Impact of operational conditions on methane yield and microbial community composition during biological methanation in in situ and hybrid reactor systems

Radziah Wahid,Svein Jarle Horn

doi:10.1186/s13068-021-02019-4

Radziah Wahid, Svein Jarle Horn

Open Access

https://doi.org/10.1186/s13068-021-02019-4

Copy DOI

Abstract

BackgroundBiogas can be upgraded to methane biologically by adding H2 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.ResultsIn 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 h) 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.ConclusionsOur results show that hybrid BM outperforms in situ BM in terms of total CH4 production and content of CH4 in the biogas. In comparison to in situ BM, the use of hybrid BM increased CH4 yield by up to 42%. Furthermore, addition of H2 at 2:1 H2:CO2 ratio in in situ BM resulted in stable reactor operation.

Highlights

Biogas can be upgraded to methane biologically by adding H2 to biogas reactors
The current work demonstrates the feasibility of the hybrid biogas upgrading concept and identifies some challenges that must be tackled for future process improvement
When hybrid biological methanation (BM) was used instead of in situ BM, it resulted in a 39% increase in CH4 yield

Summary

Introduction

Biogas can be upgraded to methane biologically by adding H2 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. H2 may be combined with CO2 produced in existing biogas plants and converted to C H4, for which largescale infrastructure and applications are in place [4]. This concept of converting electrical into chemical energy is known as power-to-methane (PtM) [7]. PtM can be achieved in two ways, either by thermochemical methanation (TM) or BM [1] Both methods are based on the Sabatier reaction (Eq 1), in which four moles of H 2 react with one mole of CO2 to produce one mole of CH4 and two moles of H2O [8]: 4H2 + CO2 → CH4 + 2H2O G0′ = −130 kJ mol−1 (1)

Objectives

Methods

Results

Conclusion