Integrated alkaline–hydrothermal pretreatment of pine nut shells: effects on methane production and modeling assessment

This study investigated, for the first time, the effect of combined hydrothermal and alkaline pretreatment on methane (CH4) generation from pine nut shells (PNS), a renewable substrate for anaerobic digestion (AD). The experiments were conducted under mesophilic conditions for 40 days in batch mode. With 5% NaOH following a hydrothermal (HT) pretreatment at 100 °C, lignin and hemicellulose removals of 48% and 37% were achieved, respectively. This resulted in a cumulative CH4 yield (CMY) of 228 mL CH4/g volatile solids (VS), which was approximately four times higher than the control (i.e., 60 mL CH4/g VS). Also, HT at 100°C + 5% NaOH achieved a net energy balance of 1.302 MJ/kg VS. The enhanced methane yield and reduced digestate mass (i.e., 56% on a VS basis) effectively offset the high hydrothermal input, demonstrating that joint pretreatment can be energetically feasible when biogas benefits outweigh operating costs. In parallel, this study proposes a comparative modeling framework by evaluating time-series approaches, i.e., autoregressive integrated moving average (ARIMA), seasonal ARIMA (SARIMA), and long short-term memory (LSTM) models for both CMY and daily CH4 yield (DMY). High predictive accuracy, with R2 values up to 0.9990 for CMY and 0.9365 for DMY, was achieved via ARIMA and LSTM, respectively. Integration of pretreatment and forecasting tools enables real-time AD optimization within a circular bioeconomy.