As　a　promising　technology　for　sustainable　nitrogen　removal　from　wastewater,　the　membrane-aerated　biofilm　reactors　(MABRs)　performing　autotrophic　deammonification　are　faced　with　the　problem　of　unwanted　production　of　nitrous　oxide　(N2O,　a　potent　greenhouse　gas).　As　a　common　tool　to　study　N2O　production　from　such　an　MABR,　the　traditional　one-dimensional　modeling　approach　fails　to　simulate　the　existence　of　longitudinal　gradients　in　the　reactor　and　therefore　might　render　N2O　production　significantly　deviated　from　reality.　To　this　end,　this　work　aims　to　study　the　influences　of　key　longitudinal　gradients　(i.e.,　in　oxygen,　liquid-phase　components,　and　biofilm　thickness)　on　the　N2O　production　from　a　typical　MABR　performing　autotrophic　deammonification　by　applying　a　modified　version　of　a　newly　developed　compartmental　model.　Through　comparing　the　modeling　results　of　different　reactor　configurations,　this　work　reveals　that　the　single　impact　of　the　longitudinal　gradients　studied　on　the　N2O　production　from　the　MABR　follows　the　order:　oxygen　(significant)　＞　liquid-phase　components　(slight)　＞　biofilm　thickness　(almost　none).　When　multiple　longitudinal　gradients　are　present,　they　become　correlated　and　would　jointly　influence　the　N2O　production　and　nitrogen　removal　of　the　MABR.　The　results　also　show　the　need　for　multispot　measurements　to　get　an　accurate　representation　of　spatial　biofilm　features　of　the　MABR　configuration　with　the　membrane　lumen　designed/operated　as　a　plug　flow　reactor.　While　the　traditional　modeling　approach　is　acceptable　to　evaluate　the　nitrogen　removal　in　most　cases,　it　might　overestimate　or　underestimate　the　N2O　production　from　the　MABR　with　at　least　one　of　the　longitudinal　gradients　in　oxygen　and　liquid-phase　components.　For　such　an　MABR,　the　longitudinal　heterogeneity　in　biofilm　thickness　and　the　number　of　biofilm　thickness　classes　to　be　included　in　the　model　would　also　make　a　difference　to　the　simulation　results,　especially　the　N2O　production.　The　work　also　proposes　that　under　the　studied　conditions,　proper　design/operation　of　the　MABR　in　consideration　of　longitudinal　heterogeneity　has　the　theoretical　potential　of　reducing　the　N2O　production　by　77%　without　significantly　compromising　the　nitrogen　removal.　Copyright　©　2020　American　Chemical　Society.