Abstract: A biological waste-water treatment using a biological reactor is described where the processes are conducted in an alternating and joint, aerobic zone, anoxic zone and aerobic zone arrangement. Raw waste-water is transferred to the aerobic zone and/or to the anoxic zone. The waste-water is also transferred from the last anoxic zone to the last aerobic zone, which acts as an exit from the biological reactor. In the process of external recirculation to the aerobic zone, sludge from the bottom of the secondary precipitation tank is moved to the aerobic zone. In the process of internal recirculation the content of the last aerobic zone is divided between anoxic zone and aerobic zone. From the last aerobic zone, waste-water is transferred with the sludge to the secondary precipitation tank, where the clean waste-water is separated from the sludge on the bottom of the secondary precipitation tank.