Enhanced lignocellulosic component removal and biomethane potential from chestnut shell by a combined hydrothermal–alkaline pretreatment

This study proposes new perspectives for the management and biorefinery of wastes deriving from the agri–food sector such as chestnut shell (CS), which was here used as an organic feedstock for biomethane production through anaerobic digestion (AD). 1–5% alkaline (i.e. NaOH and KOH), hydrothermal (i.e. at 100 °C) and combined hydrothermal–alkaline pretreatments were employed to enhance the CS biodegradability prior to biochemical methane potential (BMP) tests conducted under mesophilic conditions. The hydrothermally–pretreated CS with 3% NaOH achieved the highest biomethane yield of 253 (±9) mL CH4·g VS−1 coupled to a volatile solid reduction of 48%. The hydrothermal–alkaline pretreatment positively affected both delignification and hemicellulose polymerization, promoting an approximately 2.4–fold higher substrate biodegradability compared to the untreated CS, which only reached a CH4 production of 104 (±5) mL CH4·g VS−1. AD proceeded via volatile fatty acid accumulation, subsequently followed by methane production that was effectively simulated via the modified Gompertz kinetic having a R2 of 0.974–0.999. Among the physical–chemical parameters characterizing the CS, the soluble chemical oxygen demand (sCOD) was highly correlated with the BMP showing a Pearson coefficient of 0.952. The cumulative biomethane yield, the sCOD and the cellulose, hemicellulose and lignin amount of CS were also processed through the least square method, obtaining a useful regression equation to predict the BMP. The economic assessment indicated that the hydrothermal–alkaline pretreatment is a cost–effective method to improve the BMP of CS, also for future full–scale applications.