Identification of key steps and associated microbial populations for efficient anaerobic digestion under high...

Ammonium (NH₄⁺) and salinity are major inhibitors of CH₄ production in anaerobic digestion. This study evaluated their inhibitory effects on CH₄ production and explored the key populations for efficient CH₄ production under high NH₄⁺ and NaCl concentrations to understand their inhibition mechanisms. Comparative batch experiments for mesophilic anaerobic digestion were conducted using three seeding sludges under different concentrations of NH₄⁺ (1-5 gNH₄-N/L) and NaCl (10-30 g/L). Although all sludges tolerated 3 gNH₄-N/L and 10 g/L NaCl, NH₄⁺ or NaCl concentrations higher than these substantially reduced CH₄ production, depending on the seeding sludge, primarily by impairing the initial hydrolysis and methanogenesis steps. In addition, propionate was found to be a deterministic factor affecting CH₄ production. Based on microbial community analysis, Candidatus Brevefilum was identified as a potential syntrophic propionate-oxidizing bacterium that facilitates the mitigation of propionate accumulation, allowing the maintenance of unaffected CH₄ production under high inhibitory conditions.