Preliminary　studies　had　been　conducted　to　determine　the　correctness　of　the　theory　and　technique　of　energy　saving　achieved　by　limited　filamentous　bulking　under　low　DO　using　a　lab-scale　A/O　reactor　with　real　domestic　wastewater　as　the　influent.　The　results　showed　that　SVI　could　be　maintained　150-230　mL/g　and　sludge　settleability　would　not　become　very　poor　under　the　condition　of　low　DO.　During　the　period　of　limited　filamentous　bulking,　COD　and　total　nitrogen　removal　efficiencies　were　improved,　and　distinct　simultaneous　nitrification　and　denitrification　(SND)　was　achieved,　while　ammonia　removal　efficiency　would　slightly　decline　with　decreasing　of　DO,　compared　with　the　period　of　good　settleability　sludge　under　high　DO.　COD,　ammonia　and　total　nitrogen　removal　efficiencies　were　86%,　70%　and　63%,　respectively.　It　was　found　that　about　10%-25%　nitrogen　would　be　removed　by　SND　based　on　the　mass　balance　of　nitrogen.　Besides,　SS　in　the　effluent　was　almost　negligible　and　the　effluent　turbidity　was　lower　than　3　NTU.　Significantly,　aeration　consumptions　would　be　decreased　by　17%　under　the　condition　with　DO　of　0.5　mg/L　compared　with　2.0　mg/L　according　to　theoretical　calculation　of　air　requirements　to　keep　different　DO　levels,　which　was　about　57%　in　lab-scale　reactor　correspondingly.