Separate collection and conversion method and plant for the reuse of used oils and greases for energy purposes, which method provides one or more of the following steps: —producing and separating the waste from other substances or compounds, —collecting the waste—gathering/storing the waste in at least a collection and/or regeneration facility, —regenerating or treating the waste, —reusing the collected or regenerated or treated waste, an oil or a grease or a mixture containing at least a natural and/or synthetic ester-based oil and/or grease being considered as waste, which oil or grease or which mixture is the by-product or waste of industrial processes.