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The operation performance of a novel micro-granule based syntrophic system of nitritation and anammox was studied by controlling the oxygen concentration and maintaining a constant temperature of 25 °C. With the oxygen concentration of around 0.11 (<0.15) mg/L, the single-stage nitritation-anammox system was startup successfully at a nitrogen loading rate (NLR) of 1.5 kgN/m³/d. The reactor was successfully operated at volumetric N loadings ranging from 0.5 to 2.5 kgN/m³/d with a high nitrogen removal of 82%. The microbial community was composed by ammonia oxidizing bacteria (AOB) and anammox bacteria forming micro-granules with an average diameter of 0.8 mm and good settleability. Results from pyrosequencing analysis revealed that Ca. Kuenenia and Nitrosomonas were selected and enriched in the community over the startup period, and these were identified as the dominant anammox …