Partial　nitrification　and　anammox　process　is　considered　to　be　a　good　choice　for　nitrogen　removal.　Nevertheless,　it　is　difficult　to　achieve　in　low　ammonia　wastewater　because　of　the　need　to　control　further　oxidation　of　nitrite.　Nitrification　combined　with　Feammox　(anaerobic　ammonia　oxidation　coupled　to　Fe　(III)　reduction)　and　NDFO　(nitrate/nitrite-dependent　Fe　(II)　oxidation)　can　achieve　autotrophic　removal　of　low　ammonia.　And　sponge　iron　can　avoid　the　disadvantage　of　continuously　adding　iron　sources　in　this　process.　Therefore,　a　sponge　iron　biofilter　(SIBF)　treating　low　ammonia　wastewater　under　aeration　was　explored　in　this　study.　The　optimal　operating　conditions　for　SIBF　were　hydraulic　retention　time　(HRT)　of　9　h　and　gas–water　ratio　(R)　of　9:1,　at　which　the　total　inorganic　nitrogen　removal　efficiency　was　up　to　77.2　%,　and　effluent　ammonia　and　total　inorganic　nitrogen　concentrations　were　less　than　5　mg/L　and　15　mg/L,　respectively.　This　met　the　1A　discharge　standard　of　municipal　wastewater　in　China.　Stable　ratio　isotope　results　indicated　that　Feammox,　NDFO　and　Anammox　resulted　in　high　nitrogen　loss　in　SIBF.　And　different　nitrogen　transformation　contribution　tests　indicated　NDFO　had　the　dominant　contribution,　which　accounted　for　55.7　%.　Besides,　the　production　of　N2O　was　minimum　under　HRT　of　9　h　and　R　of　9:1,　in　which　the　N2O　emission　factor　was　0.7　%.　Stable　isotope　tracing　analysis　showed　that　92.8　%　of　N2O　was　derived　from　ammonia　and　the　dominant　production　pathway　was　nitrifier　denitrification.　The　results　of　microbial　community　showed　that　the　nitrification　bacteria-Nitrosomonas,　Nitrotoga,　Nitrospira,　Feammox　bacteria-Dechloromonas,　NDFO　bacteria-Thiobacillus　and　anammox　bacteria-Brocadia　played　major　roles　in　SIBF.　Therefore,　SIBF　shows　superior　performance　and　application　potential　in　autotrophic　nitrogen　removal　from　low-ammonia　nitrogen　wastewater　under　aeration.　©　2023　Elsevier　B.V.