ethanol is produced efficiently by employing a drive - through pit in which carbohydrate - containing material is fermented by hydrolyzing agents such as enzymes . the configuration and depth of the pit are such as to permit conventional farm vehicles to have ready access to the pit , thereby enabling the pit to be loaded and unloaded conveniently . the pit is insulated on its sides and bottom and is closed at the top by a heat - insulating cover . the pit is heated to assist the fermentation process . after several days of heating , the pit is drained through a sediment trap and the filtered pit drainage is separated into concentrated ethanol and a mixture of water and enzymes . the solid residue left in the pit is removed for use as feed or fertilizer . the separated water and hydrolyzing agents are stored in a reservoir for subsequent flooding of the pit when a new quantity of carbohydrate - containing material is added to the pit .