Describes different methods of removing various contaminants from water

http://www.howitworks.net/how-water-filters-work.html

Summary:
Most water filtration systems today use activated charcoal to remove many poisonous substances. This is due to the porous quality of the charcoal, which absorbs many impurities. Another method to purify water is reverse osmosis, though it likely wastes too much water to be a viable option for Kenyans. Ion exchange is used to remove dissolved salts in water. Finally, distillation is the process of boiling water so that the condensed steam is mostly contaminate-free.