If an existing underground tank that stored another type of fuel will be converted to store E85, the tank should be carefully cleaned. Storing gasoline underground causes some particulates to settle out and form a petroleum sludge; introducing alcohol into the tank without cleaning it will place this sludge into suspension and lead to serious problems with vehicle drivability.
There are several methods that can be used to clean the tank. Using a “filter agitator” device lowered into the tank moves the product and allows a filter to catch any suspended particles. The tank can also be cleaned by physically entering and steam cleaning it. A robotic cannon can liquefy the sludge so it may be pumped out and disposed, or a chemical cleaner may be placed in the tank to clean the walls, and any remaining particulates may then be pumped out. With any of these methods, consult with a certified and bonded company familiar with cleaning petroleum storage tanks.