Garden uses for 55-gallon plastic barrels include rain barrels, compost barrels and container gardens, but the barrel must be cleaned to remove any trace of its former use. Food-grade barrels work best to avoid any potential contaminants, particularly if you plan to use the barrel for any purpose related to edible foods, such as a rain barrel or strawberry barrel garden. Do not clean or use a plastic barrel that held toxic or hazardous materials.
Chlorine bleach acts as a disinfectant to kill any germs and bacteria that might be present inside the plastic barrel. It can also kill algae, mold and mildew growth, and rid the barrel of any unpleasant odors if the barrel has been left untouched for extended periods. Remove the lid from the barrel, if possible, to allow access to the inside, and clean the barrel with a solution of 1 to 2 tablespoons of bleach added to 1 gallon of water. If the lid is not removable and you don't want to cut off the top, fill it about one-fourth full of water and add 1 cup of bleach. Secure the small bung cap to the barrel top, tilt the barrel on its side and roll it back and forth so the solution sloshes on all sides. Rinse the barrel several times with clean water to remove any residual bleach.
Soap is an effective cleaning agent for 55-gallon plastic barrels, but should only be used when necessary because it requires repeated rinsing to get rid of the soap residue. Dish detergents cut through grease and oil, making them a natural choice for cleaning barrels that were used to store vegetable oil, for example. Try propping up the barrel upside down and spraying the inside with a garden hose to rinse the soap residue as quickly as possible. Castile soap mixed with vinegar also works to clean the inside of a 55-gallon plastic barrel. This mild soap won't cut through oil as easily as dish detergent, but it is easier to rinse when finished. Add a couple of tablespoons of castile soap and distilled vinegar to 1 gallon of water and swish it around the inside the barrel.
Food-grade plastic barrels often have lingering odors from whatever was previously stored inside the barrel. Sprinkle a box of baking soda inside the barrel to help neutralize the odor. Baking soda is also a mild abrasive, so you can use it to scrub away any stuck-on debris or algae from the sides of the barrel. For best results as an odor neutralizer and abrasive, mix the baking soda with water to form a paste and use a cleaning rag to rub the paste all over the inside of the barrel. Rinse the barrel thoroughly with a garden hose; wipe off the residual paste with a cleaning rag to ensure the baking soda is completely gone. Avoid using vinegar to clean 55-gallon barrels intended for use as planters because residual vinegar can kill plants.
The suds of various cleaning solutions for plastic barrels can clean algae, mold, dirt and debris from the sides of the barrels, but a scrub brush or cleaning rag is usually necessary to remove stuck-on debris. A basic hand-held scrub brush with stiff bristles, an abrasive sponge or even a washcloth is all you need if you can reach the inside and bottom of the barrel. Try a long dryer lint brush if your only access to the inside is through the small hole on top of the barrel. These brushes feature stiff bristles on a flexible wire handle that allows you to bend and scrub all sides of the barrel. If you want to clean around the faucet hole in a rain barrel, a small lint brush, a bottlebrush or even pipe cleaners work to access the smaller hole.