The Katadyn Micropur Purification Tablets (Pack of 20) meet the US EPA water purification standards. Each tablet treats 1 quart (1 liter) of water and destroys bacteria and viruses in 15 minutes, giardia in 30 minutes and cryptospordia in 4 hours. Yes, four hours is a long wait, but most purification tablets don't deal with cryptospordia at all. Just fill up the spare bottle when it's empty so you'll have drinking water ready when you want it. And the best bit? These tablets don't leave any chemical aftertaste.