In　this　study,　a　novel　combined　strategy　was　successfully　established　by　partial　nitritation/anammox　(PN/A)　within　a　step-feed　A/A/O　process　integrated　with　fixed-biocarriers　to　treat　municipal　sewage　for　200　days.　The　excellent　nutrients　removal　performance　of　this　system　compared　with　national　level　of　discharging　standard　were　achieved:　low　total　inorganic　nitrogen　(7.1　mg/L)　and　phosphorus　(0.3　mg/L)　in　the　effluent　with　the　influ-ent　(51.1　and　4.2　mg/L)　at　C/N　ratios　of　3.4　+/-　0.5,　mainly　attributed　to　the　stable　PN　(oxic　zone)　and　subsequently　anammox　effect　(anoxic　zone).　Nitrogen　mass　balance　indicated　that　anammox　contribution　in　anoxic　zones　to　nitrogen　loss　could　be　up　to　42%　at　stable　phase.　Therefore,　aeration　and　carbon　cost　could　be　greatly　reduced　under　low　DO,　low　C/N　and　aerobic　hydraulic　retention　time　(HRT)　of　7.4　h　condition.　The　low　DO　and　anammox　bacteria　retention　in　anoxic　chambers　promoted　the　washout　of　NOB　and　combination　of　anammox　and　partial　nitritation　process.　During　long-term　operation,　the　activity　of　AOB　effectively　maintained　while　that　of　NOB　drastically　reduced　to　0.1　mg　N　/　g　MLSS　/　h　resulting　in　high　and　stable　nitrite　accumulation　ratios　(about　90%).　The　achievement　of　partial　nitritation　was　mainly　due　to　low　DO　(0.4-0.5　mg/L)　and　effective　retention　of　anammox　bacteria　even　with　a　low　temperature　(14.5　degrees　C).　Notedly,　anammox　activity　gradually　increased　both　on　the　biocarriers　and　in　the　flocs　while　a　higher　anammox　abundance　was　observed　on　the　biocarriers　(2.48%)　than　that　in　suspend　flocs　(0.03%).　As　above,　this　study　indicated　that　the　novel　combined　strategies　could　be　applicable　to　mainstream　anammox,　and　a　pilot-scale　reactor　will　be　established　to　verify　and　promote　the　industrial　application　of　mainstream　anammox.　(c)　2021　Elsevier　B.V.　All　rights　reserved.