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SRT, days

Figure 7.3 Effect of SRT on the total steady-state oxygen requirement and solids wastage rate for the CAS or HPOAS system depicted in Figure 7.1 operating under the conditions listed in Figure 7.2. For comparison, the dashed curves represent the performance of a single CSTR with a volume of 250 m".

5, even when a tanks-in-series reactor is to be used. Furthermore, these similarities tell us that the mass of MLSS in the two systems will be similar at a fixed SRT. Consequently, the simple equations for a CSTR can also be used to estimate the Xm-t product. This ability to use the simple equations is very helpful during process design, as seen in Chapter 10.

7.2.3 Dynamic Performance

Figures 7.2 and 7.3 present only the steady-state responses of the chain of CSTRs, but, as seen in Chapter 6, biochemical operations are often subjected to diurnal changes in loading. Thus, it is important to consider how bioreactor configuration influences dynamic response. Because 10 days is a commonly used value for the SRT in activated sludge systems, the dynamic response of the system in Figure 7.1 was studied at that SRT by imposing the diurnal loading pattern shown in Figure 6.2. The average flow rate to the system was 1000 m'/day, the same as in the steady-state simulations, and the flow-weighted average concentrations of the various constituents were the same as those in Table 6.6. Because the alkalinity of a wastewater is determined primarily by the characteristics of the carriage water, rather than by the waste constituents, the alkalinity was assumed to be constant at the value in Table 6.6.

The output response of the tanks-in-series system to repeated application of the loading pattern in Figure 6.2 is shown in Figure 7.4 by the solid curves. For comparison, the response of a single CSTR with the same SRT and average HRT is shown by the dashed curves. Only the soluble constituents are shown because the particulate constituents are relatively constant due to the length of the SRT relative to the HRT. Examination of the curves shows that the variability in the effluent concentrations of the two reactants, organic substrate and ammonia-N, was much less for the chain of CSTRs than for the single CSTR. This is a direct result of the hydraulic differences between the two systems. Any change in the influent to a single CSTR is seen instantly in the effluent because the bioreactor and the effluent have the same concentration. The chain experiences time delays, however, as the fluid moves from tank to tank. This gives a greater opportunity for degradation or transformation of the substrates. The effect is particularly significant for ammonia because of the small maximum specific growth rate of nitrifying bacteria.

An important assumption in all of the simulations was that the dissolved oxygen concentration was constant at 2.0 mg/L, a situation that can be achieved in practice through DO control. We see in Figure 6.13, however, that imposition of the typical diurnal loading pattern causes considerable variation in the oxygen requirement within a single CSTR and that failure to meet that requirement causes a deterioration in system performance. Thus, it is important to consider how the diurnal loading influences the oxygen requirement in each bioreactor of the chain so that appropriate provisions can be made for delivering the needed amount. This is shown in Figure 7.5. In this figure, the vertical lines represent the range in the oxygen consumption rate experienced in each bioreactor as the loading varies over a 24-hr period. As expected, because the first bioreactor receives the variable waste load directly, it experiences the greatest variability, exhibiting a four-fold range in oxygen consumption rate. The second bioreactor receives only the substrate that is not removed by the first, and the magnitude of the oxygen requirement and the variability associated

Figure 7.4 The time dependent response from the CAS or HPOAS system depicted in Figure 7.1 when subjected to the diurnal loading patterns shown in Figure 6.2. For comparison, the dashed curves represent the performance of a single CSTR with a volume of 250 m\ Average influent flow = 1000 m'/day. Average influent concentrations are given in Table 6.6. Biomass recycle flow = 500 m'/day; volume of each reactor = 50 m'; SRT = 10 days. Parameters are listed in Table 6.3. The dissolved oxygen concentration was held constant at 2.0 mg/L.

Time, hrs

Figure 7.4 The time dependent response from the CAS or HPOAS system depicted in Figure 7.1 when subjected to the diurnal loading patterns shown in Figure 6.2. For comparison, the dashed curves represent the performance of a single CSTR with a volume of 250 m\ Average influent flow = 1000 m'/day. Average influent concentrations are given in Table 6.6. Biomass recycle flow = 500 m'/day; volume of each reactor = 50 m'; SRT = 10 days. Parameters are listed in Table 6.3. The dissolved oxygen concentration was held constant at 2.0 mg/L.

with it are lower, and so on down the chain. This means that any system that uses a tanks-in-series type reactor must be designed to handle much different oxygen requirements at different points in the system. The dots on the bars represent the steady-state oxygen requirement in each bioreactor caused by the entrance of a constant flow and concentration of wastewater to the system. They clearly show two things: (1) a system designed only on the basis of the steady-state requirement would be inadequate during periods of peak loading, and (2) equalization of influent flow and concentration allows use of a smaller oxygen transfer system. The figure also shows that the peak to average ratio changes from bioreactor to bioreactor, becoming particularly large in bioreactor three. This is primarily due to nitrification, which occurs further downstream than organic substrate removal during periods of high loading. This is because nitrifying bacteria have a much smaller maximum specific growth rate than heterotrophic bacteria. As a consequence, the nitrifiers cannot increase their metabolism as much in response to the increased input rate of substrate, allowing a larger fraction of the ammonia-N to pass through to the downstream c a> E

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