SRT, days

Figure 6.10 Effect of SRT on the AS/AN ratio. The reference numbers refer to the sources of the data. (From D. J. Engberg and E. D. Schroeder, Kinetics and stoichiometry of bacterial denitrification as a function of cell residence time. Water Research 9:1051-1054, 1975. Copyright © Elsevier Science Ltd.; reprinted with permission.)

electron donor that must be added to reduce a given amount of nitrate-N. In other words, it will increase the AS/AN ratio.

The impact of the input of oxygen into a denitrification reactor can best be seen through use of the model in Table 6.1 because it considers both aerobic and anoxic growth of the heterotrophic bacteria and the effect of dissolved oxygen on each. To illustrate this point, a wastewater like that in Table 6.6 was assumed to be entering a CSTR operated at an SRT of 240 hr. The kinetic and stoichiometric coefficients describing the biomass in the bioreactor were assumed to have the values given in Table 6.3, except for p.Awhich was set equal to zero to ensure only heterotrophic reactions. Two situations were considered. In one. the influent nitrate-N concentration was set equal to 50 mg/L, which gave a mass input rate equivalent to 143 kg/day of oxygen. Given the values of the stoichiometric coefficients in Table 6.3, the amount of biodegradable COD entering the bioreactor (265 kg/day) was in excess of that needed to meet the required AS/AN ratio. This case is called the excess COD case. In the other, the influent nitrate-N concentration was set equal to 60 mg/L, giving a mass input rate equivalent to 172 kg/day of oxygen, which was more than could be completely removed by the available COD. This is called the limiting COD case. Simulations were then conducted in which the rate of oxygen transfer into the bioreactor was set at various values and the results are shown in Figure 6.11. For the excess COD case, the input of a significant amount of oxygen could be tolerated without having an effect on the effluent nitrate-N concentration because the oxygen simply acted to allow more removal of COD. Eventually, however, a point was reached at which nitrate-N removal deteriorated because the total input of electron acceptor, i.e., nitrate-N plus oxygen, exceeded the amount of electrons available from the donor. For the limiting COD case, the entrance of even a small amount of oxygen caused the nitrate-N concentration to increase because any oxygen entering the bioreactor accepted electrons that otherwise would have gone for nitrate-N

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