Abstract: Process and plant for biological purification of polluted water wherein the polluted water is successively subjected to an anaerobic, anoxic and aerobic treatment in the presence of microorganisms, and wherein the anaerobic and the anoxic treatment are carried out in the presence of one or more microorganism cultures which are different from the microorganism culture used in the aerobic treatment. Hereby better exploitation of organic matter in the water is obtained thus improving the removal of nitrogen, and an increased reaction rate per unit mass of biomass is obtained thereby allowing the amount of biomass to be reduced or the total reaction rate of the plant to be increased.