To improve the energy efficiency of biological methanation of CO₂ and H₂, this study evaluates the performance of a thermophilic (55 °C) anaerobic trickle-bed reactor (700 ml). A novel inoculation strategy is presented, involving the use of unfiltered and uncentrifuged anaerobic sludge from a municipal wastewater treatment plant (WWTP) as a mixed microbial culture for methanation. The results demonstrate rapid, stable, and complete conversion of H₂ and CO₂, achieving methane (CH₄) concentrations exceeding 95%. This level of purity enables direct injection into existing natural gas infrastructure without the need for extensive biogas upgrading. Furthermore, a high specific methane production rate of 10 Nm³ /m³·day was achieved, indicating strong potential for system scale-up.