In order to upgrade the conventional mesophilic anaerobic (MD) of waste activated sludge (WAS), a thermophilic pre-fermentation and a recycle system were introduced to form continuous thermophilic-mesophilic temperature-phased anaerobic digestion (TPAD) and TPAD with recycling capability (TPAD-R). The synchronous operation, with the MD as control, indicated significant improvements in reduction of solids and the ability to produce methane in the TPAD and TPAD-R, with a similar amount of improvement in both systems. VS reduction was improved from about 40% in the MD to 50%, and the methane recovery rates were improved from 0.53 L/g VS destroyed in the MD to 0.63 L/g VS destroyed accordingly. The thermophilic stage in the TPAD and TPAD-R made a large contribution to organic matter degradation and solubilization, and the specific hydrolysis rate in thermophilic stage attained 0.2 gCOD/g VS/d. Furthermore, under experimental conditions, the thermophilic stage in the TPAD and TPAD-R also played an important role in acidogenesis and methanogenesis. The thermophilic pre-fermentation made Firmicutes and Methanosarcina become the main phylum and genus in the mesophilic stage, accounting for 44% and 54%, respectively. The recycle system improved the diversity of bacteria and archaea in the thermophilic stage of the TPAD-R. The TPAD and TPAD-R also achieved about twice the net energy of the MD.
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