Hydrogen addition can accelerate propionate degradation kinetics during in situ biomethanation

In situ biomethanation, which involves the injection of H 2 into anaerobic digesters, is often associated with the accumulation of volatile fatty acids (VFAs). Some metabolic pathways responsible for VFA degradation also produce H 2 . Therefore, high H 2 concentration can lead to reversible thermodynamic inhibition if critical thresholds are exceeded. However, microbial adaptation to H 2 can result in faster H 2 consumption rates, potentially mitigating these inhibitory effects. This study aims to assess the kinetics of VFA degradation under high H₂ partial pressures (> 0.2 bar) using an H 2 -adapted inoculum. The degradation of propionate and a mixture of acetate and butyrate was monitored in distinct batch reactors, both with or without H 2 addition. Opposed to previous literature results assessing the impact of H 2 addition in non-adapted digesters, H 2 injection did not inhibit VFA consumption. In the acetate/butyrate-fed condition, H 2 injection resulted in a slightly longer time prior to total VFA degradation (13 vs 9 days, with vs without H 2 injection, respectively). Notably, H 2 addition significantly promoted propionate degradation, with overall consumption rates of 0.17 ± 0.02 vs 0.03 ± 0.01 g COD·L −1 ·d −1 with vs without H 2 injection, respectively. These results, together with microbial analysis and biochemical mechanistic modeling efforts, indicate that microbial adaptation to H 2 injection enhances propionate removal by accelerating H 2 consumption. H 2 injection likely stimulated the growth of high- and low-rate hydrogenotrophic methanogens, which consumed H 2 at high and low concentrations, respectively, relieving thermodynamic constraints. The mechanisms underlying these interactions offer new perspectives for optimizing in situ biomethanation via microbial acclimation strategies. • H 2 addition stimulates the growth of high-rate hydrogenotrophic methanogens. • H 2 -adapted consortia can avoid thermodynamic inhibitions due to H 2 accumulation. • H 2 addition can promote syntrophic propionate oxidation in adapted consortia. • Syntrophic butyrate-oxidizing bacteria grow with and without H 2 addition.