Despite　the　well-reported　ability　of　ammonium-oxidizing　bacteria　(AOB)　to　cometabolize　non-growth　substrates,　most　relevant　studies　were　substantiated　with　the　ammonium　oxidation　activity　as　the　driving　force　and　didn＇t　further　explore/verify/distinguish　the　contribution　of　the　hydroxylamine　oxidation　process　of　AOB.　Therefore,　using　highly　enriched　AOB-dominated　sludge,　a　series　of　dedicated　batch　tests　were　conducted　in　this　work　to　investigate　the　hydroxylamine-based　cometabolic　degradation　of　chloroquine　(CLQ),　a　widely　applied　drug　with　a　significant　environmental　concern,　and　its　associated　impacts　on　the　nitrogen　conversions　of　AOB.　The　results　indicated　that　the　hydroxylamine　oxidation　process　of　AOB　could　trigger　the　degradation　of　CLQ　which　achieved　0.4-1.2　mg/L　CLQ　removal　under　the　studied　conditions,　despite　the　potentially　preferred　utilization　of　CLQ　degradation　products　towards　mineralization　by　AOB　over　CLQ　itself.　CLQ　and　its　degradation　products　wouldn＇t　affect　the　hydroxylamine　oxidation　process　or　the　associated　N2O　production　through　the　hydroxylamine　oxidation　pathway　of　AOB.　We　hope　this　work　will　not　only　provide　new　perspectives　for　prospective　research　on　the　cometabolic　capacity　and　features　of　AOB　but　also　inspire　the　revisit　of　the　literature-reported　research　on　such　an　important　topic.