Outer Radius, cm
Figure 17.5 Effect of disc size on the performance of a single RDR with constant hydraulic loading and constant peripheral velocity.
performance. This possible effect is not reflected in the model, which assumes a constant value for the biofilm thickness, L,.
Scale-up of RDRs is complicated because of the many factors involved. One approach that found use early in the development of RDRs was to maintain a constant peripheral velocity while maintaining the same hydraulic loading, probably as a result of recommendations based on lightly loaded systems."' To investigate the efficacy of this practice, simulations were performed to investigate the effect of disc size while maintaining the peripheral velocity and the hydraulic loading constant as the disc diameter was increased, and the results are presented in Figure 17.5. Maintenance of a constant peripheral velocity required that the rotational speed be decreased as the disc diameter was increased, so the rotational speed corresponding to each disc size is shown on the upper abscissa. Examination of Figure 17.5 shows clearly that the scale-up strategy is not effective since the percent substrate removal decreases as the disc size is increased, an observation that has been made in practice as well." This is a result of two effects. First, because the rotational speed is decreased to maintain a constant peripheral velocity as the disc size is increased, the external mass transfer coefficient in the submerged sector is decreased. This has the effect of decreasing the overall effectiveness factor for that sector, reducing the substrate removal rate. Second, the decrease in rotational speed reduces the volume of liquid carried through the aerated sector relative to the influent flow rate, which decreases the mass of substrate removed there, in spite of the increase in the external mass transfer coefficient in the aerated sector associated with the decrease in the liquid film thickness, Consequently, the loss in performance associated with an increase in disc size is primarily due to the effect of rotational speed on the rates of mass transfer in both sectors and the movement of liquid through the aerated sector. In addition to the problems discussed above, several other effects complicate the problem of scale-up." As a result, and because no suitable scale-up strategy has been found, it is recommended that pilot studies be performed with full-scale discs.
The effect of the number of discs on the performance of an RDR is shown in Figure 17.6. An increase in the number of discs causes a corresponding increase in the submerged biofilm area (and associated quantity of biomass) and in the volume of fluid carried with the discs into the aerated sector. Consequently, the substrate removal rates in both the submerged and the aerated sectors will increase with an increase in the number of discs, causing a reduction in the effluent substrate concentration and an increase in the percent removal.
Figure 17.7 shows the effect of fractional submergence of the discs on substrate removal in an RDR with a fixed number of discs rotating at a constant speed. By reducing the inner radius, r„ the discs can be submerged to any fraction up to 0.5. An increase in the fractional submergence increases the submerged area. A^, and allows more microorganisms to grow on a disc of fixed size. Consequently, it causes the substrate removal rate in the submerged sector to increase. It also increases the volume of fluid carried with the rotating discs into the aerated sector, thereby causing the substrate removal rate in that sector to increase as well. Therefore, the percent substrate removal will increase as the degree of submergence is increased. Although not reflected in the model, submergence in excess of 0.5 will decrease the rate of oxygen transfer in the system, thereby hurting bioreactor performance.
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