Long-term evaluation of bioaugmentation to alleviate ammonia inhibition during anaerobic digestion: Process monitoring, microbial community response, and methanogenic pathway modeling

[Display omitted] •Bioaugmentation was confirmed stable to mitigate ammonia inhibition in long-term operation.•49% increment was achieved in MSH + SS + MBS with both two pathways enhanced.•Bioaugmentation stains should be properly selected to achieve a synergistic effect.•Methanosarcina spp. performed different methanogenic pathway in different bottles.•Methanogenic pathway simulated by ADM1 is consistent with the microbial analysis. The effect of different bioaugmentation strategies on anaerobic digestion related to the alleviation of ammonia inhibition was investigated in a long-term operation. The long-term operation confirmed that bioaugmentation is a stable method. A 35% increase in methane production (MP) was observed in bottles bioaugmented with Methanosarcina barkeri (MSB) or Syntrophaceticu schinkii (SS) + Methanobrevibacter smithii (MBS), and a 49% increment was obtained from the bottles bioaugmented with Methanosaeta harundinacea (MSH) + SS + MBS. Results suggest that the enhancement in both aceticlastic and hydrogenotrophic methanogenic pathways should be considered, and bioaugmentation strain should be properly selected to achieve a synergistic effect. The microbial community analysis indicated Methanosarcina spp. was the dominant archaea. Combined with specific methanogenic activity and carbon isotope fractionation analysis, it was suggested that Methanosarcina spp. performed differently in methanogenic pathways in different bottles. The abundance of COG and total enzymes in the bottles with high MP (MSB and MSH + SS + MBS) was higher than that in the other bottles. The ratio of the functional enzyme tetrahydromethanopterin S-methyltransferase subunit H (EC 2.1.1.86) to formylmethanofuran dehydrogenase subunit E (EC 1.2.99.5) and the relative abundance of enolase (EC 1.2.1.2) confirmed the aceticlastic methanogenic pathway of MSB in Group 1 and the pathway enhancement balance in MSH + SS + MBS. The modified Anaerobic Digestion Model No.1 including syntrophic acetate oxidation was used for simulation with R2 > 0.96. Simulated contribution rate data indicated that the hydrogenotrophic methanogenic pathway was dominant while both two pathways got strengthened, which is consistent with the findings of the microbial analysis.