Enhanced　nitrogen　removal　by　hydroxylamine　(NH2OH)　on　anammox-related　process　recently　received　attention.　This　study　investigated　the　impact　of　NH2OH　on　the　partial-denitrification/anammox　(PDA)　biosystem.　Results　show　that　NH2OH　(≤10　mg　N/L)　immediately　induced　nitrite　accumulation　and　provided　sufficient　NO2–　to　anammox,　achieving　a　18.1　±　4.3　%　increase　of　nitrogen　removal　efficiency　compared　to　the　absence　of　NH2OH.　Long-term　exposure　to　NH2OH　accelerated　the　functional　microbial　community　transformation　to　PDA.　Thauera　was　highly　enriched　(6.1　%　→　26.9　%)　along　with　Candidatus　Brocadia　increased　in　the　biofilms,　which　mainly　favor　the　coupling　process　of　nitrate　reduction　and　anammox.　Although　the　migration　mechanism　of　anammox　and　denitrifier　revealed　by　CLSM-FISH　alleviates　the　adverse　effects　of　NH2OH,　the　anammox　was　inhibited　when　NH2OH　exceeding　15　mg　N/L　through　destroying　the　inner　reduction　of　NO2–.　These　results　suggested　appropriate　NH2OH　addition　favors　the　synergy　between　denitrifying　and　anammox　bacteria,　providing　a　promising　option　for　wastewater　treatment.　©　2024　Elsevier　Ltd