Impact of hydrothermal pretreatment at different temperatures on biomethane yield in anaerobic digestion of rice husk

The inadequate conversion of rice husk (RH) into biogas due to its recalcitrant lignocellulosic structure and high silica content necessitates the requirement of pretreatment prior to anaerobic digestion (AD) of RH. This study evaluated the impact of hydrothermal pretreatment on the solubilization of complex lignocellulosic matrix of RH and its subsequent influence on biogas and biomethane yield. The RH was pretreated at six different temperature levels between 200 and 250 °C, at an interval of 10 °C with 20% total solids (TS) loading and retention time of 10 min. The solubilization of RH at different pretreatment temperatures was evaluated by characterizing the liquid and solid fractions of the pretreated RH. The AD of untreated and pretreated RH revealed that the highest biogas and biomethane yields of 355 ± 34 mL/g VS_input and 153 ± 20 mL/g VS_input were observed for RH pretreated at 230 °C, 4.5 and 5.7 times higher than untreated RH. This increase in biogas and biomethane yield for RH230 was confirmed by observing enhanced volatile solid (VS) reduction (38.72%) and biodegradability (39.40%) compared to VS reduction (8.87%) and biodegradability (7.02%) for untreated RH. Further, correlation matrix analysis revealed a strong relation between pretreatment temperature and its severity on the fate of utilizing RH as an AD substrate and obtaining maximal biogas yield. Conclusively, pretreatment up to 230 °C could be recommended to enhance biogas and biomethane yield from RH effectively. Further investigation in pilot-scale reactors is recommended to validate these findings.