Abstract: A rotating disc-type biological waste water treatment apparatus comprises rotating disc assemblies rotatively driven while their lower halves are immersed in waste water. This rotating disc assemblies include a plurality of impellers, each comprises two parallel discs fixed to a main shaft, one of the discs being formed at its center with a suction opening, and impeller blades radial to the main shaft between the two discs, thereby increasing the surface areas of the rotating disc assemblies without increasing the driving power. Besides, only by closing the outer peripheries of the two discs the apparatus can perform the anaerobic denitrification.According to the invention the impeller is divided into a plurality of sectors consisting of two sector side plates and two impeller blades which are integrally formed by blow molding and then arranged side by side to form a sector block. Thus produced sector blocks are then arranged about the main shaft and fixed thereto to form a rotating disc assembly.