Anaerobic co-digestion of kitchen waste with hyperthermophilically pretreated grass for biohydrogen and biomethane production

•Co-digestion of kitchen waste with grass at 55 °C produced 293 NmlCH4/gVS.•Methane yield from grass mono-digestion did not exceed 131 NmlCH4/gVS.•Digestion process at 35 °C turned into dark fermentation.•Ruminococcaceae, Atopobiaceae and Lactobacillaceae families were found at 35 °C.•Petrotogaceae, Synergistaceae and Methanosarcinaceae families were reported at 55 °C.