was maintained long-term 8-3 Rotating Biological Contactors Rotating biological contactors(RBCs) were first installed in West Germany in 1960 and later introduced in the United States. Hundreds of RBC installations were installed in the 1970s and the process has beer reviewed in a number of reports. An RBC consists of a series of closely spaced circular disks of polystyrene or polyvinyl chloride that are submerged in wastewater and rotated through it(see Fig. 8-7) The cylindrical plastic disks are attached to a horizontal shaft and are provided at standard unit sizes of approximately 3.5 m in diameter and 7.5 m in length. The surface area of the disks for a standard unit is about 9300 m2, and a unit with a higher density of disks is also available with approximately 13, 900 m? of surface area. The RBC unit is partially submerged(typically 40 percent) in a tank containing the wastewater,and the disks rotate slowly at about 1.0 to 1. 6 revolutions per minute(see Fig 8-11a) Mechanical drives are normally used to rotate the units, but air-driven units have also been installed. In the air-driven units, an array of cups(see Fig 8-1lc)is fixed to the periphery of the disks and diffused aeration s used to direct Radel to the cups to cause rotation. as the rbc disks rotate wastewater Wastewater flows down through the 70-90% sloughing occurs. trickling filter. rbc ondary arircauon liquid/solids separation A submerged RBC design early 1980s but has laical RBC units: (a) conventional RBC with mechanical drive and optional air input, (b) conventional RBC in enclosed reacto seen wbmerged-rype RBC equipped with air capture cups (air is used both to rotate and to oerate the biodisks), and (d) typical applications. The 70 air-drive units are used to provide oxygen and rotation The advantages claimed for the submerged unit are reduced loadings on the shaft and bearings, improved biomass control by air agitation, the ability to use larger bundles of disks, and ease of retrofit into existing aeration tanks. However, because of the comparatively low levels of dissolved oxygen in the liquid, biological degradation activity by the submerged units may be oxygen-limited. To prevent algae growth, protect the plastic disks from the effects of ultraviolet exposure, and to prevent excessive heat loss in cold weather, RBC units are covered (see Fig. 8-7b) The history of RBC installations has been troublesome due to inadequate mechanical design and lack of full understanding of the biological process. Structural failure of shafts, disks, and disk support systems has occurred. Development of excessive biofilm growth and sloughing problems has also led to mechanical shaft, bearing, and disk failures. Many of these problems were related to a lack of conservatism in design and scal e-up issues from pilot-plant to full-scale units. Many of the problems 8-128-12 was maintained long-term. 8-3 Rotating Biological Contactors Rotating biological contactors (RBCs) were first installed in West Germany in 1960 and later introduced in the United States. Hundreds of RBC installations were installed in the 1970s and the process has been reviewed in a number of reports. An RBC consists of a series of closely spaced circular disks of polystyrene or polyvinyl chloride that are submerged in wastewater and rotated through it (see Fig. 8-7). The cylindrical plastic disks are attached to a horizontal shaft and are provided at standard unit sizes of approximately 3.5 m in diameter and 7.5 m in length. The surface area of the disks for a standard unit is about 9300 m2 , and a unit with a higher density of disks is also available with approximately 13,900 m2 of surface area. The RBC unit is partially submerged (typically 40 percent) in a tank containing the wastewater, and the disks rotate slowly at about 1.0 to 1.6 revolutions per minute (see Fig. 8-11a). Mechanical drives are normally used to rotate the units, but air-driven units have also been installed. In the air-driven units, an array of cups (see Fig. 8-1lc) is fixed to the periphery of the disks and diffused aeration is used to direct air to the cups to cause rotation. As the RBC disks rotate out of the wastewater, aeration is accomplished by exposure to the atmosphere. Wastewater flows down through the disks, and solids sloughing occurs. Similar to a trickling filter, RBC systems require pretreatment of primary clarification or fine screens and secondary clarification for liquid/solids separation. A submerged RBC design was also introduced in the early 1980s but has seen limited applications. The submergence is 70 to 90 percent and air-drive units are used to provide oxygen and rotation. The advantages claimed for the submerged unit are reduced loadings on the shaft and bearings, improved biomass control by air agitation, the ability to use larger bundles of disks, and ease of retrofit into existing aeration tanks. However, because of the comparatively low levels of dissolved oxygen in the liquid, biological degradation activity by the submerged units may be oxygen-limited. To prevent algae growth, protect the plastic disks from the effects of ultraviolet exposure, and to prevent excessive heat loss in cold weather, RBC units are covered (see Fig. 8-7b). The history of RBC installations has been troublesome due to inadequate mechanical design and lack of full understanding of the biological process. Structural failure of shafts, disks, and disk support systems has occurred. Development of excessive biofilm growth and sloughing problems has also led to mechanical shaft, bearing, and disk failures. Many of these problems were related to a lack of conservatism in design and scale-up issues from pilot-plant to full-scale units. Many of the problems Fig. 8-7