How to Clean, Prepare and Fill a Water Barrel Now that you have purchased your 30-gallon water barrels, do you know how to store water so that it will be clean and safe to drink? Do you just stick the hose into the barrel and fill? No, not a good idea. Here's a step-by-step method to prepare them that will ensure clean, potable water when and if you need it in an emergency. Various Water Storage Container Sizes & How to Purify the water.