| Summary: | In this study, the feasibility of hydrogen and methane production from grass and leaves via hyperthermophilic anaerobic digestion was investigated. The hyperthermophilic treatment of grass at 70 °C resulted in the highest concentrations of volatile fatty acids (TVFA) and reducing sugars in the supernatant of over 21 and 6.5 g/L reported on day 3 and 4 of the experiment. In contrast, hydrolysis and acidification of leaves performed slower and with lower efficiency, as the peak concentrations of TVFA and reducing sugars were observed at the end of the process. However, the highest cumulative hydrogen and methane yields of 69.64 mLH<sub>2</sub>/gVS and 38.63 mLCH<sub>4</sub>/gVS were reported for leaves digested at 70 °C, whereas the corresponding maximum productions observed for grass were 50 mLH<sub>2</sub>/gVS and 1.98 mLCH<sub>4</sub>/gVS, respectively. A temperature increase to 80 °C hampered hydrogen and methane production and also resulted in lower yields of volatile fatty acids, reducing sugars and ammonia as compared to the corresponding values reported for 70 °C.
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