http://www.survivalnewsonline.com/index.php/2014/05/disinfecting-water-with-the-sun/

Survival Tips S2 E7 — Harness the power of the sun to disinfect ground water. Note that this process does not sterilize the water; algae may still live in the container, but it is usually harmless. Neither does this process remove chemical contaminants. But it is an effective way to kill pathogens in pond water, for example.