Co

Figure 16.4 Effect of influent flow rate and the associated total hydraulic loading on the performance of a packed tower with fixed cross-sectional area without recirculation. The values of the kinetic parameters, stoichiometric coefficients, and system variables are given in Table 16.1 unless otherwise specified.

erally require greater tower depths to achieve a fixed effluent concentration, as shown in Figure 16.5.

Recirculation of clarified effluent has a complicated effect on tower performance. First, depending on the amount, it will reduce the applied substrate concentration by dilution of the feed with the treated effluent, as indicated by the inlet concentrations in Figure 16.6a. It will also result in flatter substrate concentration profiles, which would be expected because a plug-flow reactor behaves more like a continuous stirred tank reactor (CSTR) as the recirculation ratio approaches infinity. Although not considered in the model of Grady and Lim,4 recirculation also acts to provide a more uniform biofilm thickness throughout a tower.17 The lower applied substrate concentration reduces the effectiveness factor at the tower inlet, although

Figure 16.5 Effect of influent flow rate and the associated total hydraulic loading on the depth of packed tower with fixed cross-sectional area required to achieve 90% reduction in substrate concentration in the absence of recirculation. The values of the kinetic parameters, stoichiometric coefficients, and .system variables are given in Table 16.1 unless otherwise specified.

Figure 16.5 Effect of influent flow rate and the associated total hydraulic loading on the depth of packed tower with fixed cross-sectional area required to achieve 90% reduction in substrate concentration in the absence of recirculation. The values of the kinetic parameters, stoichiometric coefficients, and .system variables are given in Table 16.1 unless otherwise specified.

the impact is ameliorated somewhat by the increased flow rate through the tower, which increases the external mass transfer coefficient. Thus, the effectiveness factor at the inlet does not decrease as much as the applied substrate concentration. Furthermore, as a consequence of the increase in the external mass transfer coefficient, the effectiveness factor in the lower portions of the tower increases as the recirculation ratio is increased, as shown in Figure 16.6b. The decreased applied substrate concentration reduces the reaction rate. When this is coupled with the reduction in the effectiveness factor in the top of the tower, the result is less substrate removal in the upper portions. Furthermore, even though the increased flow rate increases the effectiveness factor in the lower portions of the tower, the increase cannot overcome

Aerobic Growth in Packed Towers

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