Biogas production and methanogenic archaeal community in mesophilic and thermophilic anaerobic co-digestion processes

•Methanobacteriales and Methanosarcinales were dominant in the anaerobic digester.•Of the genera Methanosaeta and Methanosarcina, only the latter was detected.•Temperature had a strong impact on the methanogen community dynamics, OLR did not.•More biogas was produced in the thermophilic process.•Biogas production capacity was achieved at the OLR of 5 kg VS m−3 d−1.

Over 258 Mt of solid waste are generated annually in Europe, a large fraction of which is biowaste. Sewage sludge is another major waste fraction. In this study, biowaste and sewage sludge were co-digested in an anaerobic digestion reactor (30% and 70% of total wet weight, respectively). The purpose was to investigate the biogas production and methanogenic archaeal community composition in the anaerobic digestion reactor under meso- (35–37 °C) and thermophilic (55–57 °C) processes and an increasing organic loading rate (OLR, 1–10 kg VS m−3 d−1), and also to find a feasible compromise between waste treatment capacity and biogas production without causing process instability. In summary, more biogas was produced with all OLRs by the thermophilic process. Both processes showed a limited diversity of the methanogenic archaeal community which was dominated by Methanobacteriales and Methanosarcinales (e.g. Methanosarcina) in both meso- and thermophilic processes. Methanothermobacter was detected as an additional dominant genus in the thermophilic process. In addition to operating temperatures, the OLRs, the acetate concentration, and the presence of key substrates like propionate also affected the methanogenic archaeal community composition. A bacterial cell count 6.25 times higher than archaeal cell count was observed throughout the thermophilic process, while the cell count ratio varied between 0.2 and 8.5 in the mesophilic process. This suggests that the thermophilic process is more stable, but also that the relative abundance between bacteria and archaea can vary without seriously affecting biogas production.