinglight.com/cgi-bin/backpackinglight/water_treatment_methods_part4.html
A Survey of Water Hazards and Water Treatment Methods – Part 5 (Filtration methods)
http://www.backpackinglight.com/cgi-bin/backpackinglight/water_treatment_methods_part5.html
A Survey of Water Hazards and Water Treatment Methods – Part 6 (UV methods)
http://www.backpackinglight.com/cgi-bin/backpackinglight/water_treatment_methods_part6.html
Cheers
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