BACKGROUND The present study compared the performance and capability of thermophilic-mesophilic, temperature-phased anaerobic digestion (TPAD) of wastewater sludge at 45 and 55 degrees C in terms of rate of hydrolysis and methanogenesis at two point interval times - December and January - with operational conditions kept constant (total solid content 6.5%, solids retention time 12.5 days). RESULTS The reduction of volatile solids (VS) was 77% at 45 degrees C TPAD-1 and 72% at 55 degrees C TPAD-II. The accumulation of ammonia and volatile fatty acids (VFAs; propionic acid) were observed to be below the inhibitory range (>3000 mg L-1), whereas the methane (CH4) production (45 degrees C, 3.55 +/- 0.47 L CH4 L-1 day(-1); 35 degrees C, 1.44 +/- 0.12 L CH4 L-1 day(-1)) remained considerably higher in TPAD-I than TPAD-II. Furthermore, the TPAD-II system suffered from certain degree of instability due to high level of total VFAs (6087 +/- 1578 mg L-1), low buffering capacity, increased level of total NH3 (2982 +/- 219 mg L-1) and free NH3 (246 +/- 25 mg L-1), and relatively reduced CH4 production (1.69 +/- 0.1 L L-1 day). The bacterial and archaeal population of the TPADs investigated by 454 pyrosequencing and Illumina sequencing showed, in both TPAD systems, a bacterial community dominated by Firmicutes, followed by Bacteriodetes, Proteobacteria, Synergistetes and Actiniobacteria. The archaeal community was dominated by Methanimicrobia (74-84% Methanosarcina) and Methanobacteria (15-27% Methanobacterium). A progression from genus Clostridium to Coprothermobacter and Tepidanaerobacter, and Methanocarcina to Methanothermobacter and Methanobacterium was observed in TPAD-II. CONCLUSIONS This study proved the processes driving the dynamics of key microbial population and its correlation with hydrolytic functionality of TPAD systems. (c) 2019 Society of Chemical Industry