Different　from　anammox,　the　combination　of　Fe　(III)　reduction　coupled　to　anaerobic　ammonium　oxidation　(Feammox)　and　nitrate/nitrite　dependent　ferrous　oxidation　(NDFO)　do　not　require　to　control　nitrite　accumulation.　Furthermore,　sponge　iron　can　avoid　continuous　iron　supplementation　in　practice　and　is　a　good　iron　source　for　the　occurrence　of　Feammox　and　NDFO　in　wastewater　treatment.　Therefore,　a　biofilter　using　sponge　iron　as　carrier　treating　low　nitrogen　wastewater　was　built.　In　this　study,　the　performances　of　nitrogen　removal　were　explored　under　different　hydraulic　retention　times　(HRT)　and　gas-water　ratios　in　sponge　iron　biofilter.　And　the　pathways　of　nitrogen　removal　were　analyzed　by　activity　tests.　The　results　showed　ammonia　removal　efficiency　reached　94.1%　and　total　inorganic　nitrogen　removal　efficiency　was　up　to　70.6%　at　HRT　of　19　h　and　gas-water　ratio　of　18.　Compared　to　nitrogen　removal　by　adsorption　under　non-aeration,　the　activity　tests　showed　that　total　inorganic　nitrogen　loss　was　caused　by　Feammox　and　NDFO　after　aeration.　The　results　of　microbial　communities　showed　that　appearances　of　nitrifier-Nitrosomonadaceae,　Feammox　bacteria-Clostridiaceae　and　NDFO　bacteria-Gallionellaceae　resulted　in　deep　nitrogen　removal　after　aeration,　in　which　Nitrosomonadaceae　and　Clostridiaceae　contributed　to　ammonia　removal　and　Gallionellaceae　contributed　to　nitrite/nitrate　reduction　to　nitrogen　gas.　Therefore,　it　was　feasible　to　achieve　deep　autotrophic　nitrogen　removal　and　Fe　(II)　and　Fe　(III)　cycle　in　sponge　iron　biofilter.