Comparison of hyper-thermophilic–mesophilic two-stage with single-stage mesophilic anaerobic digestion of waste activated sludge: Process performance and microbial community analysis

• Hyperthermia at 70 °C and recycling were employed to upgrade mesophilic digestion.
• The hyper-thermophilic–mesophilic process improved the solid reduction by over 10%.
• Hyperthermia solubilized 34.4% of the COD in waste activated sludge.
• Firmicutes was the most dominant in the mesophilic stages of two-stage systems.
• Recirculation improved the diversity of species in the hyper-thermophilic stage.

The conventional mesophilic digestion of waste activated sludge (WAS) is challenged by the updated wastewater treatment process. A hyper-thermophilic (70 °C)–mesophilic (35 °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.