Rotating Biological Contactors

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Rotating biological contactors contain a number of rotating discs on a shaft submerged in a tank partially or completely filled with liquid (Figure 9). Biofilm grows in immobilized form on the surface of a large number of closely spaced discs or inside corrugated packing units that slowly rotate in a trough, partially immersed in liquid and partially in the air space above the reactor (Figure 9). During the passage in the air or gas space, the liquid drains from the plates or packing and oxygen can diffuse in the remaining thin film of liquid and ultimately reach the biomass itself, and simultaneously CO2 can escape. Then the surface rotates further back in the liquid entraining air in the liquid, effectively aerating the fluid as well. The rotation and resulting mixing lead to very efficient mass transfer of nutrients and products to and from the film. As the film grows thicker, it will eventually inactivate and detach. The released biomass can be recovered from the bottom of the reactor where it accumulates. The discs or packing are rotated at only a few rpm, and this limits the shear but is enough to control the film thickness to below 1-2 mm. As obviously some axial mixing will occur in this reactor configuration, the reactor can be compartmented with baffles to separate groups of discs, yielding a cascade of stirred tanks. This allows for complete conversion and still high concentrations and high conversion rates in a large part of the equipment. This is especially useful for dilute streams that are well described by a first-order conversion rate and hence benefit from a plug flow pattern, such as in dilute waste stream treatment. It should also be remarked that a fully submerged operation is possible in the case of anaerobic operation. Some of the advantages of good mass transfer are then lost but gas disengagement is easy and plug flow can be approached. RBCs have been applied in denitrification of wastewater.

Figure 9. Schematic view of an RBC reactor.

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