BACKGROUND:　In　this　study,　a　plug-flow　A(2)O　(anaerobic/anoxic/oxic)　reactor,　with　a　working　volume　of　52.5　L,　was　employed　to　investigate　the　performance　of　biological　nutrients　removal　and　microbial　population　variations　when　treating　low　C/N　ratio　domestic　wastewater.　RESULTS:　Results　showed　that　TN　removal　was　significantly　affected　by　the　shortage　of　carbon　source　while　phosphorus　removal　was　only　slightly　affected.　The　effluent　soluble　orthophosphate-phosphorus　(SOP)　concentration　was　lower　than　0.50　mg　L-1　but　the　TN　concentration　was　over　20　mg　L-1　when　the　C/N　ratio　was　4.43.　There　was　denitrifying　phosphorus　removal　in　the　anoxic　reactor　and　this　was　enhanced　by　increasing　the　volume　ratio　of　anoxic　reactor　and　maintaining　appropriate　mixed　liquor　recycle　rate.　More　than　60%　of　the　SOP　were　removed　in　anoxic　reactors　by　denitrifying　phosphorus　removal　when　the　volume　ratio　of　anaerobic/anoxic/oxic　was　1/1.4/1.6　and　the　mixed　liquor　recycle　rate　was　250%.　The　TN　concentration　of　effluent　decreased　to　11.34　mg　L-1　and　SOP　concentration　was　still　lower　than　0.5　mg　L-1　in　this　condition.　The　main　microorganisms　found　in　the　process　by　polymerase　chain　reaction-denaturing　gradient　gel　electrophoresis　(PCR-DGGE)　and　the　functional　biodiversity　are　discussed.　CONCLUSION:　Traditional　design　and　operating　parameters　of　A(2)O　are　not　appropriate　for　treating　low　C/N　wastewater.　Enhancing　the　denitrifying　phosphorus　removal　ratio　in　an　A(2)O　process　is　an　effective　way　to　increase　the　removal　rate　of　N　and　P　from　low　C/N　wastewater.　(C)　2010　Society　of　Chemical　Industry