When we were in the MTC we were instructed on how important it was to drink only purified water. Only bottled water should be used, even for brushing our teeth. When we got to Poland, the mission doctor emphasized that even the water that we rinse our dishes in should be purified. Now of course we don't use bottled water for washing and rinsing dishes, so they taught us how purify it ourselves. You can boil it and then put it through a coffee strainer, or the more simple option is just to add a few drops of Clorox to the water. Naturally that sounded like the easier and more practical solution.