Integrating hydrogen delivery and in situ water removal for biomethanation in a hollow fiber membrane biofilm reactor (HF-MBfR)

Biomethanation offers a sustainable pathway for renewable energy storage and for the utilization of captured CO 2 . Nevertheless, challenges remained with H 2 mass transfer limitations from the gas to the liquid phase and nutrient dilution caused by metabolic water production, which will increase the demand for nutrient supplementation. This study demonstrates the use of a hollow fiber membrane biofilm reactor (HF-MBfR) to advance biomethanation by integrating H₂ delivery through gas-permeable polydimethylsiloxane (PDMS) membranes with in situ water removal. This design enables high-rate thermophilic operation (55°C) without gas-liquid mixing or frequent nutrient addition. The system achieved a record volumetric methane production rate of 3.2 L·L⁻¹·d⁻¹ in biomethanation MBfR along with 95% CH₄ purity and near-complete CO₂/H₂ conversion. Re-inoculation rapidly and sustainably recovered performance losses due to nutrient limitations, outperforming the addition of synthetic supplements. Periodic lumen side purging removed a water volume equivalent to 60–65% of the metabolic water and mitigated excessive bulk liquid dilution, while minimal ammonium concentration > 0.6 g·L⁻¹ was found essential for pH buffering (6.8–8.3) against CO₂ partial pressure variation (0.004–0.21 atm) in the HF-MBfR. The biofilm was dominated by Methanothermobacter and Coprothermobacter , which facilitate the high-rate and high-efficiency conversion of CO 2 /H 2 into methane. These results demonstrate that HF-MBfR is a robust and scalable platform for efficient CO₂ biomethanation and set the stage for further optimization of membrane layouts and surface modifications to support robust biofilm development and enhanced methane productivity.