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Anaerobic ammonium oxidation (Anammox) granules with the maximum specific Anammox activity (SAA) of 2.14 kg N/kgVSS d, the short doubling time of 6.9 days and the maximum endurable NO2−-N concentration of 130 mg/L were successfully formed in a continuously fed and completely mixing submerged anaerobic membrane bioreactor (SAnMBR). With specific input power ranging from 0.08 to 0.02 kW/m3, mean particle size (volume-weighted mean) of the Anammox flocs did not change a lot. With a specific input power of 0.003 kW/m3, the mean particle size increased rapidly from 180 μm of the flocs to 863 μm of the formed granules in 90 days. With maximum influent total nitrogen (TN) up to 4.1 g/L, a stable nitrogen removal efficiency of 88% was accomplished by the formed granules. The nitrite degradation kinetics was also investigated and it showed that the formed Anammox granules reached its maximum activity at 30 mg NO2−-N/L but was strongly inhibited at 150 mg NO2−-N/L. With granule size increasing from 287 to 896 μm, the membrane operation circle prolonged by 2–2.3 times due to the relief of membrane fouling. The cultivated Anammox granules with high activity and shock resistance capacity, fast growth rate and relieved membrane fouling shows the appealing potential of the Anammox-SAnMBR in treating high-strength nitrogen wastewaters.