Circular bioenergy approach: Digestate-based biochar for enhancing anaerobic digestion performance under ammonia stress

To address the dual challenges of ammonia inhibition and digestate accumulation in high-load anaerobic digestion (AD), this study investigated digestate-based biochar (RBC) as a functional additive. RBC was produced by pyrolysis at 500 °C for 2 h and tested in synthetic high-ammonia wastewater. Under ammonia stress, cumulative methane production decreased by 39.77%, whereas the addition of RBC improved microbial adaptation and substrate utilization, resulting in a 20.02% increase in methane yield. These enhancements were associated with the high specific surface area, developed porosity and redox activity of RBC, which promoted ammonia adsorption, pH buffering, microbial colonization and interspecies electron transfer. Microbial community analysis and functional prediction showed that RBC enriched Firmicutes and Synergistota, supporting acetogenesis, while the relative abundances of Methanobacterium and Methanomassiliicoccus increased by 11.01% and 23.79%, respectively. In addition, the abundance of the co-methyltransferase gene increased by 107%, indicating stimulation of hydrogenotrophic and methylotrophic methanogenesis pathways. Overall, RBC effectively alleviated ammonia inhibition, enhanced methane production and improved process stability. This strategy also valorizes digestate and provides a sustainable pathway for bioenergy production within circular economy frameworks.