NaClO,　as　a　common　membrane　cleaning　agent　were　introduced　from　the　permeate　side　across　membrane,　which　could　diffuse　into　MBR　membrane　pool.　Microplastics　would　also　accumulate　in　MBR　membrane　pool　during　wastewater　treatment,　thus,　NaClO　and　MPs　are　co-exposed　to　activated　sludge.　This　study　investigated　the　effects　of　PET-MPs　(0,　50,　100　and　300　μg/g-ss)　on　activated　sludge　with　or　without　NaClO　stress.　The　presence　of　PET-MPs　had　no　significant　impact　on　nitrification　and　denitrification　rates,　but　when　exposed　to　NaClO,　the　increasing　level　of　PET-MPs　(i.e.,　from　0　to　300　μg/g-ss)　inhibited　the　nitrification　(94.85　%　to　65.90　%)　and　denitrification　(88.68　%　to　60.09　%)　rates.　For　microbial　metabolism　(key　enzymes)　evaluated,　the　key　enzymes　related　to　organic　matter　biodegradation,　nitrification　as　well　as　denitrification　were　both　inhibited　with　PET-MPs　level　rise　under　NaClO　stress.　The　generation　of　reactive　oxygen　species　(ROS)　and　the　release　of　lactate　dehydrogenase　(LDH)　increased　significantly　indicating　NaClO　caused　the　toxicity　of　MPs　to　activated　sludge　by　enhancing　the　oxidative　stress　level.　Correspondingly,　a　variation　in　abundance　related　to　membrane　pollution,　nitrification,　and　denitrification　after　exposure　to　PET-MPs　with　NaClO　stress　accompanied　these　adverse　effects.　©　2024　Elsevier　B.V.