A circulating moving bed biofilm method for biological purification of waste water in a continuous process includes receiving a pretreated waste water inlet stream in an anaerobic zone of a reactor wherein micro-organism culture exist on free flowing biofilm carriers, letting the waste water stream with the biofilm carriers into an aerobic zone aerating the waste water stream and carriers received from the anaerobic zone, at an end of the aerobic zone, transferring the biofilm carriers to the anaerobic zone without transfer of water, and discharging the water through an outlet to a sludge separation process.