Sustained methane production enhancement by magnetic biochar and its recovery in semi-continuous anaerobic digestion with varying substrate C/N ratios

Abstract

In anaerobic digestion (AD), an imbalance of C/N ratio in the substrate may lead to accumulations of ammonia and/or acid, and cause system instability. This study introduced magnetic biochar (MB) into AD systems with various ratios of corn straw and pig manure and further investigated its effects on methane production and microbial community structure during semi-continuous AD. The results indicated that the enhancement of methane production by MB application was highly affected by the C/N ratio of substrate. Particularly, MB application alleviated acid inhibition in the carbon-rich reactor, where an increased methane production by 14.0 % was obtained. As for the nitrogen-rich reactor, MB application mitigated the ammonia accumulation and promoted volatile fatty acids (VFAs) production as well as H₂S removal. The microbial community indicated that the MB promoted methanogenesis in the carbon-rich reactor by enhancing the hydrogenotrophic methanogenesis pathway, whereas the acetoclastic methanogenesis pathway dominated the nitrogen-rich reactor. The metabolic pathway involved in VFAs synthesis and methanogenesis were enhanced due to MB addition. Additionally, recovered MB was proved to retain rich dominant microbial communities for AD system. This study could provide comprehensive insights into the application of MB in long-term AD systems and offer guidance for the stable operation of biogas plants.