In　this　study,　enhanced　nitrogen　removal　through　in　situ　enrichment　of　anammox　bacteria　was　successfully　obtained　in　a　full-scale　municipal　wastewater　treatment　plant　(WWTP).　The　WWTP　was　an　anaerobic-anoxic-oxic　(AAO)　process　and　upgraded　by　adding　moving　carriers　into　the　anoxic　zone.　Enhanced　nitrogen　removal　was　obtained　during　almost　two　years　of　operation.　The　significant　nitrogen　removal　might　be　associated　with　the　in　situ　enrichment　of　anammox　bacteria　on　the　adding　carriers,　as　revealed　by　the　comprehensive　results　of　molecular　analysis　and　N-15-stable　isotope　tracing　tests.　Quantitative　PCR　results　indicated　that　anammox　bacteria　in　the　anoxic-carrier　biofilms　presented　a　higher　abundance　than　flocculent　sludge　(16S　rRNA:　P　＜　0.005;　HzsB:　P　＜　0.042).　The　16S　rRNA　amplicon　sequencing　showed　significant　differences　in　the　phylum　Planctomycetes　(P　＜　0.001)　between　anoxic-carrier　biofilms　and　flocculent　sludge.　And　metagenomic　sequencing　analysis　further　revealed　the　anammox　relative　abundance　in　the　anoxic-carrier　biofilms　was　significantly　higher　than　the　reported　level　in　the　flocculent　sludge　of　conventional　WWTPs.　In　addition,　N-15-stable　isotope　tracing　tests　showed　that　anammox　could　be　combined　with　nitrate　reduction　by　the　anoxic-carrier　biofilms.　Thus,　enriched　anammox　bacteria　might　contribute　to　nitrogen　loss　and　lead　to　improvements　in　the　nitrogen　removal,　which　was　also　supported　by　the　mass　balance　analysis　of　organic　carbon,　nitrogen,　and　phosphorus　of　the　WWTP.　Overall,　this　study　suggests　that　anoxic-carrier　biofilms　might　be　a　candidate　to　enhance　nitrogen　removal　through　partial　anammox　in　municipal　WWTPs.　(C)　2019　Elsevier　Ltd.　All　rights　reserved.