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Partial nitritation is necessary for the implementation of the mainstream anammox (anaerobic ammonium oxidation) process in wastewater treatment plants. However, the difficulty in outcompeting nitrite-oxidizing bacteria (NOB) at mainstream conditions hinders the performance of partial nitritation. The present work aimed to develop a high-rate partial nitritation process for low-ammonium wastewater treatment at low temperatures by seeding aerobic granules. Experimental results suggested that both stratified structure of nitrifiers developed in the granules and sufficient residual ammonium concentration (18-35 mg N L -1 ) in the bulk liquid contributed to efficient NOB repression. With the hydraulic retention time progressively shortened from 1.0 to 0.17 h, the influent nitrogen loading rate of the partial nitritation process reached 6.8 ± 0.4 kg N m -3 d -1 even at 10-15 °C. The high concentration (7.5 gVSS L -1 ) and activity (0.48 g N g -1 VSS d -1 at 11 °C) of granular sludge made the reactor possess an overcapacity evaluated by the ratio between the actual ammonium oxidation rate of the granules and their maximum potential. The overcapacity helped the reactor to face the adverse effect of decreasing temperatures. Overall, this work indicated the great potential of applying aerobic granules to achieve high-rate partial nitritation at mainstream conditions. Moreover, anammox bacteria with a relative abundance of 2.8% was also identified in the partial nitritation granules at the end of this study, suggesting that the granules provided a habitable niche for anammox bacteria growth. Note that these results cannot fully relate to the treatment of real domestic/municipal wastewater, they are a source of important information increasing the knowledge about low temperature partial nitrification., (© 2021. The Author(s), under exclusive licence to Springer Nature B.V.)