In　order　to　achieve　advanced　nitrogen　and　phosphorus　removal　in　low　C/N　ratio　urban　sewage　without　the　addition　of　external　carbon　source，this　study　focused　on　the　treatment　of　actual　urban　domestic　wastewater，and　constructed　a　pilot-scale　A2/O-BCO　system　（pre-denitrification　phosphorus　removal）with　a　daily　treatment　capacity　of　100　m3‧d-1.　The　research　encompassed　aspects　including　rapid　startup　of　the　pilot　system，operational　adjustments　and　optimization，and　analysis　of　the　microbial　community.　The　rapid　startup　of　the　pilot　scale　A2/O-BCO　system　was　achieved　by　first　enriching　phosphorus-accumulating　organisms（PAOs）in　the　A2/O　unit　and　nitrifying　bacteria　in　the　BCO　unit　separately，then　increasing　the　return　flow　of　nitrified　effluent　to　enrich　denitrifying　phosphorus-accumulating　organisms（DPAOs）.　In　the　A2/O　unit，the　anaerobic/anoxic/aerobic　volume　ratio　was　adjusted　to　3∶5∶1，and　when　the　return　flow　ratio　of　nitrified　effluent　was　set　at　250%，the　average　phosphorus　removal　efficiency　in　the　anoxic　stage　reached　95%，and　the　average　effluent　PO43--P　and　TN（total　nitrogen）concentrations　were　less　than　0.2　and　11　mg‧L-1，respectively.　Molecular　biology　analysis　revealed　a　significant　increase　in　the　abundance　of　PAOs　and　nitrifying　bacteria.　The　relative　abundance　of　Candidatus　Accumulibacter　increased　from　2%　in　the　seed　sludge　to　14%.　And　Dechloromonas，a　confirmed　organism　with　denitrifying　and　phosphorus-accumulating　functions，increased　from　2%　to　7%.　In　the　biofilm　of　the　BCO　unit，Nitrosomonas　accounted　for　5.91%　of　the　microbial　community，and　Nitrospira　accounted　for　7.58%.　©　2024　Science　Press.　All　rights　reserved.