O

0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Volume of the Aerobic Reactoras a Fraction of Total System Volume

Figure 7.33 Effect of the relative volumes of the two reactors on the steady-state concentrations of various constituents in each reactor of the MLE system described in Figure 7.29. The total system volume was constant at 250 m\ SRT = 10 days. The solid curves represent the anoxic (first) reactor and the dashed curves the aerobic (second) reactor.

determined by the minimum allowable aerobic SRT. It will be recalled that ASM No. 1 was not developed for simulating prolonged anaerobic periods. However, once washout of the autotrophic biomass has occurred, no nitrate will be present, which means that bioreactor 1 will be anaerobic rather than anoxic. Because the model is not valid under those conditions and because it was apparent that failure of nitrification was occurring at an aerobic fraction of 0.35, lower aerobic fractions were not investigated.

The rise of ammonia-N in both bioreactors and the drop in nitrate-N in the aerobic bioreactor as the aerobic fraction is reduced reflects the washout of the autotrophic biomass discussed above. Furthermore, the rapid rise in the concentration of soluble organic matter as the aerobic fraction is reduced below 0.4 also reflects this washout. When the amount of nitrate-N being returned to the first bioreactor is greatly reduced, the reactions in it became severely limited by the availability of electron acceptor, preventing complete utilization of the organic matter. This limitation is also reflected in the heterotrophic biomass and MLSS concentrations. The former drops due to curtailed growth in the anoxic bioreactor whereas the latter rises due to the accumulation of particulate organic matter.

As the aerobic fraction is increased above 0.7 the nitrate-N and soluble organic matter concentrations in the anoxic bioreactor begin to rise because the HRT is insufficient to allow complete reaction. This effect becomes especially severe at aerobic fractions above 0.8, and both soluble organic matter and nitrate-N leave the anoxic bioreactor unreacted.

Aerobic fractions between 0.5 and 0.7 produce effluents with about the same total nitrogen concentrations (ammonia-N plus nitrate-N), although a greater fraction of that nitrogen is in the form of nitrate-N when the aerobic fraction is larger. This suggests that designers of MLE systems have some latitude in the distribution of the system volume between the anoxic and aerobic zones. Furthermore, that latitude increases as the system SRT is increased, suggesting that systems with longer SRTs have greater operational flexibility.

Figure 7.34 shows the effects of the aerobic fraction of the system volume on the nitrate and oxygen utilization rates. The curves are consistent with previous explanations. Nitrate utilization is low at small aerobic fractions because little is being produced and is low at large aerobic fractions because insufficient time is available in the anoxic bioreactor for its utilization. Likewise, oxygen utilization is low at small aerobic fractions because little nitrification is occurring, increases as more nitrification is achieved, and finally increases again as denitrification is curtailed, requiring more of the electrons associated with organic matter in the influent to be transferred to oxygen as the terminal acceptor.

Finally, it was pointed out at the beginning of this section that system HRT has little impact on performance.This point should be emphasized. The curves in Figures 6.33 and 6.34 change little when the system HRT is changed for a constant SRT. The important variable is the fraction of the HRT that is anoxic or aerobic, not the actual residence time in each zone.

7.6 BARDENPHO PROCESS 7.6.1 Description

As discussed in Section 7.1.1, a disadvantage of the MLE reactor configuration is that the effluent will always contain appreciable quantities of nitrate-N because nitrification occurs in the last bioreactor and the MLR is from that bioreactor. The Bardenpho1 process overcomes this by adding an anoxic bioreactor after the aerobic one in which denitrification can occur by biomass decay and the utilization of slowly-biodegradable substrate. In addition, to prevent biomass settling problems associated with denitrification in the final settler, a small aerobic bioreactor is usually used as the final zone.1 This reactor configuration is illustrated in Figure 7.35. Selection of the best combinations of bioreactor sizes is a complex question that requires the use of optimization techniques"s and it will be discussed in Chapter 11. For the purpose of this discussion, however, the total bioreactor volume was kept at 250 m', the value used in all of the simulations in this chapter, and the aerobic fraction was kept at 50%, the value used in Figures 7.29-7.32. The sizes of bioreactors one through four were selected as 50, 100, 75, and 25 m', respectively. The MLR ratio from bioreactor two to bioreactor one was maintained at 2.0, the value used in Figures 7 29, 7.30, 7.33, and 7.34.

Figure 7.34 Effect of the relative volumes of the two reactors on the total steady-state oxygen requirement and nitrate utilization rate in the MLE system described in Figure 7.29. The total system volume was constant at 250 m\ SRT = 10 days.

Fractional Volume in Aerobic Reactor

Figure 7.34 Effect of the relative volumes of the two reactors on the total steady-state oxygen requirement and nitrate utilization rate in the MLE system described in Figure 7.29. The total system volume was constant at 250 m\ SRT = 10 days.

7.6.2 Effect of SRT on Steady-State Performance

The effect of SRT on the performance of the Bardenpho system is shown by the solid curves in Figures 7.36 and 7.37. For comparison, the performance of the MLE system from Figures 7.29 and 7.30 is shown by the dashed curves. Comparison of the curves shows that both systems contain similar quantities of biomass, but that the Bardenpho system achieves an effluent with less ammonia-N and less nitrate-N than the MLE system by achieving more denitrification. This is done even though the aerobic and anoxic fractions are the same as in the MLE system. Even lower effluent concentrations could be attained by proper selection of the aerobic fraction,

Figure 7.35 Schematic diagram of four CSTRs in series with all influent and all biomass recycle to the first reactor, in which the first and third reactors are anoxic and the second and fourth are aerobic. The first reactor receives mixed liquor recirculation flow from the second. Although not shown, solids wastage is directly from all reactors. This configuration simulates the Bardenpho process.

Figure 7.35 Schematic diagram of four CSTRs in series with all influent and all biomass recycle to the first reactor, in which the first and third reactors are anoxic and the second and fourth are aerobic. The first reactor receives mixed liquor recirculation flow from the second. Although not shown, solids wastage is directly from all reactors. This configuration simulates the Bardenpho process.

Soluble Organics

Soluble Organics

1,000

Heterotrophic Biomass

1,000

Heterotrophic Biomass

Nitrate-N I.I.I

Nitrate-N I.I.I

Autotrophic Biomass I . I ■ I i I

8 12 16 SRT, days

0 0

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