This fellow does all sorts of testing on tools, oil filters, and about anything else he and his listeners can think of. Based on what I've seen of his videos, he is a reliable source for information.
This video compares various well known and some not so well known small scale filtration systems. The information could be invaluable for your emergency stash, or even for your day to day wants and needs:
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Harvesting Rainwater for your Homestead in 9 Days or Less by Renee Dang