Rotating Biological Contactors (RBCs) are used in the treatment of wastewater as a secondary treatment process. The RBC process involves allowing wastewater to come in contact with a biological medium in order to remove contaminants in sewage before discharge of the treated wastewater to the environment, usually a river.

The construction of an RBC consists of a series of plastic discs, the media, mounted on a drive shaft that is contained in a tank or trough. Commonly used plastics for the media are polythene, PVC, and expanded polystyrene. The shaft is aligned with the flow of sewage so that the discs rotate at right angles to the flow, with several rotors usually combined to make up a treatment train. About 40% of the disc area is immersed in the sewage.

The biological growth that becomes attached to the discs assimilates the organic materials in the wastewater. Aeration is provided by the rotating action, which exposes the media to the air after contacting them with the wastewater, facilitating the digestion of the organic compounds that need to be removed. The degree of wastewater treatment is related to the amount of media surface area and the quality and volume of the inflowing wastewater.