Towards the understanding of hyperthermophilic methanogenesis from waste activated sludge at 70 °C: Performance, stability, kinetic and microbial community analyses

•Hyperthermophilic digestion of waste activated sludge at 70 °C was characterized.•Methane yield obviously decreased when the hydraulic retention time was below 10 d.•More than 10.2 g VS/L/d resulted in the instability of the methanogenic system.•The kinetic analysis showed that some non-degradable fraction could be converted.•The genera Methanospirillum and Coprothermobacter played significant roles at 70 °C.