Mitigating ammonia inhibition in anaerobic digestion of swine manure using hydrogen nanobubble water acclimation:...

Methane recovery from anaerobic digestion (AD) of nitrogen-rich swine manure (SM) is severely restricted by ammonia inhibition, which disturbs microbial metabolism and syntrophic interactions. In this study, a hydrogen nanobubble water (H₂-NBW)-assisted acclimation strategy was developed by introducing H₂-NBW during ammonia acclimation rather than during the subsequent AD stage. Two rounds of H₂-NBW-assisted acclimation resulted in a 17% increase in both methane production and electron transport system activity, indicating improved metabolic activity in the acclimated inoculum. During subsequent ammonia-rich SM digestion, volatile fatty acid turnover was promoted, buffering recovery was improved, hydrolytic enzyme and coenzyme F420 activities were enhanced, and methane yield was increased by 19-31% under recoverable ammonia stress. These improvements occurred without significant reductions in total ammonia nitrogen or free ammonia nitrogen, suggesting enhanced ammonia tolerance rather than reduced ammonia exposure. Microbial community analysis showed that key hydrolytic, syntrophic, and methanogenic populations, including Spirochaetes, Treponema, Methanothrix, and Methanobacterium, were better maintained after H₂-NBW -assisted acclimation. Functional gene and transcriptomic analyses further indicated that pathways related to carbohydrate hydrolysis, carbon metabolism, membrane transport, protein synthesis, electron transfer, and methanogenesis were preserved under ammonia stress. Overall, H₂-NBW-assisted acclimation improved ammonia-rich SM-AD by maintaining metabolic coordination, providing a practical and scalable inoculum-level strategy for enhancing process stability, ammonia resilience, and methane recovery under ammonia inhibition in SM digestion systems. These findings highlight H₂-NBW-assisted acclimation as a promising strategy for mitigating ammonia inhibition and improving the sustainability of SM-AD.