Improving methane generation from pistachio skin: integrated environmental engineering assessment and kinetics-driven scale-up

Pistachio skin is an agro-industrial residue that can be bioenergetically valorized via anaerobic digestion (AD). However, this biomass is rich in lignocellulosic compounds, thus limiting its methane potential. This study investigated, for the first time, the use of sodium dodecyl sulphate (SDS), ultrasounds, and SDS-mediated sonication as pretreatments before AD. Batch tests were conducted for 30 days under anaerobic and mesophilic conditions (i.e., N2 environment and 38 °C, respectively) for different conditions, i.e., 0.01 – 0.1 kg SDS/kg TS (R2 – R4), 150 W at 20 kHz (R5), and their combination (R6 – R8). The SDS-ultrasonic pretreatment (R7) resulted in the highest methane yield (i.e., 322 L/kg VS) coupled with a volatile fatty acid accumulation up to approximately 2059 mg HAc/L. Methane production was forecasted using Long-Short Term Memory and Auto-Regressive Integrated Moving Average models, which can improve temporal prediction compared to sigmoidal models due to the model flexibility required for real-time applications. A preliminary energy balance and life cycle assessment were conducted to evaluate the feasibility of pretreatment, achieving a positive balance in R3 in terms of energy and economic benefits (i.e., 1.01 kWh/kg VS and 0.10 USD/kg VS, respectively). Finally, by coupling batch-derived Modified Gompertz kinetics with continuous reactor mode, approximately 1 ton per day of pistachio skin could be treated in a mesophilic anaerobic digester of about 196 m3, with a hydraulic retention time and organic loading rate of 20 days and 4.38 kg VS/m3 d, respectively.