Rotating biological contactors (RBC) operate with what basic arrangement?

• A. Long shafts with plates of media that rotate through wastewater; bacteria grows on plates and removes BOD; first stages about 100,000 ft2 per shaft; later stages 120-180,000 ft2.
• B. A fixed-film system with rotating discs.
• C. A series of activated sludge basins with air diffusers.
• D. An anaerobic digestion unit.

Answer: (A)

Rotating biological contactors use long shafts that carry a stack of media plates (discs) which rotate through the wastewater. Microorganisms grow as a biofilm on those disc surfaces, and as the discs turn, the biofilm is alternately exposed to wastewater and air, providing both contact with the organics and a supply of oxygen. This setup creates a very large effective surface area for biological treatment, which is why the design emphasizes large surface areas per shaft—about 100,000 ft2 in the early stages and 120,000–180,000 ft2 in later stages—to boost treatment capacity. The rotating discs and biofilm model explain how RBCs accomplish BOD removal without relying on mixed-liquid aeration or suspended-growth systems.