Rapid　achievement　of　nitritation　of　mainstream　municipal　wastewater　in　a　continuous-flow　process　is　attractive　since　it　favors　the　involvement　of　the　anammox　process　and　reduces　the　operational　costs.　In　this　study,　a　feasible　and　economical　strategy　is　proposed　to　rapidly　achieve　the　nitritation　of　municipal　wastewater.　By　aggressively　discharging　excess　sludge　during　the　seasonal　warming　period　(temperature　increasing　from　18　degrees　C　to　22　degrees　C),　nitritation　was　established　in　15　days　with　a　nitrite　accumulation　ratio　of　85.09%　in　a　continuous-flow　anaerobic/oxic　(An/O)　reactor.　Meanwhile,　qPCR　results　revealed　that　amoA　abundance　increased　from　(1.78　+/-　0.10)　x10(8)　copies/(g　VSS)　to　(1.05　+/-　0.11)　x　10(10)　copies/(g　VSS)　while　the　abundance　of　nitrite-oxidizing　bacteria　decreased　from　(1.1　+/-　0.02)　x10(10)　copies/(g　VSS)　to　(5.01　+/-　0.02)　x10(8)　copies/(g　VSS).　The　temperature　gradually　stabilized　at　26　degrees　C　during　the　following　operational　period　and　stable　nitritation　was　maintained　with　a　nitrite　accumulation　ratio　above　90%,　which　was　mainly　attributed　to　a　short　sludge　retention　time　(SRT)　of　4.3　days　and　a　low　dissolved　oxygen　of　0.86　+/-　0.5　mg/L.　Falling　temperature　negatively　impacted　the　stability　of　nitritation,　but　nitritation　could　be　restarted　by　aggressively　discharging　excess　sludge　during　another　temperature　increase　period.　Overall,　this　study　provides　a　feasible　strategy　to　start-up　nitritation　that　has　great　potential　applications　for　municipal　wastewater　treatment.　(C)　2021　Elsevier　Ltd.　All　rights　reserved.