Figure 7.1 presents a schematic diagram of the system used to simulate conventional and high purity oxygen activated sludge systems. All influent and all biomass recycle enters the first bioreactor and passes from bioreactor to bioreactor down the chain. For the purposes of this chapter, the bioreactors were considered to be of equal volume, but different residence time distributions can be attained by using bioreactors of different size.1* The influent flow rate used in simulations was 1000 m'/day and the volume of each bioreactor was 50 m,' giving a total system volume of 250 m' and a system hydraulic residence time (HRT) of 6 hr, which is a value commonly used in practice. The biomass recycle flow rate was fixed at 500 m'/day. unless otherwise specified, giving a recycle ratio, a(F, /F), of 0.5, which is also commonly used in practice. All bioreactors are aerobic and the dissolved oxygen concentration was controlled at 2.0 mg/L in each, thereby eliminating denitrification. This concentration was chosen because it is easily and economically achieved in practice and allows unhindered nitrification, as shown in Figure 6.7.
To allow direct comparison to the results in Chapter 6, the characteristics of the influent were assumed to be those listed in Table 6.6 and the values of the kinetic and stoichiometric coefficients of the biomass growing on that influent were assumed to be those in Table 6.3.
The effect of SRT on the steady-state performance of the tanks-in-series reactor system is given by the solid curves in Figure 7.2, which show the concentrations in the last bioreactor of the chain, and represent the concentrations entering the final settler. For comparison, the dashed curves show the performance of a single CSTR with a volume equal to the total system volume of the chain, i.e., the systems have the same HRT. Because the influent to the systems contains particulate organic matter, simulations were only performed at SRTs in excess of one day.
The most striking thing about the curves in Figure 7.2 is that although the biomass and MLSS concentrations in the two systems are essentially the same, the tanks-in-series system achieves slightly better effluent quality, with lower concentra-
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