Biological　aerated　filter　(BAF)　is　a　widely　applied　biofilm　process　for　wastewater　treatment.　However,　characteristics　of　nitrous　oxide　(N2O)　production　in　BAF　are　rarely　reported.　In　this　study,　two　tandem　BAFs　treating　domestic　wastewater　were　built　up,　and　different　gas-water　ratios　were　controlled　to　explore　N2O　production　pathway.　Results　showed　that　N2O　production　increased　with　increasing　gas-water　ratio　in　both　BAFs;　higher　gas-water　ratio　promoted　more　N2O　releasing　from　hydroxylamine　oxidation　process.　To　improve　nitrogen　removal　performance　and　reduce　N2O　emission,　the　optimal　gas-water　ratios　for　BAF1　and　BAF2　were　5:1　and　1.5:1,　respectively.　Most　of　N2O　was　produced　from　ammonia　oxidizing　bacteria　(AOB)　denitrification　and　hydroxylamine　oxidation　in　BAF1,　and　heterotrophic　denitrification　contributed　to　relieve　N2O　emission.　In　BAF2,　N2O　was　emitted　from　AOB　denitrification　and　hydroxylamine　oxidation　by　87.8%　and　12.2%,　respectively.　Heterotrophic　denitrification　is　a　N2O　sink　in　BAF,　causing　BAF1　produced　less　N2O　than　BAF2　with　the　same　gaswater　ratio.　Enhancing　heterotrophic　denitrification　and　anaerobic　ammonium　oxidation　(Anammox)　activity　could　reduce　the　release　of　N2O　in　BAFs.　(c)　2020　Published　by　Elsevier　B.V.