Effect of Methanol-Organosolv Pretreatment on Anaerobic Digestion of Lignocellulosic Materials

Lignocellulosic materials are the most abundant biomass on the planet, representing a great opportunity for energy valorization. This work investigated the effect of methanol-organosolv pretreatment on the methane production from hazelnut skin, spent coffee grounds, and almond shell. A pretreatment on the three lignocellulosic materials was performed at 130, 160, and 200 degrees C for 60 min using a 50% (v/v) methanol solution, with and without the addition of sulfuric acid as a catalyst. The biomethane potential of raw and pretreated substrates was evaluated under wet-mesophilic conditions in batch reactors. The biomethane potential after 45 days of anaerobic digestion from raw hazelnut skin, spent coffee grounds, and almond shell was 17.3 (+/- 32.3), 293.4 (+/- 46.6), and 23.2 (+/- 9.6) mL CH4/g VS, respectively. The methanol-organosolv pretreatment was particularly effective on hazelnut skin, by increasing its biomethane potential up to 310.6 (+/- 22.2) CH4/g VS. On the contrary, all pretreatment conditions were ineffective on spent coffee grounds and almond shell in terms of cumulative methane production. Among the three substates, only hazelnut skin showed significant composition changes due to the pretreatment, reducing the lignin content from 39.66 to 34.73 % and leading to an increase of bioavailable sugars. An energy assessment confirmed the pretreatment efficacy on hazelnut skin, with a net positive energy recovery of 1.35 kWh/kg VS deriving from the extra biomethane produced under the optimal pretreatment condition. Further studies are recommended to look into the optimization and utilization of the liquid waste stream produced after pretreatment.