In　order　to　optimize　the　nitrogen　removal　efficiency　of　the　anoxic/oxic　(A/O)　process　in　a　state　of　limited　filamentous　bulking,　both　an　A/O　reactor　and　lab-scale　sequencing　batch　reactors　were　used　to　perform　experiments　on　anoxic　denitrification.　Methanol,　ethanol,　sodium　acetate,　and　sodium　propionate　were　used　as　external　carbon　sources.　The　results　showed　that　the　highest　specific　denitrification　rate　(7.06　mgN/(gSS.h))　was　achieved　when　sodium　acetate　was　added　to　SBRs.　When　external　carbon　sources　were　added　to　the　anoxic　zone　during　the　A/O　process,　the　average　total　nitrogen　(TN)　removal　efficiency　rose　to　56.5%　and　the　mean　concentration　of　NO2--N　in　effluent　from　the　anoxic　zone　was　6.73　mg/L.　When　the　hydraulic　retention　time　(HRT)　was　extended　to　11　h　10　min　(the　actual　retention　time　of　anoxic　zone　was　45　min),　the　average　TN　removal　efficiency　reached　70.4%　and　the　mean　concentration　of　NO2--N　in　effluent　fell　to　2.61　mg/L.　The　results　of　this　study　indicated　that　massive　nitrite　accumulation　was　caused　by　some　carbon　sources　and　the　lack　of　actual　HRT　in　the　anoxic　zone.