Effect of nano-Fe3O4 biochar on anaerobic digestion of chicken manure under high ammonia nitrogen concentration

In order to improve the stability and CH4 production efficiency of the anaerobic digestion (AD) system under high ammonia nitrogen (AN) concentration, this study investigated the effect of biochar loaded with nano-Fe3O4 particles to generate nano-Fe3O4 biochar on AD of chicken manure (CM). The results showed that biochar can be used as an excellent carrier for nano-Fe3O4 particles, solving the problem of easy agglomeration and loss. The inhibition of the AD system caused by high concentrations of AN was greatly mitigated after loading nano-Fe3O4 particles on biochar, and the CH4 yield was substantially increased. The highest CH4 yield was achieved when the amount of nano-Fe3O4 biochar was 15%, which was 62.61% higher compared to the control group. Microbial community analysis showed that the porous structure of biochar was capable of immobilizing and enriching microorganisms, and the relative abundance of Firmicutes and Methanobacterium increased by 28.67-54.44% and 52.62-87.70%, respectively. Firmicutes acted as an important acetate bacterium, contributing to syntrophic ac-etate oxidation (SAO) with hydrotropic methanogenesis. SAO was an effective alternative pathway for acetate degradation under ammonia stress involving the syntrophic activities of SAO bacteria and hydrogenotrophic archaea. The load of nano-Fe3O4 particles increased the relative abundance of Chloroflexi from 0.52% to 3.51% and the Methanosarcina from 27.50% to 49.30%. Chloroflexi and Methanosarcina could act as potential direct electron transfer (DIET) partners for facilitating methane production by promoting the SAO pathway.

Automated summary

This study found that loading nano-Fe3O4 particles on biochar can improve the stability and CH4 production efficiency of the AD system under high ammonia nitrogen concentration. The porous structure of biochar can immobilize and enrich microorganisms, while the loading of nano-Fe3O4 particles helps mitigate the inhibition caused by ammonia nitrogen and substantially increase CH4 yield.