A　pilot-scale　Orbal　oxidation　ditch　was　operated　for　17　months　to　optimize　nitrogen　removal　from　domestic　wastewater　of　average　COD　to　total　nitrogen　ratio　of　2.7,　with　particular　concern　about　the　roles　of　dissolved　oxygen　(DO),　mixed　liquor　suspended　solids　(MLSS)　and　return　activated　sludge　(RAS)　recycle　ratio.　Remarkable　simultaneous　nitrification　and　denitrification　(SND)　was　observed　and　mean　total　nitrogen　(TN)　removal　efficiency　up　to　72.1%　was　steadily　achieved,　at　DO　concentration　in　the　out,　middle　and　inner　channel　of　0.1,　0.4　and　0.7　mg/L,　respectively,　with　an　average　MLSS　of　5.5　g/L　and　RAS　recycle　ratio　of　150%.　Although　the　out　channel　took　the　major　role　in　TN　removal,　the　role　of　middle　channel　should　never　be　ignored.　The　denitrification　potential　could　be　fully　developed　under　low　DO,　high　MLSS　with　adequate　RAS　ratio.　The　sludge　settleability　was　amazingly　improved　under　low　DO　operation　mode,　and　some　explanations　were　tried.　In　addition,　a　series　of　simplified　batch　tests　were　done　to　determine　whether　novel　microorganisms　could　make　substantial　contribution　to　the　performance　of　nitrogen　removal.　The　results　indicated　that　the　SND　observed　in　this　Orbtal　oxidation　ditch　was　more　likely　a　physical　phenomenon.