The　effect　of　dissolved　oxygen　(DO)　concentration　on　nitrite　accumulation　was　investigated　in　a　pilot-scale　pre-denitrification　process　at　room　temperature　for　100　days.　In　the　first　10　days,　due　to　the　instability　of　the　system,　the　DO　concentration　fluctuated　between　1.0　and　2.0　mg/L.　In　the　next　14　days,　the　DO　concentration　was　kept　at　0.5　mg/L　and　nitrite　accumulation　occurred,　with　the　average　nitrite　accumulation　rate　at　91%.　From　the　25th　day,　the　DO　concentration　was　increased　to　2.0　mg/L　to　destroy　the　nitrite　accumulation,　but　nitrite　accumulation　rate　was　still　as　high　as　90%.　From　the　38th　day　the　nitrite　accumulation　rate　decreased　to　15%-30%　linearly.　From　the　50th　day,　DO　concentration　was　decreased　to　0.5　mg/L　to　resume　nitrite　accumulation.　Until　the　83rd　day　the　nitrite　accumulation　rate　began　to　increase　to　80%.　Dissolved　oxygen　was　the　main　cause　of　nitrite　accumulation,　taking　into　account　other　factors　such　as　pH,　free　ammonia　concentration,　temperature,　and　sludge　retention　time.　Because　of　the　different　affinity　for　oxygen　between　nitrite　oxidizing　bacteria　and　ammonia　oxidizing　bacteria　when　DO　concentration　was　kept　at　0.5　mg/L,　nitrite　accumulation　occurred.　©　2008　Higher　Education　Press　and　Springer-Verlag　GmbH.