In the present work, quick start-up ultrastable nitritation was successfully demonstrated in a pilot-scale sequencing batch reactor (SBR; V = 6.5 m3) treating real municipal wastewater (5.9°C–29.8°C). The SBR was operated under a high influent loading rate by aggressively discharging surplus sludge and controlling a short aeration time, resulting in a significant decrease of both ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) abundances. Subsequently, the influent loading rate sharply dropped from 312 mg COD/(g VSS∙d) to 55 mg COD/(g VSS∙d) with the extension of aeration time. According to qPCR and batch test results, the AOB abundance increased 106-fold and 136% of its initial bioactivity level was recovered. The NOB abundance was maintained at a low level, resulting in incomplete nitrification and nitrite accumulation. Nitritation with an ultra-high nitrite accumulation ratio (NAR) of 97% started in seven days. Moreover, despite the decrease of temperature from 17.5°C to 5.9°C, a high NAR (98.1 ± 1.1%) was maintained for over 450 days. The NOB abundance was below the detection level, and a high AOB (Nitrosomonas; 0.95%) level was effectively maintained. Importantly, the average levels of ammonium and nitrite effluents were maintained at 8.3 mg N/L and 13.8 mg N/L, respectively, favoring the application of an anammox process. Therefore, the proposed nitritation start-up strategy provides a practical alternative for extending anammox into sewage treatments.

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