SteriPEN utilizes UV (Ultraviolet light) which destroy the DNA of microbes in seconds. It also comes with a filter and water bottle. Run the water through the filter to remove dirt. Then put in the SteriPEN. The ultraviolet light will destroy microbes and you can drink the water.
So even if you didn't store water, you can still drink any water you might find. This is a great tool for camping or traveling abroad. And really great for rehydrating freeze dried food!