The CDC's recommendations for making tap water safe to drink in emergencies also can be used to treat water from a rain barrel.   Methods of treating water to make it potable (safe for drinking) include: -- Boiling: Bring water to a boil for 1-3 minutes to kill viruses, bacteria, and parasites. Let the water cool for a few hours before drinking.  -- Add disinfectants: Add a small amount of unscented household liquid chlorine bleach or chlorine dioxide tablets to decontaminate the water. See the CDC's website for specific instructions.  -- Use a water filter: If your household water filter is labeled as certified for NSF/ANSI Standard 53 or Standard 58, that means it can remove disease-causing parasites from water. However, filters don't get rid of bacteria or viruses.