LAUSR

Background Biogas can be upgraded to methane biologically by adding H 2 to biogas reactors. The process is called biological methanation (BM) and can be done in situ in a regular biogas reactor or the biogas can be transferred to a separate ex situ upgrading reactor. The hybrid BM concept, a combination of in situ and ex situ BM, has received little attention, and only a few studies have been reported. The hybrid BM has the advantage of resolving the issue of pH increment during in situ BM, while the size of the ex situ BM reactor could be reduced. Results In this study, the efficiency of in situ and hybrid biological methanation (BM) for upgrading raw biogas was investigated. The hybrid BM system achieved a CH 4 yield of 257 mL g VS −1 when degrading a feedstock blend of manure and cheese waste. This represented an increase in methane yield of 76% when compared to the control reactor with no H 2 addition. A 2:1 H 2 :CO 2 ratio resulted in stable reactor performance, while a 4:1 ratio resulted in a high accumulation of volatile fatty acids. H 2 consumption rate was improved when a low manure–cheese waste ratio (90%:10%) was applied. Furthermore, feeding less frequently (every 48 h) resulted in a higher CH 4 production from CO 2 and H 2 . Methanothermobacter was found to dominate the archaeal community in the in situ BM reactor, and its relative abundance increased over the experimental time. Methanosarcina abundance was negatively affected by H 2 addition and was nearly non-existent at the end of the experiment. Conclusions Our results show that hybrid BM outperforms in situ BM in terms of total CH 4 production and content of CH 4 in the biogas. In comparison to in situ BM, the use of hybrid BM increased CH 4 yield by up to 42%. Furthermore, addition of H 2 at 2:1 H 2 :CO 2 ratio in in situ BM resulted in stable reactor operation.