Scientists at the Swiss Institute for Environmental Science and Technology say that this simple ‘solar water disinfection’ works by using both the sun’s UV rays and heat. They say it can kill almost all germs that cause cholera and diarrhoea. It can save on fuel for boiling water for drinking.

What you need:
• plastic bottle with a lid (no bigger than three litre bottle, or it will not work well)
• sunshine

1. Wash the bottle and lid well with soap and rinse.
2. Fill the bottle with filtered water and close the lid.
3. Stand it in the sunshine on the roof or on a piece of tin, to maximise heat. In hot sun six hours is enough. If it is cooler leave it for up to two days. It is now ready to drink.
You can also use this method to disinfect clear, unfiltered water – such as from a clear-flowing stream in which there may be germs.