Great for backpacking, or other outdoor pursuits, these are the only EPA registered water purification tablets on the market. Micropur Tablets release chlorine dioxide into the water which is effective against viruses, bacteria, cryptosporidium and giardia. One tablet treats one liter (1.05 quart) of water. In addition to eliminating harmful microbes, the taste and odor of treated water is improved. Treatment requires up to four hours. Package of 20 tablets.