Ex-situ biogas enrichment by hydrogenotrophic methanogens at low H2/CO2 ratios: Effect of empty bed residence time

• Continuous ex-situ biogas upgrading was achieved at a lower H 2 /CO 2 than 4:1. • The biogas upgrading system performed better at the lower H 2 /CO 2 ratio tested. • Biogas upgrading to 80% CH 4 was demonstrated at a low H 2 /CO 2 ratio of 4:3. • EBRT reduction increased the CH 4 productivity to 1.7 L/L d at 4:3. Biohydrogen is a good candidate for biogas upgrading. However, the system will operate at a lower H 2 /CO 2 ratio than the stoichiometric one due to the CO 2 present in the bioH 2 , challenging the process. Some operational parameters can improve the performance. Therefore, this study evaluated the effect of reducing the empty bed residence time (EBRT) on ex-situ biogas upgrading using low H 2 /CO 2 ratios. Hydrogen was added to synthetic biogas (65 % CH 4 , 35 % CO 2 ) to obtain the H 2 /CO 2 ratios of 3:1 and 4:3. The EBRTs tested were 12, 6, and 3 h. The operation at 3:1 H 2 /CO 2 ratio and the subsequent EBRT reduction negatively affected methane production. In contrast, the highest methane concentration (80 ± 1 %, EBRT: 12 h) and CH 4 productivity (1.7 L/L d, EBRT: 3 h) were obtained with the 4:3 H 2 /CO 2 ratio. The hydrogenotrophic archaea Methanobacterium and Methanolinea, as well as the acetoclastic archaeon Methanosaeta , dominated the archaeal community. Reducing the H 2 /CO 2 ratio and the EBRT positively affected methane concentration and productivity; their optimization could enhance the system’s performance. Biohydrogens’ use in biogas upgrading diversifies its use without the need for purification to eliminate the CO 2 in the biohydrogen.