A system and method for converting animal, vegetable, and food byproduct materials into useful bio-gas and fertilizer, and/or soil amendment products has an anaerobic digester, a liquid-solids separating unit, an ammonia stripper, an ammonia absorption unit, a solids processing unit, and an equilization tank. The system and method use anaerobic digestion to break down the fats, oils, and greases, proteinaceous, and carbonaceous substances contained in the byproducts. Anaerobic digestion creates bio-gas, mainly methane and carbon dioxide, waste biosolids, which can contain phosphorous, nitrogen and other trace minerals, and a liquid effluent with significant ammonia levels. Ammonia, a useful ingredient for fertilizer formulation, is recovered from the effluent. The solids and the extracted ammonia and/or phosphorous can also be used to make fertilizer and/or soil amendment materials. The bio-gas can be used to fuel various energy recovery devices, such as boilers for steam generation, gas engine generator systems for power generation.