MSR Guardian PurifierIntroducing the world's most advanced portable purifier: the Guardian. Co developed with the US military, it's been designed to be the safest, easiest way to purify water wherever you are in the world. With a Hollow Fibre cartridge it removes viruses, protozoa, bacteria and sediment from even the dirtiest water. On each stroke the purifier uses 10% of its water to flush the contaminants from its filter, ensuring a fast flow rate regardless of water