In　this　study,　a　novel　strategy　of　zero-valent　iron　(ZVI)　combined　with　acetic　acid　was　proposed　to　optimize　partial-denitrification/anammox　(PD/A)　process,　and　enhanced　nitrogen　removal　mechanism　was　elucidated　through　metagenomics.　Results　showed　that　the　optimal　nitrogen　and　phosphorus　removal　were　as　high　as　99.50%　and　98.37%,　respectively,　with　vivianite　being　precipitated　as　the　main　byproduct.　The　occurrence　of　Feammox　was　a　crucial　link　for　enhanced　ammonia　removal　and　vivianite　recovery.　Metagenomic　analysis　further　certified　that　long-term　acclimation　of　optimization　strategy　triggered　DNRA-based　nitrate　reducing　genes　(narY/Z　and　nrfA)　assigned　to　Candidatus　Brocadia,　which　allow　direct　uptake　of　nitrate　by　the　anammox.　Additionally,　ZVI　might　act　as　a　new　electron　donor　to　decrease　organics　dependence　of　PD　by　reducing　the　abundance　of　genes　for　electron　production　involved　in　carbon　metabolism.　However,　FA　addition　enhanced　the　relative　abundances　of　genes　involved　in　anammox　including　nitrogen　reduction　and　oxidation,　thereby　accel-　erating　nitrogen　removal.