Unraveling the effects of temperature on mass transfer and microbiology in thermophilic and extreme thermophilic trickle bed biomethanation reactors

• Long-term high-rate biomethanation with mixed cultures required VFA management. • The CH 4 productivity decline at temperatures >70 °C and halted at 85 °C. • CH 4 productivity of 21.01 L CH4 L R -1 d -1 was achieved in a 70 °C trickle bed reactor. • TBR operation at 70 °C compared to 50 °C increased CH 4 productivity by 24.6 %. • The H 2 mass transfer was modeled to increase to 65.4 °C, whereafter it declined. Numerous initiatives are currently being initiated to substitute fossil fuels with renewable alternatives. Biomethanation is one of these emerging initiatives that presents a novel platform for valorizing carbon dioxide (CO 2 ) to produce methane (CH 4 ) by utilizing renewable hydrogen (H 2 ). Process temperature is a critical factor affecting CH 4 productivity and selectivity, which previously has been ascribed solely to either biological or physiochemical changes. For the first time, this study demonstrated the temperatures effect on the intertwined biological, physiochemical, and process-engineering factors in novel trickle bed reactors (TBR). It was demonstrated that CH 4 selectivity was enhanced by gradually ramping temperature from 55 °C to 70 °C resulting in 62 % reduction in acetate levels. However, further temperature increases > 70 °C deteriorated biocatalytic activity, for which the activity completely stopped at 85 °C. A comparative analysis of a thermophilic TBR (50 °C) and extreme-thermophilic TBR (70 °C) demonstrated 24.6 % improvement in CH 4 productivity at 70 °C. Hereto, the effect of temperature on the H 2 gas–liquid mass transfer rate was modeled, which indicated an increasing trend in mass transfer up to 65.4 °C, whereafter the driving force became too impaired by reduced H 2 solubilities and elevated moisture contents. A contribution of only 6.4 % enhancement in the CH 4 productivity from 50 °C to 70 °C could be attributed to the increased H 2 mass transfer rate, which made the temperature effect on the biocatalyst the most pronounced factor for the enhanced process performance and selectivity. Hereto, Methanothermobacter was identified as the dominant CO 2 -fixing biocatalyst, and Acetomicrobiaceae as the major bacterial family correlating with acetate accumulation.