Nope, I have no idea.
But since I don't I was forced to go look it up out of curiosity.
A rotating biological contactor consists of a series of closely spaced plastic circular disks. The disks are submerged in wastewater and rotated slowly through it. In operation, biological growths become attached to the surfaces of the disks and eventually form a slime layer over the entire wetted surface are of the disks. The rotation of the disks alternately contacts the biomass with the organic matter in the wastewater and then with the atmosphere for adsorption of oxygen. As with the trickling filters, the biomass uses the oxygen and the organic matter for food thereby reducing the BOD and or the nitrogen content in the wastewater.