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Figure 7.20 Effect of SRT on the steady-state concentrations of various constituents in the contact (second) reactor of the CSAS system depicted in Figure 7.19. For comparison, the dashed curves represent the performance of a single CSTR with a volume of 250 m'. Influent flow = 1000 mVday. Influent concentrations are given in Table 6.6. Biomass recycle flow = 500 mVday; volume of each reactor = 125 m '. Parameters are listed in Table 6.3. The dissolved oxygen concentration was held constant at 2.0 mg/L.

The reason becomes apparent when we consider the distribution of biomass within the system, as we did for the SFAS system. Examination of Figure 7.21 reveals that in spite of the slightly poorer performance, the CSAS system has almost the same excess biomass production as a single CSTR. As was argued for the SFAS system, this suggests that the CSAS system contains approximately the same mass of MLSS as the CSTR. However, the only How entering the first bioreactor of the CSAS system is biomass recycle flow, which contains a much higher concentration of ML.SS than the flow entering the settler. Since the CSAS system contains the same mass of MLSS as the CSTR, but the first bioreactor in it contains MLSS at a very high concentration, the MLSS concentration in the second bioreactor must be less than the concentration in the single CSTR, as shown in Figure 7.20. Furthermore, since the second bioreactor in the CSAS system receives the same influent wastewater flow rate as the single CSTR, but has only half the volume of the CSTR and contains a lower concentration of MLSS, the CSAS system cannot perform as well as the single CSTR. In other words, the process loading factor for the contact tank is much higher than it is for the CSTR, so less substrate will be removed. However, as we will see later, performance of the CSAS process can be changed by altering both the relative volumes of the two bioreactors and the recycle flow rate. This means that the degree of difference in performance between the two systems depends on the configuration chosen for the CSAS system. In addition, wastewater characteristics influence system performance. For example, colloidal and particulate organic matter are entrapped in the MLSS in the contact tank and undergo more complete biodégradation in the stabilization tank. This suggests that the CSAS system is better suited for wastewaters containing a higher fraction of their organic content in the colloidal form than in the soluble form. These and other factors influencing system performance will be discussed later when we consider the choice of system configuration

Figure 7.21 Effect of SRT on the total steady-state oxygen requirement and solids wastage rate for the CSAS system depicted in Figure 7.19 operating under the conditions listed in Figure 7.20. For comparison, the dashed curves represent the performance of a single CSTR with a volume of 250 m\

SRT, days

Figure 7.21 Effect of SRT on the total steady-state oxygen requirement and solids wastage rate for the CSAS system depicted in Figure 7.19 operating under the conditions listed in Figure 7.20. For comparison, the dashed curves represent the performance of a single CSTR with a volume of 250 m\

for activated sludge systems. For now, however, we will concentrate on understanding system performance for the standard wastewater being used in all of these simulations.

Consider first Figure 7.20a where the removal of soluble organic matter is considered. The most striking thing about the performance of the CSAS system is how closely it parallels the single CSTR. The shapes of the curves are the same; only the magnitudes are different. This follows from the fact that the mass of heterotrophic biomass in the two systems is similar, which is caused by the nature of the organic substrate in the influent. Recall from Table 6.6 that the influent contains more particulate than soluble organic substrate. This particulate substrate is entrapped in the MLSS, making it available for microbial attack in both the contact and stabilization tanks. Because the SRT is the same in the CSAS system and the single CSTR, and because particulate substrate is attacked in both tanks, the opportunity for degradation of the particulate substrate is essentially the same in the two systems. That degradation results in the growth of heterotrophic biomass, which can then attack the soluble organic matter in the contact tank, resulting in even more biomass. Since more than half of the organic substrate is particulate and will be removed totally in the contact tank, and a substantial portion of the soluble organic substrate is removed at the SRTs studied, there is relatively little difference in the mass of heterotrophic biomass formed in the two systems, and thus they perform in a similar manner. The concentration of soluble substrate in the effluent from the CSAS system is higher simply because it is removed only in the contact tank, which has a smaller volume and a lower biomass concentration than the single CSTR, giving the contact tank a higher process loading factor.

In contrast, the shape of the ammonia-N curve for the CSAS system is quite different from that of the single CSTR. This difference is also reflected by the shapes of the autotrophic biomass curves for the two systems and is primarily due to the fact that most of the influent nitrogen is present in the soluble form, but also in part to the low maximum specific growth rate of autotrophic nitrifying bacteria. Examination of Table 6.6 reveals that the wastewater contains some particulate organic-nitrogen, which will be converted to ammonia-N as the particulate organic matter undergoes biodégradation. Because the particulate organic nitrogen is entrapped in the MLSS, it is present throughout the entire system and its biodégradation provides ammonia-N that is available to the nitrifying bacteria in both tanks. In addition, that portion of the mass inflow of soluble nitrogen that is recycled through the stabilization tank is also available to the nitrifiers in both bioreactors. For the recycle ratio used (0.5), this is about one third of the influent soluble nitrogen. The remainder is only available to the nitrifiers in the contact tank. When the SRT is short, but long enough to prevent washout of the nitrifiers, they will grow using the nitrogen that is available throughout the entire system and provide the basis for ammonia-N oxidation in the contact tank. However, the quantity of nitrifiers formed will be limited primarily by the amount of nitrogen that is available in the stabilization tank, because the residence time of the biomass is greater in it and there is greater opportunity for degradation of the particulate organic nitrogen, thereby making ammonia-N available to the nitrifiers. Nitrifiers grown in the stabilization tank will then pass to the contact tank where they can oxidize a portion of the ammonia-N entering from the feed. This limitation forms the break in the ammonia-N curve at short SRTs. As the SRT is increased, further reductions in the ammonia-N concentration will be due to its greater utilization in the contact tank. Because of the low half-saturation coefficient associated with autotrophic biomass growth, the autotrophs will be growing at their maximal rate in the contact tank and thus the mass of ammonia-N removed will be governed by the mass of autotrophs present. As long as excess ammonia-N is available, the mass of autotrophs will increase almost linearly with the SRT, which means that the ammonia-N concentration will decrease almost linearly, as shown. Only when the ammonia-N concentration drops sufficiently to cause the specific growth rate of the autotrophs in the contact tank to be governed by that concentration does the curve depart from linearity.

Figure 7.21 shows that significantly less oxygen is used in the CSAS system than in the single CSTR even though the solids wastage rates are very similar. TfTis difference is due to the differences in the amount of nitrification. It will be recalled from the discussion in Section 6.3 that nitrification has a major impact on oxygen utilization, but almost no impact on biomass production.

7.4.3 Dynamic Performance

Because the influent enters only the contact tank, because the concentration of biomass in that tank is low, and because its volume is half that of the single CSTR, we might expect the dynamic performance of this bioreactor system to be worse than any we have encountered so far, and that is the case, as shown in Figure 7.22. The nitrification performance of the system is particularly poor. Reexamination of Figure 7.20 shows that at an SRT of 10 days, the value used for the dynamic simulation, steady-state nitrification is incomplete, with an effluent ammonia-N concentration of about 7 mg/L. This means that even at steady-state, the nitrifying bacteria are growing near their maximal rate in the contact tank. Consequently, when the diurnal load is applied, no excess nitrification capacity exists to oxidize the additional ammonia-N that enters during peak loading periods, causing most of it to pass through to the effluent. Relatively complete nitrification only occurs when the influent mass flow rate of ammonia is sufficiently low for the mass of nitrifiers in the system to handle it.

The diurnal oxygen requirements in each of the bioreactors of the CSAS system are shown in Figure 7.23, along with the requirement in a single CSTR. The surprising thing about the curves is that the oxygen utilization rate in the stabilization tank is almost as dynamic as the utilization rate in the contact tank. Because the influent only flows through the contact tank and the stabilization tank receives a constant flow rate, one might expect the stabilization tank to show a less severe response. There are two reasons why it does not. One is that over half of the organic loading to the CSAS system is due to particulate organic matter, which is degraded in both bioreactors. Since its input varies in a diurnal manner, so does its degradation. The other is the transport of ammonia-N into the stabilization tank through the biomass recycle flow. Since the ammonia-N concentration gets quite high in the contact tank, an appreciable quantity enters the stabilization tank where the longer HRT and higher nitrifier mass allow its oxidation. The time lag associated with the transport of these materials causes a shift in the times at which the maximum and minimum uptake occur in the two tanks, however.

10 ■ ' ■ ' ■ ' ■ ' ■ ' ' ■ ■ ' ■ ' ■ ' ■ ' ■ ' ■ 0 2 4 6 8 10 12 14 16 18 20 22 24

Time, hrs

Figure 7.22 The time dependent response of the effluent from the CSAS system depicted in Figure 7.19 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 mVday; volume of each reactor - 125 m1; SRT = 10 days. Parameters are listed in Table 6.3. The dissolved oxygen concentration was held constant at 2.0 mg/L.

7.4.4 Effects of System Configuration

We saw earlier that the recycle ratio influences the performance of the SFAS system because of its effect on the distribution of biomass in the system. Thus, we would expect the recycle ratio to also affect the performance of the CSAS system, which it does, as shown in Figure 7.24. In this figure, which was generated for an SRT of 10 days, the dashed lines represent the concentrations in the contact (second) reactor, and thus represent the concentrations entering the settler, whereas the solid lines represent the concentrations in the stabilization (first) reactor.

Consider first the concentrations of heterotrophic biomass. The mass of heterotrophic biomass in the system is essentially independent of the recycle ratio between 0.1 and 1.0 because organic substrate removal is almost complete (relative to the influent) for all of those values. Thus, the differences in the concentrations in the two bioreactors shown in Figure 7.24e reflect primarily the effect of the recycle ratio on the concentrations of biomass entering the settler and leaving in the biomass

8 10 12 14 Time, hrs

Figure 7.23 The time dependent variability in the oxygen requirement in each reactor of the CSAS system described in Figure 7.22. The solid curve represents the stabilization (first) reactor and the dashed-dot curve the contact (second) reactor. For comparison, the dashed curve shows the requirement in a single CSTR with a volume of 250 m\

8 10 12 14 Time, hrs

Figure 7.23 The time dependent variability in the oxygen requirement in each reactor of the CSAS system described in Figure 7.22. The solid curve represents the stabilization (first) reactor and the dashed-dot curve the contact (second) reactor. For comparison, the dashed curve shows the requirement in a single CSTR with a volume of 250 m\

recycle stream as given by Eq. 7.7. Because the microbial mass is fixed, an increase in the recycle ratio simply shifts heterotrophic biomass from the stabilization tank to the contact tank. An increase in the mass of heterotrophic biomass in contact with the wastewater allows more soluble organic constituents to be removed, thereby improving system performance, as shown in Figure 7.24a. As might be expected, almost all soluble organic matter is gone from the stabilization tank regardless of the recycle ratio and the residual simply reflects a balance between its utilization and its production through biomass death and lysis.

The response of the autotrophic biomass is very different from that of the heterotrophic biomass and reflects the fact that the mass of autotrophic bacteria increases as the recycle ratio is increased. When the recycle ratio is small, only a small percentage of the biomass is in the contact tank. As a consequence, only a small fraction of the ammonia-N is oxidized. Furthermore, because the recycle flow rate is small, only a small portion of the ammonia-N in the contact tank effluent is transported to the stabilization tank for oxidation. Thus, only a small percentage of the total mass of nitrogen passing through the system is oxidized and only a small mass of autotrophic biomass is formed. An increase in the recycle ratio has two effects, however. First, it shifts more of the biomass from the stabilization tank to the contact tank, allowing more ammonia-N oxidation in the contact tank, thereby forming more autotrophic biomass. Second, it transports a greater fraction of the unreacted ammonia-N into the stabilization tank, allowing formation of even more autotrophic biomass. By the time the recycle ratio is around 0.8, the majority of the nitrogen flowing into the system is being oxidized, giving a relatively constant mass of nitrifiers, so that further changes in the recycle ratio simply act to redistribute them in the same manner as the heterotrophic biomass.

One other point about nitrification needs clarification, and that concerns the concentration of nitrate-N in the stabilization tank. It can be seen in Figure 7.24c

Heterotrophic Biomass

» Soluble Organics \

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