Step Feed Activated Sludge

7.3.1 Description

Figure 7.10 presents the schematic diagram for the configuration used to simulate an SFAS system. As in Figure 7.1, five equal sized CSTRs in series were used, with all biomass recycle to the first bioreactor, but in this case the feed was distributed evenly among the bioreactors. All other characteristics of the system, including the flow rates, feed concentrations, etc., were the same as those used to simulate the performance of the tanks-in-series system.

7.3.2 Effect of SRT on Steady-State Performance

The effect of SRT on the steady-state performance of the system in Figure 7.10 is shown by the solid curves in Figure 7.11. For comparison, the performance of a single CSTR with volume equal to the total volume of the SFAS system is shown by the dashed curves. Examination of parts a, b, and c of the figure reveals that the effluent concentrations of soluble constituents from the SFAS system are slightly higher than those from the single CSTR. The differences are relatively small, however, and would probably be difficult to distinguish in practice. Further evidence for the similarity in performance between the SFAS system and a single CSTR is provided in Figure 7.12, where it is seen that the oxygen requirements and solids wastage rates are almost identical in the two systems. This suggests that the mass of MLSS in the SFAS system is approximately the same as that in a single CSTR with the same SRT.

Figure 7.10 Schematic diagram of five CSTRs in series with all biomass recycle to the first reactor and the influent distributed evenly among the reactors. Although not shown, solids wastage comes directly from all reactors. This configuration simulates step feed activated sludge (SFAS).

-4 Soluble Organics -

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