The conventional mesophilic digestion of waste activated sludge (WAS) is challenged by the updated wastewater treatment process. A hyper-thermophilic (70 degrees C)-mesophilic (35 degrees C) temperature-phased two-stage systems without and with recirculation were employed to upgrade the single-stage mesophilic digestion of waste activated sludge in this study. The solid reduction increased by more than 10% in the two-stage systems. Similar methane production was observed in the single-stage system and two-stage systems, with a methane yield of 0.24 L/g VS added. In the case of two-stage system without recirculation, hyper-thermophilic stage played an important role in COD solubilization, with 34.4% of the WAS solubilized in that stage, while mesophilic stage contributed more in methane production, with 36.2% of COD in the WAS produced as methane in the mesophilic stage. Firmicutes became the most dominant bacteria phylum in the mesophilic stages of two-stage systems as compared to the dominant phylum Proteobacteria in single-stage system, with the genus Methanothermobacter cardinal in the hyper-thermophilic stage. The recirculation in the two-stage system increased the microbial community diversity in the hyper-thermophilic stage, which was one factor to result in improved methane production in the hyper-thermophilic stage. (C) 2016 Elsevier B.V. All rights reserved.