Process for biological removal of nitrogen from wastewater

The invention relates to a process for biological removal of nitrogen from wastewater comprising: (a) providing a stream of wastewater comprising ammonium; (b) continuously supplying the stream of wastewater to a reactor containing granular sludge comprising granules having a core of anammox bacteria and an outer rim of ammonia oxidizing bacteria; (c) subjecting the wastewater in the reactor to ammonium oxidation under ammonium oxidizing conditions comprising a temperature in the range of from 5 to 25 DEG C, a concentration of dissolved oxygen in the wastewater in the range of from 0.4 mg/L to 4.0 mg/L, and a hydraulic retention time of the wastewater in the reactor in the range of from 0.5 hours to 1.5 days, to obtain a gaseous stream comprising nitrogen and a dispersion of granular sludge and non-granular sludge in treated wastewater; and (d) continuously separating the dispersion obtained into a stream comprising granular sludge and a stream comprising treated wastewater and non-granular sludge and recycling the stream comprising granular sludge to the reactor and discharging the non-granular sludge from the process, wherein the granular sludge has a retention time in the reactor of at least ten times the hydraulic retention time, and wherein the retention time of any non-granular sludge in the reactor is equal to or at most three times the hydraulic retention time.