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Figure 7.17 Effect of the biomass recycle ratio (a) on the steady-state concentrations of various constituents in each reactor of the SFAS system described in Figure 7.11. SRT = 10 days.

Figure 7.18 Effect of the biomass recycle ratio (a) on the steady-state oxygen requirement in each reactor of the SFAS system described in Figure 7.11. SRT = 10 days.

Reactor Number

Figure 7.18 Effect of the biomass recycle ratio (a) on the steady-state oxygen requirement in each reactor of the SFAS system described in Figure 7.11. SRT = 10 days.

makes the system more homogeneous, it will make the utilization of oxygen more uniform, as shown in Figure 7.18.

Although the effects associated with changes in the recycle ratio are relatively small for this system configuration, they demonstrate clearly that the performance of suspended growth biological reactors is not always independent of the recycle ratio. In fact, it has an even stronger effect on the next reactor configuration.

7.4 CONTACT STABILIZATION ACTIVATED SLUDGE 7.4.1 Description

As described previously, in the contact stabilization version of the activated sludge process, the biomass recycle flow enters a bioreactor (stabilization basin) where it undergoes aeration and reaction prior to its return to the bioreactor receiving the influent wastewater (contact tank). This system can be simulated as a two bioreactor chain, as shown in Figure 7.19, in which all biomass recycle enters the first bioreactor and all influent enters the second. To maintain consistency with previous simulations, a total bioreactor volume of 250 m' was assumed, split evenly between the two bioreactors. The influent flows and concentrations were the same as those used in previous simulations.

7.4.2 Effect of SRT on Steady State Performance

The effect of the SRT on the steady-state performance of a CSAS system is shown by the solid curves in Figure 7.20, while the dashed curves present the performance of a single CSTR for comparison. Examination of the figure reveals that the effluent concentrations of soluble organic matter and ammonia-N are both higher than that of the CSTR, showing that the contact stabilization system does not perform as well.

Figure 7.19 Schematic diagram of two CSTRs in series with all influent to the second (contact) reactor and all biomass recycle to the first (stabilization) reactor. Although not shown, solids wastage is directly from both reactors. This configuration simulates contact stabilization activated sludge.
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