Nano zerovalent iron boosts methane content in biogas and reshapes microbial communities in long-term anaerobic digestion of pig slurry

Adding nanoscale zero-valent iron (nZVI) to the anaerobic digestion (AD) process for livestock manure has been shown to significantly enhance methane content by influencing microbial communities and metabolic pathways. However, the long-term effects of nZVI on metabolically active microbial communities remain largely unexplored. This study explored these microbial shifts in nZVI-supplemented AD systems and biogas composition under both mesophilic and thermophilic conditions over extended operation. To this end, three lab-scale continuous stirred tank reactors were operated for 265 days using raw pig slurry as the feedstock. Two mesophilic reactors received 84 mg gSSV−1 of nZVI, while a third thermophilic reactor received 42 mg gSSV−1. High-throughput sequencing and quantitative PCR were employed to monitor changes in bacterial and archaeal communities following nZVI addition. The results demonstrated a notable increase in methane content in the biogas, reaching 88 % in mesophilic and 87 % in thermophilic conditions with nZVI. Microbial responses differed between reactors, including increased copy numbers of metabolically active archaeal mcrA and bacterial 16S rRNA genes, as well as a rise in the relative abundance of specific genera, such as Methanosaeta. These findings underscore the potential of nZVI to enhance AD performance through targeted shifts in microbial community structure and function.