The　usage　of　triclosan　(TCS)　has　increased　with　the　COVID-19　virus　outbreak,　causing　more　TCS　are　released　into　wastewater　treatment　systems.　However,　the　difference　in　TCS　removal　pathway　and　TCS　degrading　bacteria　between　nitrification　and　denitrification　systems　was　still　unknown.　In　this　study,　batch　tests　of　TCS　biodegradation　mechanism　and　DNA　stable　isotope　probing　(DNA-SIP)　technique　were　applied　to　decrypt　the　different　TCS　removal　pathway　and　the　corresponding　degrading　taxon　between　two　nitrification　and　two　denitrification　systems.　The　main　TCS　degradation　pathway　in　both　nitrification　and　denitrification　systems　were　the　metabolism　of　heterotrophic　bacteria,　only　a　little　TCS　was　degraded　by　the　co-metabolism　of　heterotrophic　or　nitrifying　bacteria,　and　higher　NH4+-N　or　NO3--N　concentration　contributed　to　more　TCS　degradation.　Moreover,　denitrification　systems　had　stronger　TCS　removal　capacity　(0.11　and　0.65　mg　TCS/g　SS)　than　nitrification　systems　(0.83　and　1.12　mg　TCS/g　SS).　DNA-SIP　assay　further　revealed　that　active　TCS　degrading　bacteria　in　both　systems　belonged　to　Sphingomonadaceae　family.　Furthermore,　the　oligotype　TATGCC,　TAATCA　and　GCCCCG　of　Sphingomonadaceae　played　important　roles　in　degrading　TCS　in　both　systems.　Moreover,　reactor　performance　and　mixed　liquor　suspended　solids　might　play　important　roles　in　shaping　the　ecotypes　of　Sphingomonadaceae,　which　caused　the　difference　in　degrading　TCS　between　nitrification　and　denitrification　systems.　This　lab-scale　research　might　provide　meaningful　opportunities　for　evaluating　the　scale-up　applications,　and　the　TCS　degrading　bacteria　identified　in　present　study　might　be　recommended　to　be　used　as　bioaugmentation　strains　in　practical　engineering.