Iron-carbon composite promotes the conversion of nitrogen-containing substances during the anaerobic digestion of protein-rich substances.

Adding composites is increasingly recognized as a promising strategy to enhance anaerobic digestion (AD) performance. However, the effects of these composites on the conversion of nitrogenous substances during AD are rarely documented. In this study, a simple ball milling method was used to prepare iron-carbon composites (ZVI/BC), and the effects on AD of protein-rich substrates were explored. The experimental results demonstrated that the addition of ZVI/BC indicated a distinct iron-carbon synergistic effect, which not only significantly enhanced methane production but also improved the soluble protein degradation ratio and the ammoniation ratio by 40.89 % and 26.99 %, respectively. In detail, ZVI/BC promoted the degradation of protein/ammonia sugar substances and inhibited the polymerization of nitrogenous organics in the liquid phase. The easily and moderately accessible parts could be fully utilized, while the hardly accessible part was the main component after AD in the solid phase. Microbiological analysis illustrated that ZVI/BC increased the diversity of microorganisms. Particularly, the relative abundances of Tissierella and Methanosarcina were increased to 12.16 % and 69.92 %, respectively. The metabolic pathways demonstrated that ZVI/BC upregulated the abundance of genes associated with amino acid metabolism, acetate metabolism, and methanogenesis. In addition, ZVI/BC promoted the production of ammonia nitrogen while inhibiting its consumption, thereby enhancing the conversion of organic nitrogen to ammonia nitrogen. This study proposes a strategy to simultaneously enhance methane production and regulate nitrogen metabolic pathways, providing a reference for the AD of protein-rich substrates.