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Fractional Distance from the Inlet, x/L

Figure 16.2 Hlfect of depth on the performance of a packed lower without recirculation receiving influents with different substrate concentrations: (a) effects on substrate concentration profiles: (b) effects on overall effectiveness factor profiles The values of the kinetic parameters, stoichiometric coefficients, and system variables are given in Table 16.1 unless otherwise specified

Figure 16.3 shows the effect of influent substrate concentration, SSo, on the removal of substrate by a tower without recirculation. The range of substrate concentrations is much greater than could be applied in practice without oxygen limitations or excessive biofilm buildup, but it is presented to illustrate some of the fundamental characteristics of towers. As the feed substrate concentration is increased, the fractional substrate removal decreases although the mass removal rate, F(Sso — Sv), increases. Because the tower has been assumed to have a constant and fixed biofilm thickness throughout, the amount of biomass in the tower is fixed. Consequently, the decrease in the fractional substrate removal is a direct result of the intrinsic kinetics of substrate removal for that fixed amount of biomass. At low substrate concentrations the fractional removal is almost constant as the feed substrate concentration is increased. This is because the Monod equation simplifies to a first order reaction with respect to substrate concentration under those conditions. Because mass transport is also first order and the amount of biomass is fixed, the substrate removal rate increases linearly with increases in substrate concentration, giving a constant fractional removal. As the feed substrate concentration is increased further, however, the Monod equation becomes zero order with respect to substrate, which means that the substrate mass removal rate becomes controlled entirely by

Figure 16.3 Effect of influent substrate concentration on the performance of a packed tower without recirculation. The values of the kinetic parameters, stoichiometric coefficients, and system variables are given in Table 16.1 unless otherwise specified.

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