The　feasibility　of　nitrite　accumulation　in　a　pilot-scale　A/O　(anoxic/oxic)　nitrogen　removal　plant　treating　domestic　wastewater　was　investigated　at　various　dissolved　oxygen　(DO)　concentrations　and　pH　levels.　The　results　showed　that　the　pH　was　not　a　useful　operational　parameter　to　realize　nitrite　accumulation.　Significant　nitrite　accumulation　was　observed　at　the　low　DO　concentration　range　of　0.3-0.8　mg/l　and　the　maximum　nitrite　accumulation　ratio　of　about　90%　occurred　at　a　DO　concentration　of　0.6　mg/l.　This　suggests　a　reduction　of　22%　in　the　oxygen　consumption,　and　therefore　a　considerable　saving　in　aeration.　However,　the　nitrite　accumulation　was　destroyed　at　the　high　DO　concentration　and　the　resumption　was　very　slow.　In　addition,　the　average　ammonia　removal　efficiency　reached　as　high　as　93%　at　the　low　DO　level.　Moreover,　experimental　results　indicated　that　nitrogen　could　be　removed　by　simultaneous　nitrification　and　denitrification　(SND)　via　nitrite　in　the　aerobic　zones　at　the　low　DO　concentration,　with　the　efficiency　of　6-12%.