Figure 6.11 Effect of oxygen input rate on denitrification in a CSTR operated at an SRT of 240 hrs. Parameter values are given in Table 6.3 and the influent conditions are given in Table 6.6. For the limiting COD case the influent nitrate-N concentration was 60 mg/L whereas in the excess COD case it was 50 mg/L. The influent flow was 1000 m'/day.
reduction. Although the magnitudes of the values given in Figure 6.11 are specific for the reactor conditions and parameter values used in the simulations, the results show clearly the importance of controlling the entrance of oxygen into a bioreactor in which denitrification is occurring.
The AS/AN ratio is calculated from process stoichiometry, and simply tells us how much of one reactant will be removed in proportion to another. If the mass input rate of biodegradable COD into a denitrifying bioreactor is greatly in excess of that needed to remove the nitrate-N present, then the effluent biodegradable COD will be high (because there is insufficient acceptor to which to transfer electrons) and the nitrate-N concentration will be low and rate controlling. In other words, the term Ss/(Ks + Ss) in the rate expression for process 2 in Table 6.1 will approach 1.0 and the term SN(,/(KN(, + SNO) will be small. Conversely, if the input rate of biodegradable COD is less than that needed to remove the nitrate-N, then the effluent nitrate-N concentration will be high (because there is insufficient donor to provide the needed electrons) and the biodegradable COD concentration will be low and rate controlling. In other words, SNO/(KM> + SN()) will approach 1.0 and Ss/(Ks + Ss) will be small. The only way for both concentrations to be low and for both to simultaneously influence the rates of activity in the bioreactor is for the influent concentrations to closely match the required AS/AN ratio. Given the influence of the entrance of small amounts of oxygen, as discussed above, and the variability associated with the values of the stoichiometric and kinetic coefficients in mixed microbial communities such as those used in wastewater treatment, this is difficult to achieve. For example, Figure 6.12,g presents results from a study in which the relative amounts of biodegradable COD and nitrate-N (expressed as the carbon:nitrogen ratio) in the influent to a CSTR were varied. There it can be clearly seen that there was only a small range of influent ratios over which the effluent concentrations of both constituents were low. To overcome this problem, it is common practice to add the
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