They’re not as sleek-looking as other manufacturer’s filters, but I’ve owned two versions of the First Need portable purifier over the past 20+ years, and they’ve served me well. Remember there’s a distinct difference between a
filter and a
purifier; personal preference, usage environment and acquisition/filter costs will influence which you choose.
A little marketing hype from their
web site: “First Need portable purifier continues to be the only chemical-free portable system independently certified to meet EPA Microbiological Guide Standard for removal of bacteria, cysts and virus - all without chemicals, hold time or double pumping.”
It’s not as well-suited as other smaller units for backpacking due to its somewhat larger bulk and weight (15 oz. total weight), but I like not having to use chemicals and avoiding the associated wait time.
Among filters, the
Katadyn Hiker (made/marketed by PUR Outdoor until acquired by Katadyn in 2001) has always gotten high marks; I haven’t personally used one. Speaking of Katadyn, their Pocket Filter is a well-regarded classic, albeit pricey at around $200. But, in fairness, they claim its ceramic cartridge will filter ~13,000 gallons under average conditions vs. 200 gallons for a Hiker cartridge, and 125 gallons for a First Need cartridge. Under heavy usage conditions, the Pocket Filter makes more economical sense.
You might find this
water filter GearFinder on Backpacker Magazine website a useful research tool.
Edit: I might have to recant on accuracy of the GearFinder. Tried it, specifying virus removal as a requirement, yet it listed several
filters. The basic distinction between filters and purifiers is that filters don't remove viruses (unless chemicals are used after filtering). REI has a decent article on their website:
How to Choose a Water Filter or Purifier.