Q O O

Figure 7.15 Effect of SRT on the steady-state concentrations of various constituents in each reactor of the SFAS system described in Figure 7.11.

The concentration profiles of soluble organic substrate and ammonia-N are the opposite of the MLSS profiles. This follows from the fact that less biomass is available in the later bioreactors to act on the substrate and ammonia entering them, thereby causing the concentrations to rise. In other words, the process loading factor increases from bioreactor to bioreactor down the chain. The tank to tank changes in substrate and ammonia-N concentrations are less at longer SRTs, however, because the larger mass of biomass in the system at longer SRTs allows more complete reaction. This is particularly evident for nitrification, which is a slower reaction than organic substrate removal.

The steady-state profiles in oxygen requirement through the system are shown in Figure 7.16. They respond more systematically to changes in SRT than the profiles through the tanks-in-series system, shown in Figure 7.7, primarily because the soluble organic substrate and ammonia-N profiles are less severe. The major impact of an increase in SRT is to increase the oxygen requirement by increasing the importance of biomass decay.

The effect of the biomass recycle ratio on the steady-state concentration profiles is shown in Figure 7.17, while the effect on the oxygen requirement is illustrated in Figure 7.18. Examination of those figures shows that changes in the recycle ratio have a greater effect on the effluent quality of this system than on that of the simple chain (Figure 7.8) or the single CSTR (upon which the recycle ratio has no effect). This is due entirely to the hydraulics of the system. All of the systems whose responses are shown in Figures 7.17 and 7.18 have the same SRT. Thus, they all contain the same mass of MLSS. Furthermore, they all have the same reactor volumes, and they all have the same average MLSS concentrations. However, as shown by Eq. 7.7, as the recycle ratio is increased, the difference between the concentration of MLSS entering the settler and the concentration leaving in the recycle flow decreases. This means that the MLSS concentration gradient along the bioreactor chain

Figure 7.16 Effect of SRT on the steady-state oxygen requirement in each reactor of the SFAS system described in Figure 7.11.

Reactor Number

Figure 7.16 Effect of SRT on the steady-state oxygen requirement in each reactor of the SFAS system described in Figure 7.11.

is reduced, as shown in Figure 7.17d. Furthermore, since the recycle ratio doesn't influence the average MLSS concentration, the MLSS concentration in the first bio-reactors must decrease with increasing recycle while the MLSS concentration in the last ones must increase. This decreases the process loading factor in the last bio-reactors, allowing more substrate and ammonia-N to be removed, thereby making their effluent concentrations lower. Finally, since an increase in the recycle ratio

Soluble Organics

Nitrate

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