What is the KSD? Plant tour Rates News & announcements Agenda Specifications Year end report & audit Home Town of Kennebunk		Biological treatment After leaving the primary clarifiers, the wastewater enters tanks each filled with three rotating drums called Rotating Biological Contactors (RBCs). Each drum is made up of layers of plastic sheets on which microorganisms are growing. As these drums rotate through the wastewater, the microorganisms digest the nutrients in the wastewater. This process helps to reduce the amount of solids in the waste water. There are three tanks of Rotating Biological Contactors in the plant.