Anaerobic　methanogenesis　from　domestic　wastewater　is　one　of　the　promising　way　to　wastewater　resource　utilization.　In　this　study,　the　effects　of　nanoscale　zero-valent　iron　(nZVI)　and　nano　ferroferric　oxide　(Fe3O4　NPs)　on　organic　matter　removal,　methanogenesis,　microbial　communities,　and　metabolic　pathways　in　the　anaerobic　biofilm　system　for　treating　domestic　wastewater　at　ambient　temperature　were　studied.　The　results　showed　nZVI　and　Fe3O4　NPs　not　only　promoted　the　hydrolytic　fermentation　process　of　the　anaerobic　biofilm　system,　but　also　facilitated　the　removal　of　refractory　organic　matter.　Especially,　Fe3O4　NPs　improved　the　methanogenic　efficiency,　shortened　the　lag　time,　and　entered　the　rapid　methanogenic　stage　rapidly.　Fe3O4　NPs　formed　direct　interspecies　electron　transfer　(DIET)　and　symbiotic　relationship　between　the　acetic　acid　oxidizing　bacteria　and　the　methanogenic　bacteria.　Furthermore,　Fe3O4　NPs　promoted　the　biosynthesis　of　F420.　In　the　Fe3O4　NPs　system,　the　CO2-reduced　methanogenesis,　as　the　main　methanogenic　pathway,　was　enhanced.　©　2022