Olive Pomace Protein Hydrolysate Waste Valorization Through Biogas Production: Evaluation of Energy Produced and Process Efficiency

The biogas production from biomasses for energy production, must be considered as last step in the ideal biorefinery supply chain, since the extraction of valuable subproducts has greater importance and must be performed earlier than the final preparation of bio-digestates. This study aims to prove the technical feasibility of carrying out both these processes, or the initial extraction of valuable subproducts and the following production of biogas. Moreover, it aims to quantify the benefits introduced, in terms of biogas production, related to the addition of pomace to the digestate inoculation. For the scope, biogas was produced, in lab-scale apparatuses, starting from different digestate samples; among them, one was enriched with untreated pomace and another with the treated one, consisting of the residual of pomace used for sub-products extraction. Based on the measured quantity of biogas achieved, the energy produced was quantified; conversely, the energy consumed was evaluated according to the plant efficiencies declared in literature. The daily energy production and consumption were compared to define the optimal production period and, according to it, the quantity of energy lost, referred to the unextracted biogas from digestates, was quantified and the efficiency was re-defined. The benefits related to the addition of pomace and, in particular, treated pomace, were defined as a function of the overall energy production, the percentage of energy lost (or produced outside from the optimal production period) and the re-defined process efficiency. Finally, the energy obtainable per unit of dry matter and volatile solids was calculated.