Volume ratios between the thermophilic and the mesophilic digesters of a temperature-phased anaerobic digestion system affect their performance and microbial communities

•Effluent recycling aided establishment of balanced acidogenesis and methanogenesis.•Balanced acidogenesis and methanogenesis bettered thermophilic digester function.•Methanosarcina abounded in thermophilic and Methanosaeta in mesophilic digesters.•Thermophilic:mesophilic digester volume ratio of 1:2 suits digesting dairy manure.

An experimental temperature-phased anaerobic digestion (TPAD) system, with the thermophilic digester operated at neutral pH and with a balanced acidogenesis and methanogenesis (referred to as NT-TPAD), was evaluated with respect to the microbial communities and population dynamics of methanogens when digesting dairy cattle manure at 15-day overall system hydraulic retention time (HRT). When fed a manure slurry of 10% total solid (TS), similar system performance, 36–38% volatile solid (VS) removal and 0.21–0.22 L methane g−1 VS fed, was achieved between a 5-day and 7.5-day HRT for the thermophilic digester. However, the thermophilic digester achieved a greater volumetric biogas yield when operated at a 5-day RT than at a 7.5-day HRT (6.3 vs. 4.7 L/L/d), while the mesophilic digester had a stable volumetric biogas yield (about 1.0 L/L/d). Each of the digesters harbored distinct yet dynamic microbial populations, and some of the methanogens were significantly correlated with methane productions. Methanosarcina and Methanosaeta were the most important methanogenic genera in the thermophilic and the mesophilic digesters, respectively. The microbiological findings may help understand the metabolism that underpins the anaerobic processes within each of the two digesters of TPAD systems when fed dairy manure.