In　a　continuous　anaerobic-anoxic-aerobic　(A(2)O)　process　treating　domestic　wastewater　at　ambient　temperatures,　nitritation　was　achieved　through　a　combination　of　short　aerobic　actual　hydraulic　retention　time　(AHRT)　and　low　dissolved　oxygen　(DO)　levels　(0.3-0.5　mg/L).　The　nitrite　accumulation　rate　was　about　90%　and　ammonia　removal　efficiency　was　over　95%.　With　respect　to　total　nitrogen　removal,　nitritation-denitritation　at　low　DO　levels　of　0.3-0.5　mg/L　was　essentially　equal　to　the　complete　nitrification-denitrification　at　DO　levels　of　1.5-2.5　mg/L　with　the　addition　of　external　carbon　sources.　Regardless　of　low　DO　operation,　sludge　bulking　did　not　occur　since　the　sludge　volume　index　was　below　150　ml/g.　Real-time　PCR　assays　showed　that　in　response　to　complete　and　partial　nitrification　modes,　the　numbers　of　ammonia　oxidizing　bacteria　population　were　5.28　x　10(9)　cells/g　MLVSS　and　3.95　x　10(10)　cells/g　MLVSS,　respectively.　Achievement　of　nitritation-denitritation　is　highly　beneficial　to　the　treatment　of　domestic　wastewater　in　terms　of　lower　carbon　requirements　and　reduced　aeration　costs.　(c)　2010　Published　by　Elsevier　Ltd.