NOVELTY - Producing biodiesel using a multi step process involves subjecting a biodiesel feedstock to an acid esterification process, a transesterification process, and a purification process.    USE - For producing biodiesel using a multi step process (claimed), which is useful in conventional diesel engines.    ADVANTAGE - The method and system for producing biodiesel increases energy efficiency by reducing the overall amount of energy associated with producing the biodiesel. Unlike most systems which use a batch procedure, the instant method and system utilize a continuous flow sonification procedure which provides for reactions to take place in tubes as the biodiesel moves along through the process. The continuous flow sonification procedure is performed in two main stages, utilizing the steps of acid esterification, transesterification, and a waterless washing. The combination of equipment and the method increases the energy efficiency of the overall production process by reducing the energy levels needed to produce the high quality biodiesel and reduces the Global Environment Energy Technology lifecycle analysis model for biodiesel production. The method reduces the energy needed to produce each gallon by up to 50%. The system even allows for processing of animal oils and fats and other solutions that are not clean, such as restaurant grease. The method and system uses low pressure, low temperature, and no open flames, thus providing a much safer design than currently used systems.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a system for producing biodiesel using the multistep, continuous process comprising: at least one first reaction unit adapted for converting a biodiesel feedstock into a biodiesel using an acid esterification process; at least one second reaction unit adapted for converting a biodiesel feedstock into a biodiesel using the transesterification process, where the second reaction unit fluidly coupled to the first reaction unit; and at least one third reaction unit adapted for converting a biodiesel feedstock into a biodiesel using a purification process, where the third reaction unit fluidly coupled to the second reaction unit.