The "drinkable book" combines treated paper with printed information on how and why water should be filtered. Its pages contain nanoparticles of silver or copper, which kill bacteria in the water as it passes through. In trials at 25 contaminated water sources in South Africa, Ghana and Bangladesh, the paper successfully removed more than 99% of bacteria. The resulting levels of contamination are similar to US tap water, the researchers say. Tiny amounts of silver or copper also leached into the water, but these were well below safety limit