Solar water disinfection is a common water purification method in the developing world, but it can be difficult to determine when the water is safe to drink. This is where the wadi comes in. When the simple (and cheap) bottle top is placed on a PET plastic bottle that's been left in the sun for a few hours, it will let the drinker know when its contents have been well and truly disinfected.
Although solar water disinfection has been endorsed by the UNICEF, the World Health Organization (WHO) and the International Red Cross, depending on the sun's position and other environmental factors, the process could take 30 minutes or 2 days. The Wadi measures the sun's rays and shows a smiley face when it is clean drinking water, avoiding further confusion or health problems.