The Rotating Biological Contactor
This system, shown in Figure 6.7, is a derivative of the biological filter. It effectively combines the advantages of this previously described approach, like the absence of a complicated settlement system for sludge return and a low maintenance requirement with the smaller footprint and long microbial exposure characteristic of the active sludge process.
They have submerged internal disc baffles which act as sites for the attached growth of biomass, which are slowly turned by electric motor causing the microbes to be alternately aerated and immersed in the effluent. Rotating biological contactors are typically used for small installations, and are particularly useful for applications with high seasonal variations like caravan site sewage systems.