Biogas Production with Residuals Deriving from Olive Mill Wastewater and Olive Pomace Wastes: Quantification of Produced Energy, Spent Energy, and Process Efficiency

At present, taking into account the sustainability of the starting matrices, the biogas production industry is continuously growing, especially in consideration of ecological transition and circularity. The present study deals with the development of anaerobic bioreactors aimed at valorizing two specific wastes of the olive oil supply chain, i.e., the residual of protein hydrolysis process of three-phases olive pomace (OP-PH) and that recovered after the extraction of bioactive molecules from olive mill wastewater (OMWW waste). The energy consumed for biogas production varied from 0.52 kJ (OP and OMWW waste) to 0.97 kJ (OP-PH), while the energy produced for OP, OP-PH and OMMW waste was equal to 1.73, 2.94 and 1.60 kJ, respectively. The optimal production period was defined by considering only the range showing energy production higher than its consumption. According to this, OMWW showed the best performances, since it required 9 days (instead of 12 of untreated and treated OP) to reach the completion. The biogas production efficiency of the threephase OP-PH waste calculated in the optimal production period, i.e., 12 days, was higher than the other samples, with a yield of 76.7% and a quantity of energy potentially producible corresponding to 1727.8 kJ/kg of volatile solids. These results pave the way for possible applications of this procedure for the planning of a multi-purpose biorefinery fed with by-products from the olive supply chain waste, thus promoting the use of sustainable waste materials from a circular economy perspective.