Sunlight can destroy pathogens in water, making it safe to drink. But how much sun is needed? The answer to this question can mean the difference between life and death.
Each day, 4,000 children around the world under the age of 5 die as a result of drinking contaminated water.
One of the cheapest and easiest ways to kill harmful pathogens in water such as cholera is to fill clean PET bottles with the suspect non-turbid water and place them in the sun. The sun’s ultra-violet (UV) rays then go to work. This process is called Solar Water Disinfection (SODIS) and is already being used quite extensively in developing nations.
As for how long the bottle must stay in the sun, the answer is at least six hours – but it could be as much as 2 days or even more depending on cloud cover. Other factors influencing the time required include the distance from the equator, time of year and altitude.
For such an important process, it would be useful to know exactly when the bottle of water has been exposed to enough UV light – and such a device would need to be robust and very easy to use.
This is where WADI comes into play. It’s a compact, solar powered UV-measurement device that requires no batteries.
WADI is placed next to the PET-bottles being exposed to sunlight, or screwed onto the top of a bottle. A flashing sad face initially displays on the WADI screen along with bar indicators showing the stage of the process. When the water is safe to drink, a flashing smiley face appears, accompanied by 5 bars.
Using WADI, a 99.9% reduction in common coliform bacteria in water has been reached in all tests. The device meets WHO (World Health Organisation) microbiological performance criteria for household water treatment.
The manufacturer of WADI, Helioz, provides a two-year guarantee; but expects the device to have a longer service life. Once the solar cell is no longer able to generate sufficient energy, the face and the progress bars on the display stop flashing, or are not visible at all.
Another device we’ve mentioned in the past that uses a smiley face disinfection indicator is Solvatten. It’s around the same price as the WADI for personal buyers (EUR 30 – around AUD $45) and is an all-in-one solar water disinfection unit.