Impact of Operational Conditions on Methane Yield and Microbial Community Composition during Biological Methanation in a Hybrid Reactor System

Abstract Background: Biogas can be upgraded to methane biologically by adding hydrogen 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 CH4 yield of 257 mL gVS-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 H2 addition. A 2:1 H2:CO2 ratio resulted in stable reactor performance, while a 4:1 ratio resulted in a high accumulation of volatile fatty acids. H2 consumption rate was improved when a low manure-cheese waste ratio (90%:10%) was applied. Furthermore, feeding less frequently (every 48 hours) resulted in a higher CH4 production from CO2 and H2. 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 H2 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 CH4 production and content of CH4 in the biogas. The application of hybrid BM increased CH4 yield up to 42%. Furthermore, addition of H2 at 2:1 H2:CO2 ratio in in-situ BM resulted in stable reactor operation.