NOVELTY - A system has primary stage for producing concentrated fats, oil and/or grease stream, secondary stage for processing concentrated fats, oil and/or grease stream, and intermediate byproduct reuse stage for recycling water, heat and/or alcohol from secondary stage to primary stage. The primary stage has processing stages comprising solid removal stage, water/oil separation stage and/or purification stage. The secondary stage consists of esterification stage and transesterification stage.    USE - System is used for converting waste stream comprising admixture of fats, oil and/or grease and waste solids into biodiesel fuel (all claimed) used in diesel engines.    ADVANTAGE - The system processes waste stream containing 100% free fatty acids, removes sulfur, and does not require caustic chemicals, and provides biodiesel economically.    DETAILED DESCRIPTION - A system has a primary stage for producing concentrated fats, oil and/or grease stream, a secondary stage for processing the concentrated fats, oil and/or grease stream for esterification of waste stream into biodiesel, and an intermediate byproduct reuse stage for recycling water, heat and/or alcohol from secondary stage to primary stage. The primary stage consists of an apparatus for at least partially dewatering fats, oil and/or grease waste and an apparatus for removing solids of fats, oil and/or grease waste, and processing stages comprising solid removal stage, water/oil separation stage and/or purification stage. The secondary stage consists of esterification stage and transesterification stage, in which at least one of stages involves removal of water, heat and/or alcohol. The energy and other byproducts from the secondary stage are reused in the primary stage to reduce the utilization of water and heat to minimize the generation of other waste byproduct from biodiesel generation. At least one of processing stages involve introduction of water, heat and/or alcohol. An INDEPENDENT CLAIM is included for method for converting waste stream comprising admixture of fats, oil and/or grease, and waste solids into biodiesel fuel.