SODIS: Disinfecting Water with Sunlight

Every rung so far needed something: alum, a kiln, graphite rods, a lamp, a pump. This one needs a clear plastic bottle and a sunny day, and it is endorsed by the World Health Organization for exactly that reason. Aftim Acra and colleagues at the American University of Beirut showed in the early 1980s that sunlight in a transparent bottle inactivates the organisms that cause diarrhoeal disease. Two things do the work together: UV-A in the sunlight damages the cells directly, and the heat makes them far more vulnerable to it — above about 45 °C the two effects multiply rather than add. You will run it properly, with a control, and measure the one variable that decides whether it worked.