Biogas upgrading by injection of hydrogen in a two-stage Continuous Stirred-Tank Reactor system

An innovative method for biogas upgrading (i.e. CH4 content more than 90%) combines the coupling of H2, which could be produced by water electrolysis using surplus renewable electricity produced from wind mills, with the CO2 of the biogas. CO2 is biologically converted to CH4 by hydrogenotrophic methanogens. In this study, a novel serial biogas reactor system is presented, in which the produced biogas from the first stage reactor was introduced in the second stage, where also H2 was injected. The effects of the H2 addition on the process performance and on the microbial community were investigated. It was shown that after the H2 addition, the CH4 rate increased by 45%, resulting in an average CH4 content of approximately 85%, with a maximum of 93.9%. The increase of the pH to 8.5, due to the CO2 conversion, was not an inhibitory factor, demonstrating the adaptation of microorganisms to these pH levels. The profiles of the microbial communities prior and after the H2 addition showed distinct differences. Changes in the archaeal community and more specifically increase in the relative abundance of Methanobrevibacter sp. and Methanoculleus sp. indicated that the methanogenic pathway was clearly shifted from aceticlastic to hydrogenotrophic.