A fixed-film biological treatment process that employs rotating disks that move within the wastewater is referred to as a rotating biological contactor (RBC). Developed in the late 1960s, the RBC employs a plastic medium configured as disks and mounted on a horizontal shaft. The shafts are rotated slowly (1 to 2 rpm) by mechanical or compressed air drive. For a typical aerobic RBC, approximately 40 percent of the medium is immersed in the wastewater. Anoxic or anaerobic RBCs (far less common) are fully immersed in the wastewater. Wastewater flows through the medium by simple displacement and gravity. Biomass continuously sloughs from the disks, and some suspended biomass develops within the wastewater channels through which the disks rotate, making the addition of a secondary clarifier necessary. The rotation of the disks exposes the attached biomass to atmospheric air and wastewater. Oxygen is supplied by natural surface transfer to the biomass. Some oxygenation of the wastewater is also created by turbulence at the disk-water interface. The use of exposed and submerged stages in multiple tanks to create aerobic and anoxic conditions is employed for nitrogen removal.
Commercially available modifications primarily address the media employed, the configuration of the tankage, and the mechanical supporting systems (e.g., supplemental aeration, programmable cycling, etc.). All excess solids are conveyed back to the pretreatment stage (septic tank) for subsequent removal. Lightweight synthetic media have greater surface area and a longer lifecycle compare to traditional materials and are used in all BioDisc applications.
RBC’s can meet secondary effluent standards (20 mg/L of BOD and TSS), but they would need a minimum of effluent disinfection to be acceptable for surface water discharges. They might meet onsite water reuse requirements as long as the effluent is distributed below the ground surface. Some data support the potential for soil absorption field infiltrative surface reduction following RBC treatment.
RBC plants have usually three treatment stages to the process. The first chamber is the PST (primary settlement tank) (1), the next chamber is the biozone (2+3), and the third stage is the FST (Final Settlement tank) (4) The PST acts as a pre-treatment tank, the "biozone" holds the rotating plastic discs, and, the FST performs a final clarification.
At first the sewage flows into the PST then through the biozone, and finally through the FST. The discs in the biozone turn slowly allowing aerobic bacteria to attach and grow. The constant rotating movement ensures that the media or discs go through a constant cycle of air contact and full submerging to pick up the liquids for digestion.
A recirculation pump fitted in the FST ensures that pollutants will run an additional cycle, before cleaned effluent leaves the unit.