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Figure 7.5 The variability of the oxygen requirement in each reactor for the CAS or HPOAS system described in Figure 7.4. 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\

reactors during periods of high loading. For comparison purposes, the dashed lines in Figure 7.5 show the high, low, and steady-state oxygen requirements in a single CSTR. They show clearly that the first tanks in the chain require much higher oxygen input rates and experience a greater variation in requirement due to the lower degree of dampening associated with the small bioreactor. One reason for use of single CSTRs is to take advantage of the smaller variability in oxygen requirement.

7.2.4 Variations Within the System

The dots in Figure 7.5 demonstrate that each tank in a chain of CSTRs has a different reaction rate associated with it even when the system is operating at steady-state. This suggests that further investigation of that aspect of system performance would be worthwhile. Thus, changes in the steady-state concentrations of three soluble constituents and the MLSS are shown in Figure 7.6 for three different SRTs.

The first thing to note in Figure 7.6 is that the MLSS concentration has little tank-to-tank variability. The same is true for the heterotrophic and autotrophic biomass concentrations, although they are not shown. This lack of change is because the SRT is large relative to the HRT, causing the biomass recycle to make the mixed liquor homogeneous, as discussed previously. A decline in the MLSS concentration as the solids move down the chain would only occur when the HRT approaches the SRT.

In contrast to the behavior of the particulate material, there are significant decreases in the concentrations of soluble organic substrate and ammonia-N as the fluid moves down the chain. All substrate is added to the first tank where it is mixed with biomass that has been returned from the settler. The presence there of high

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