Intervention Strategies for the “Inhibited Steady State” Dry Thermophilic Anaerobic Digesters: The Mitigation Mechanisms of Tailored Biochar

Compared with the single inhibitory condition, the "inhibited steady state" caused by the coaccumulation of volatile fatty acids and ammonia is more common in actual biogas projects. However, the application of tailored biochar to full-scale anaerobic digester samples under the "inhibited steady state" still remains elusive. The present study evaluated the effectiveness of regularly prepared biochar (2 cm biochar) and tailored biochar (<5 μm biochar and high-electron-donating-capacity (high-EDC) biochar) in alleviating the "inhibited steady state" with double stress from high concentrations of ammonia and propionic acid. It showed that tailored biochar reduced the lag phase (31.7–41.9%) and increased the cumulative CH4 production (5.6–37.1%) compared with the traditional intervention strategy of high-ratio inoculation. Moreover, high-EDC biochar significantly promoted the utilization and conversion of propionic acid by the colonization of Syntrophobacter, Smithella, and its methanogenic partners (Methanobacterium and Methanosaeta). Comparative analysis of the microbial diversity and the amount in the adaptive, steady, and later stages showed that hydrogenotrophic Methanobacterium and Methanosarcina were actively resistant to the "inhibited steady state". In addition, tailored biochar promoted the enrichment of biochar-biofilm consortia. The outcome provides a crucial reference for exploring tailored biochar to be a promising antidote for rescuing a seriously failed digester in actual biogas plants.