The BBC has a story on using the sun to sterilise water. The low-tech process, which is currently in use in Africa, seems simple enough, using regular disposable soda bottles.
Here’s how it works : fill the plastic bottles, put the lids on, then put them on a black-painted roof where they stay for a whole day. The sun heats the water, helped by the black roof, which helps to absorb the heat.
Solar radiation means a combination of ultra-violet rays and heat destroys the bacteria which cause common water-borne diseases like cholera, typhoid, dysentery and diarrhoea. After eight hours in the sun, the water is ready to drink. If the water reaches more than 50 degrees celsius, it is safe in just one hour.