Doubt has been cast on a much-lauded method of disinfecting water using only sunlight, after a study found that it doesn't reduce diarrhoea among children in families using the technique. Solar Water Disinfection (SODIS), a low-cost water purification method that uses only sunlight to disinfect water, is currently used by about three million people in 30 countries, according to the SODIS Reference Center in Switzerland.