The following comes from National Geographic“s April 2010 special issue on water, written by Mark Jenkins:

“Retrieve a discarded water bottle. Tear off the label and fill with any water that”s not too murky from a creek, standpipe, or puddle. Place the bottle on a piece of metal in full sun. In six hours the UVA radiation will kill viruses, bacteria, and parasites in the water, making it safe to drink.

SODIS, the acronym for this Swiss-pioneered water-disinfection program, is now being used all over the world to provide drinking water for some four million people. “It”s simple, it”s free, and it”s effective,” says Ibelatha Mhelela, principal of the Ndolela Primary School in Tanzania. In 2006 her school started using SODIS to disinfect its contaminated tap water, placing bottles on the building”s corrugated metal roof. The result? Absenteeism due to diarrhea droped considerably, and examination scores soared. “Before we started SODIS, only 10 to 15 percent of the children passed the national sixth-grade exams,” says Mhelela. “Now 90 to 95 percent of the students pass.”