Solar disinfection
Disinfecting water using the natural UV rays of the sun is a common practice in the developing world. However the time needed for water to be exposed in the sun varies depending on the climate conditions and this can become a major problem, when people consume water that has not been sufficiently exposed. To address this problem, Martin Wesian developed the Wadi, an affordable device that measures the UV rate of the sun and notifies the user when the water is disinfected. The Wadi can replace a normal plastic bottle cup and shows a happy face when the water is drinkable.
Read more about this amazing invention at Fast Co.Exist.