The combined flow from the primary clarifiers is split between two parallel series of Rotating Biological Contactors (RBC or Bio-Discs). Each of the two sets of RBC's contains two covered tanks. The first stage RBC is a single-disc module and has supplemental aeration within the tank provided from blowers. The second stage RBC contains two disc modules, but no supplemental aeration is provided.
The RBC's provide secondary, biological treatment of the overall treatment process. The RBC's consist of corrugated plastic media mounted on shafts, which are slowly rotated in a tank with approximately 40 percent of the media surface submerged in the wastewater. Bacteria and other organisms adhere to the rotating surface and multiply until the entire surface is covered with a layer of biomass.
While rotating, the media carries a film of wastewater into the air where dissolved oxygen is absorbed. Organisms in the biomass remove organic (waste) materials in the wastewater, and use dissolved oxygen in the process. Some of the biomass from the RBC's continuously sloughs and is contained in the wastewater in the tanks. The bacteria and living organisms on the bio-discs and in the tanks use waste material dissolved in the wastewater as their food, and thus provide the means for removal of the pollutant materials.