Just checked Cabella's web site. Here's what they say:

"removes 99.9% of giardia, cryptosporidium and other contaminants down to 3 microns," (that's without a decimal point)

REI says the contaminants can be as small as .5 (with decimal) microns so that leaves a lot of room for bad things to go through the filter.

The words in the quote above don't really say the filter takes out 99.9% of the contaminants. It says it takes out 99.9% of the contaminants down to 3 microns in size. I could say, similarly, that a chain link fence fabric takes out 99.9% of the contaminants down to 2" in size.

Soooo, I like the gravity system idea but I would use a finer filter (and probably not bother with the micropur tablets in the Pacific Northwest).