Boiling: Handy to know the answer if you are out backpacking or taking a trip to the less civilized parts of the world. Common options are to use chlorine, a ceramic filter, iodine tablets or to boil the water to treat it. Boiling water is the only known method that is 100% effective in destroying pathogens known to cause sickness. The Centers for Disease Control have a very handy chart comparing the effectiveness of various water treatments. It may not be as convenient as the other methods but it works and is effective.
What could you teach this civilization?