Figure 7.14 The variability of the oxygen requirement in each reactor for the SFAS system described in Figure 7.13. The vertical lines represent the range observed over a 24 hr period and the dots represent the requirement when the influent flow and concentrations are constant at the flow-weighted average values (steady-state). For comparison, the dashed lines show the extreme diurnal requirements, as well as the steady-state requirement, for a single CSTR with a volume of 250 m\

alleviate the problem and produce an acceptable effluent, even though it may not be as good as that from a properly designed tanks-in-series system.

These simulations have only examined the case of distributing the feed evenly along the chain. Obviously, many possible distribution patterns exist and each will have a different impact on system performance. A study of the impact of alternative patterns will be left as an exercise for the reader.

7.3.4 Variations Within the System

Tank-to-tank variations in concentration within an SFAS system are shown in Figure 7.15 for steady-state operation. Part d of the figure shows the gradient in MLSS concentration discussed earlier, which is steeper at longer SRTs. This gradient is the other major reason for using the SFAS system." Occasionally the settling characteristics of activated sludge will deteriorate so that the mass loading rate of solids entering the final settler must be reduced to prevent the settler from failing. Since the flow rate of wastewater being treated cannot be reduced, this requires reduction of the MLSS concentration. One way to do this is to reduce the SRT, but this may negatively impact other aspects of process performance, such as causing loss of nitrification. If the system were operating in the tanks-in-series mode, however, and contained the appropriate piping, switching to the SFAS mode would establish the MLSS concentration gradient, thereby reducing the concentration of MLSS entering the final settler while maintaining the same system SRT. Then, after the biomass settling characteristics had returned to normal, operation could be returned to the tanks-in-series mode.

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