This　study　applied　a　novel　complete　ammonium　oxidation　(comammox)-inclusive　biological　nitrogen　removal　(BNR)　model　to　simulate　total　nitrogen　(TN)　removal　and　nitrous　oxide　(N2O)　production　in　mainstream　membrane-aerated　biofilm　reactor　(MABR)　performing　simultaneous　nitrification　and　denitrification　(SND).　The　results　were　compared　with　those　from　a　conventional　comammox-exclusive　BNR　model　under　the　same　operational　conditions.　The　findings　revealed　that　the　MABR　involving　the　functionality　of　comammox　bacteria　capable　of　two-step　nitrification　demonstrated　superior　TN　removal　performance　compared　to　the　MABR　performing　the　conventional　ammonium-oxidizing　bacteria　(AOB)-based　SND.　Although　comammox　bacteria　cannot　produce　N2O　directly,　the　unexpected　functional　shift　of　heterotrophic　denitrifying　bacteria　from　an　N2O　sink　in　the　MABR　with　AOB-based　SND　to　an　N2O　source　in　the　MABR　with　comammox-based　SND　resulted　in　even　higher　N2O　production　factors　in　the　latter　driven　by　the　abundant　nitrite/nitrate　supply　provided　by　comammox　bacteria　performing　two-step　nitrification　under　specific　conditions　(e.g.,　O2　surface　loading　≥　3.53　g-O2/m2/d　and　hydraulic　retention　time　(HRT)　≥　0.7　d).　By　precisely　regulating　operational　conditions　(e.g.,　O2　surface　loading　=　3.16　g-O2/m2/d,　influent　C/N　ratio　=　6,　HRT　=　0.60　d,　and　steady-state　biofilm　thickness　=　300　μm),　the　MABR　performing　comammox-based　SND　could　achieve　efficient　TN　removal　(e.g.,　81.3　%)　with　minimal　N2O　production/emissions　(e.g.,　0.06　%).　This　study　demonstrated　the　feasibility　of　establishing　comammox-based　SND　in　MABR　and　provided　critical　insights　for　advancing　further　research　and　design/control　of　sustainable　BNR　systems.　©　2025　Elsevier　Inc.