Lignocellulose biomass for bioethanol production is one way for solving the environmental problems of the agricultural residues, besides the production of renewable fuel for the transportation sector, however, the bioethanol production is still inefficient due to a large amount of remnant waste left. So the anaerobic digestion (AD) of this remnant could be an effective solution to increase the overall energy output from the biomass. This study aims to explore the overall energy output and sustainability from the olive mill solid waste (OMSW) through bioethanol production and the utilization of the remnant residues for biogas production by exploiting the organic waste-energy nexus as a circular economy model. The used remnant for biogas production was obtained following bioethanol production from OMSW subjected to microwave-assisted maleic acid, formic acid, or distilled water which were used for the pretreatment, and was followed with enzymatic hydrolysis and fermentation process. The biochemical methane potential results fitted the modified Gompertz model. No influence of pretreatment was observed on the maximum methane production. A clear difference was observed on the kinetic constant and on the lag phase. The maleic acid pretreatment showed the highest energy conversion efficiency of 71.0% and the energy yield for the combined ethanol production and biomethane production was 12.92 GJ fuel per ton of dry weight of substrate (10.3 MJ/kg from methane and 2.6 MJ/kg from ethanol). This study shows that the synergy between bioethanol production and AD process could exploit the OMSW – energy nexus.
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