From Low‐Loaded Mesophilic to High‐Loaded Thermophilic Anaerobic Digestion: Changes in Reactor Performance and Microbiome

This study investigated temporal dynamics in reactor performance and microbial community structure during anaerobic digestion of sewage sludge when the temperature was changed from 37°C to 55°C, followed by an increase in organic loading rate (OLR). Performance instability was observed immediately following the temperature increase and in the end of the study when the OLR was 11.1 ± 0.3 kgVS m^-3d^-1. The specific methane production peaked at 0.31 ± 0.06 Nm³ kg^-1 volatile solids (VS) during thermophilic operation and when the OLR was 3.5 ± 0.9 kgVS m^-3d^-1. Using metagenomic sequencing, 304 species-representative genome bins (SGB) were assembled. Network analysis revealed that 186 SGB were associated with thermophilic conditions and several new species putatively involved in key reactor functions were identified. When reactor function initially stabilised, two hydrogenotrophic and one aceticlastic methanogen (Methanothermobacter spp. and Methanosarcina thermophila), the hydrolytic Coprothermobacter proteolyticus, and putative syntrophic propionate oxidisers (e.g., Pelotomaculaceae) had high relative abundance. During the peak in specific gas production, the community was dominated by one hydrogenotrophic Methanothermobacter species coexisting with syntrophic acetate oxidising bacteria (Thermacetogenium phaeum and other species). Finally, when the reaction function deteriorated due to high OLR, new hydrolytic taxa emerged and the same aceticlastic methanogen as seen during the initial acclimatisation phase returned.