•A continuous flow reactor for treating actual municipal wastewater was studied•Nitritation was quickly initiated in 15 d via UL-SRT during seasonal warming•Stable nitritation could be achieved by using joint UL-SRT and tapered aeration•The AOB (amoA) abundance increased approximately 50 times after nitritation 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°C to 22°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) × 108 copies/(g VSS) to (1.05±0.11) × 1010 copies/(g VSS) while the abundance of nitrite-oxidizing bacteria decreased from (1.1±0.02) × 1010 copies/(g VSS) to (5.01±0.02) × 108 copies/(g VSS). The temperature gradually stabilized at 26°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. Display omitted