Enhancement　of　anaerobic　ammonium　oxidation　(anammox)　process　and　enrichment　of　anammox　bacteria　in　the　oxic　zone　of　mainstream　sewage　treatments　are　complex.　Also,　quantification　of　the　anammox　contribution　for　nitrogen　removal　in　the　oxic　zone　is　hindered　owing　to　the　simultaneous　occurrence　of　anammox　and　nitrification.　An　alternating　anaerobic/oxic/anoxic/oxic　bioreactor　whose　oxic　zone　boosted　partial　nitrification　coupling　anammox　(PN/A)　and　anoxic　zone　boosted　partial　denitrification　coupling　anammox　(PD/A),　respectively,　was　operated　to　treat　real　sewage　for　＞380　days.　Desirable　nitrogen　removal　(effluent　total　inorganic　nitrogen　(TIN)　of　4.7　+/-　1.9　mg　N/L)　was　obtained　from　low　carbon/nitrogen　(3.6　+/-　0.5)　sewage　with　ammonium　concentration　of　52.5　+/-　2.2　mg　N/L　in　the　influent.　Under　the　condition　of　dissolved　oxygen　(DO)　of　1.5-3　mg/L,　anammox　bacteria　was　still　enriched　within　the　aerobic　biofilms,　with　the　relative　abundance　increasing　to　8.2%(day　345)　from　0%(no　biomass　on　day　1),　which　was　higher　than　the　value　in　the　flocculent　sludge　(0.03　%)　(P　＜　0.001).　PN　driven　by　flocculent　sludge　with　high　activity　of　ammonium　oxidized　bacteria　(AOB)　ensured　sufficient　nitrite　supply　for　the　anammox　process　with　the　existence　of　continuous　DO.　During　the　steady　operation　period,　the　maximum　anammox　contribution　in　the　oxic　zone　was　quantified　to　be　10.6　%　by　withdrawing　aerobic　biofilms　from　the　system.