Hydrothermal pretreatment of rice straw at relatively lower temperature to improve biogas production via anaerobic digestion

Although the degradation mechanism of straw anaerobic digestion is still obscure, lower temperature thermophysical pretreatment might be a feasible way to improve biogas fermentation efficiency and net energy production in whole slurry. In this study, the performances of rice straw (RS) degradation and biogas production were examined at different pretreatment temperatures from 90℃ to 130℃ to identify the optimal process. With increasing pretreatment temperature, the change in cellulose and hemicellulose degradation in all the tests was directly related to volatile fatty acids (VFAs) content, but did not correspond to the variation in cumulative methane production. Both 100℃ and 130℃ presented similar ideal digestion performances with highest methane yields of 127.6 and 124.6 mL/g TS, respectively, which were 22.80% and 19.83% higher than that noted in the control. Although test at 100℃ pretreatment, which achieved 12.8% higher net energy production from RS than that observed in the control, could be considered as the optimal choice, the surplus biogas could only meet 75.9% of energy requirement for pretreated water temperature shift. Nevertheless, mesophilic or thermophilic fermentation, lower pretreatment water input, and reuse of excess heat are recommended as feasible working conditions for improving net biogas production.