AbstractThe efficient biosynthesis of methane from renewable biomass resources is discussed in this paper. Herbal-extraction process residues (HPR) are an excellent raw material for anaerobic digestion because of their abundant trace elements and fermentation stability. Anaerobic co-digestion of wheat straw with HPR was evaluated at HPR/wheat straw ratios (based on total solids (TS), of wheat straw) of 3%, 5%, and 10 % with anaerobic sludge at 35±1 °C during 30-d anaerobic digestion. The best performance was achieved with 5% HPR added to the reactor, with cumulative methane production of 13,130 mL and cumulative methane yield of 260.5 mL/g TSadded, respectively. Cumulative methane production increased by 31.4% compared to the 9995 mL achieved in mono-digestion with wheat straw. Furthermore, higher activities of protease and total dehydrogenase and higher ATP levels were displayed during the co-digestion process. The high methane yield in this study demonstrates the great potential of co-digestion of renewable biomass as a feedstock for the economical production of methane.