Process for biological removal of nitrogen from wastewater

A process is provided, comprising: (a) supplying ammonium-containing wastewater to a reactor containing granular sludge comprising granules having a core of anammox bacteria and an outer rim of ammonia oxidizing bacteria; (b) subjecting the wastewater to ammonium oxidation at a temperature between 5 to 25° C., a dissolved oxygen concentration between 0.4 mg/L to 4.0 mg/L, and a hydraulic retention time between 0.5 hours to 1.5 days, obtaining a nitrogen gaseous stream and a dispersion of granular and non-sludge in treated wastewater; and (c) separating the dispersion into a granular sludge stream and a stream of treated wastewater and non-granular sludge, and (d) recycling the granular sludge to the reactor while discharging the non-granular sludge, wherein the granular sludge has a reactor retention time at least ten times the hydraulic retention time, and wherein the retention time of non-granular sludge is no more than three times the hydraulic retention time.