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Wastewater treatment plants (WWTPs) face great challenges globally, especially in China, for increasingly strict discharge standard. Abundance of AOA was found dramatically higher in a SBR WWTP in South China (ZSH) recently, which achieved extremely low ammonium concentration in effluents. Moreover, AOA equips lower half-saturation constant (K m) for dissolved oxygen (DO) and ammonia in comparison with AOB, which indicate AOA can oxidize ammonia to nitrite more thoroughly than AOB. Thus, AOA may play an important role for nitrogen removal in SBR WWTP. To understand the key factors for AOA dominance in SBR WWTP is of importance for designing and operating WWTP in future. In this study, a Nitrosocosmicus sp. AOA was enriched from the ZSH SBR WWTP. Growth, pH, temperature, ammonium and oxygen tolerance of AOA enrichments were explored. Moreover, ammonia-oxidizing pathway was investigated with presence of PTIO as inhibitor, and the production of middle products (NH 2 OH and N 2 O) during ammonia oxidation were monitored over time. Growth of Nitrosocosmicus sp. AOA was observed with a doubling time of 4.5 days, which was active under a wide pH range (6.0–9.0), with optimal pH of 7.5. Activity of Nitrosocosmicus sp. AOA increased with increasing of ammonium concentration below 40 mM, which was suppressed above 40 mM ammonium. Moreover, K m for ammonia and oxygen were 0.14 μM and 4.1 μM, respectively, which suggested the enriched AOA could thoroughly oxidize ammonia under low dissolved oxygen conditions. Activity of Nitrosocosmicus sp. AOA was completely inhibited by PTIO. Emission of N 2 O by AOA (0.05%) was lower than most reported AOB and Comammox bacteria. Our results demonstrated that the long sludge retention time, intermittent aeration, high ammonium and pH resistance were most probably the controlling factors for AOA dominance in WWTPs, which benefitted to lowing ammonium in effluents, saving energy and reducing greenhouse gas in future. [Display omitted] • A Nitrosocosmicus sp. AOA was enriched from SBR WWTP, with grape-like shape. • The enriched AOA equips high Km for ammonia (0.14 μM) and oxygen (3.47 μM). • Emission of N2O by AOA (0.05%) was lower than most reported nitrifying bacteria. • Key factor to dominate AOA: long SRT, intermittent aeration, high ammonium and pH resistance. [ABSTRACT FROM AUTHOR]