## Tf iN7Ta

Figure 10.19 is a plot of Eq. 10.43 over the range of recycle ratios and number of equivalent tanks likely to be encountered in practice. To determine whether a proposed SFAS system will work, the fraction of MLSS in the last tank should be determined with Eq. 10.42 or Figure 10.19 for the smallest anticipated recycle ratio and that value should be used to determine if Eq. 10.41 is satisfied. As long as that fraction is greater than the right side of Eq. 10.41, the desired effluent quality will be met and a SFAS system can be used.

### Example 10.3.5.1

Consider the wastewater that was the subject of the examples in Section 10.3.3 and 10.3.4. Consideration is being given to using a SFAS system with an SRT of 3 days that is equivalent to four tanks in series with equal distribution of the influent to all tanks. The effluent quality objective is 10 mg/L as COD of readily biodegradable organic matter. Can that objective be met if the recycle ratio is 0.5?

What specific growth rate is required in the last tank of the SFAS system? Calculate the specific growth rate in the last tank, p.n N. using Eq. 10.28 and the kinetic parameters for winter conditions, since they will control. The desired substrate concentration is 10 mg/L. Using the kinetic parameters from Table E10.2 gives:

b. What is the smallest fraction of the MLSS that can be in the last tank? Use of in Eq. 10.41 gives the smallest fraction of the MLSS that can be in the last tank.

Thus, as long as more than 8.8r/r of the MLSS is in the last tank, the effluent quality goal can be met.

c. Will the proposed system be capable of meeting the effluent quality goal? The actual fraction of MLSS in the last tank can be calculated with Eq. 10.43, or read from Figure 10.19. For a system equivalent to four tanks in series with a recycle ratio of 0.5, the fraction of MLSS in the last tank can be seen from the figure to be 0.175. Thus, the SFAS system is capable of meeting the effluent quality goal. In fact, examination of the figure reveals that the goal can be met no matter what the recycle ratio is.

The next task in the design is to distribute the steady-state oxygen requirement to each of the equivalent tanks. This can be done using the techniques described for spatially distributing the oxygen requirement in CAS systems, but in a simpler manner. The oxygen requirements for heterotrophic biomass synthesis from slowly and readily biodegradable substrate can be calculated with Eqs. 10.23 and 10.24. respectively. Likewise, the requirement for synthesis of autotrophic biomass can be cal-

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