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Figure 7.12 Effect of SRT on the total steady-state oxygen requirement and solids wastage rate for the SFAS system depicted in Figure 7.10 operating under the conditions listed in Figure 7.11. For comparison, the dashed curves represent the performance of a single CSTR with a volume of 250 m\

a MLSS concentration gradient through the system. It will be recalled from Figure 7.12, however, that for a given SRT, the SFAS system has the same mass wastage rate of biomass as a single CSTR, suggesting that both systems contain the same mass of MLSS. Because of this, and because the volume of the single CSTR is equal to the total volume in the SFAS chain, the MLSS concentration in the single CSTR must equal the average concentration in the SFAS system. Consequently, since the SFAS system contains a MLSS concentration gradient, the MLSS concentration in its last tank must be less than the concentration in the single CSTR. That this is true is shown in Figure 7.11. Now, since the influent flow rate into the last bioreactor of the chain is one-fifth the flow rate into the single CSTR, and since the volume of the last bioreactor is one-fifth the volume of the single CSTR, the mass flow rate of substrate per unit volume into the last bioreactor of the chain is the same as that into the single CSTR. However, since the biomass concentration is less, there is less biomass per unit of substrate added, i.e., the process loading factor is higher, allowing less substrate to be removed, as observed. Thus, the very configuration of the SFAS system prevents it from performing as well as a single CSTR or a simple chain. This raises the question as to why such a system would be used. One reason can be seen by examining the dynamic performance of the system.

7.3.3 Dynamic Performance

The dynamic performance of the SFAS system when subjected to the same diurnal loading pattern as the tanks-in-series system is shown in Figure 7.13. Again, the response of a single CSTR is shown for comparison. As would be expected from the entrance of feed directly into the last bioreactor with its low biomass concentration, the dynamic performance of the SFAS system is worse than that of the CSTR. Furthermore, although the difference is relatively small for soluble organic substrate,

Figure 7.13 The time dependent response from the SFAS system depicted in Figure 7.10 when subjected to the diurnal loading patterns shown in Figure 6.2. For comparison, the dashed curves represent the performance of a single CSTR with a volume of 250 m\ Average influent flow = 1000 m'/day. Average influent concentrations are given in Table 6.6. Biomass recycle flow = 500 m'/day; volume of each reactor = 50 m'; SRT = 10 days. Parameters are listed in Table 6.3. The dissolved oxygen concentration was held constant at 2.0 mg/L.

Time, hrs

Figure 7.13 The time dependent response from the SFAS system depicted in Figure 7.10 when subjected to the diurnal loading patterns shown in Figure 6.2. For comparison, the dashed curves represent the performance of a single CSTR with a volume of 250 m\ Average influent flow = 1000 m'/day. Average influent concentrations are given in Table 6.6. Biomass recycle flow = 500 m'/day; volume of each reactor = 50 m'; SRT = 10 days. Parameters are listed in Table 6.3. The dissolved oxygen concentration was held constant at 2.0 mg/L.

it is substantial for nitrification, with the maximum effluent ammonia-N concentration being almost twice as high as that from the CSTR. This is a direct result of the low maximum specific growth rate of nitrifying bacteria, which prevents them from responding rapidly enough to the changing input rate of nitrogen into the last bioreac-tor. In addition, comparison of the effluent ammonia-N concentration to that from the simple chain in Figure 7.4 reveals that a SFAS system subjected to diurnal loading produces an effluent that is much worse than the chain. Thus, the dynamic performance of the SFAS system does not justify its use.

A major reason for employing a SFAS system, however, is revealed by Figure 7.14, where it can be seen that the distribution of the load along the length of the chain gives an oxygen consumption pattern that is much closer to that of a single CSTR than was the simple chain. This makes it much easier to provide the needed oxygen. Consequently, if a tanks-in-series system is not performing properly because of an inability to supply sufficient oxygen to the first tanks in the system, redistribution of a portion of the influent to tanks further down the chain may be able to

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