A　modified　two-sludge　system　consisting　of　AAO　and　BCO　(Anaerobic　anoxic　oxic-biological　contact　oxidation)　was　developed.　The　unique　advantage　was　the　high　efficient　utilization　of　influent　carbon　sources　in　AAO　reactor　with　NO3--N　produced　in　BCO　reactor,　which　achieved　the　simultaneous　removal　of　both　nitrogen　and　phosphorus.　With　real　municipal　wastewater　at　low　carbon/nitrogen　ratio　(COD/TN　=　3.50),　the　influence　of　hydraulic　retention　time　(HRT　=　6　-　12　h)　in　the　AAO　reactor　on　denitrifying　phosphorus　removal　was　observed.　The　mass　balance　analysis　demonstrated　that　the　system　saved　about　30-35%　carbon　sources　compared　with　traditional　biological　nutrient　removal　(BNR)　systems.　HRT　of　10　h　was　found　to　be　optimal　for　denitrifying　phosphorous　removal　due　to　the　limitation　of　poly-beta-hydroxyalkanoates　(PHA)　utilization　and　denitrification　capacity　(NO3--N/PO43--P　ratio).　It　was　also　determined　that　the　NO3--N　concentration　of　the　middle　settler　should　be　kept　around　1.50-2.50　mg/L　to　prevent　secondary　phosphorus　release.　Finally,　anoxic　duration　and　NO3--N　loading　were　the　major　limiting　factors　for　denitrifying　phosphorus　accumulation　organisms　(DNPAOs)　growth.　(C)　2015　Elsevier　B.V.　All　rights　reserved.