Hydrogenotrophic methanogens are the key for a successful bioaugmentation to alleviate ammonia inhibition in thermophilic anaerobic digesters

Bioaugmentation to alleviate ammonia inhibition under thermophilic anaerobic digestion has never been reported, as well as the working mechanism that allows a fast and successful bioaugmentation. Thus two bioaugmentation inocula (an enriched culture, and a mixed culture composed 50/50 by Methanoculleus thermophilus and the enriched culture) on the recovery of ammonia-inhibited thermophilic continuous reactors was assessed. The results showed that bioaugmentation improved methane yield by 11–13% and decreased the volatile fatty acids (VFA) by 45–52% compared to the control reactor (abiotic augmentation). Moreover, the importance of hydrogenotrophic methanogens to a fast and successful bioaugmentation was recognized. Specifically, the instant hydrogen partial pressure reduction by the bioaugmented hydrogenotroph created thermodynamically favourable conditions for the acetate oxidation process and consequently, the catabolism of other VFA. High-throughput sequencing results strengthened this explanation by showing that the bioaugmented M. thermophilus stimulated the growth of syntrophic acetate oxidising bacterium Thermacetogenium phaeum, immediately after bioaugmentation.