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Oxidation-reduction potential and organic carbon sources as two control parameters for simultaneous nitrification and denitrification in biological nutrient removal processes Zhao, Hong Wang


The main objective of this study was to demonstrate the feasibility of achieving carbon (C), nitrogen (N) and phosphorus (P) removal from domestic sewage, in a two-stage intermittent aeration (IA) process, under conditions favorable to simultaneous nitrification and denitrification (SND). It was demonstrated in this study that, under the above operating conditions, the twostage process achieved levels of N, P and C removals, similar to the 3-Bardenpho process operated at a dissolved oxygen (DO) concentration of 3 mg/L. It was possible to simultaneously reduce the influent total nitrogen concentration from 24-32 mgN/L to 6-12 mgN/L and the influent total phosphorus concentration from 3.0-6.0 mgP/L to less than 1 mgP/L, in the two-stage process. Compared to the 3-Bardenpho process, the two-stage process removed the same amount of total COD (75-90%) from the influent, but it produced more solids (average 480 mg/L higher) containing a 1-3% higher volatile content. Also compared to the 3-Bardenpho process, the two-stage process produced sludge with higher SVIs; however, this did not lead to wash-out of solids. It was calculated through nitrogen balances that the unaccounted for nitrogen loss in the aeration tank (i.e., the amount of SND) accounted for up to 50% of the influent TKN for the two-stage process under low DO conditions and an average of 15% for the 3-Bardenpho process at a DO concentration of 3 mg/L. The experimental results suggested that aerobic denitrification and heterotrophic nitrification were the main causes of the loss in the current systems. However, anoxic microzone denitrification cannot be precluded. According to the pared t-tests, significant differences in process performance (e.g., the percentages of nitrification and denitrification in the IACM tank) were observed for the two-stage process, when different ORP ranges were used to control the intermittent aeration; this proved that ORP range can be used as a control parameter for SND (i.e., nitrogen removal) in the IACM tank. As confirmed by the independent t-tests, acetate and methanol additions improved both N and P removals in all three processes at acetate dosages less than 50 mgCOD/L and methanol dosages less than 30 mgCOD/L, but not at high dosages (e.g., 100 mgCOD/L for acetate and 60 mgCOD/L for methanol). It was suggested that the key factor in optimizing N and P removal in the two-stage process is to maximize carbon storage in the anaerobic zone by using ORP to control the degree of nitrification in the IACM tank. Process dynamic behavior, in response to instantaneous ammonium and nitrate shock loads, was also investigated for the three experimental systems. The dynamic responses more clearly showed that the nitrification and denitrification in the IACM tank occurred simultaneously. Based on the dynamic responses, a technique was developed to determine maximum specific nitrification rate (SNR) and maximum specific denitrification rate (SDNR). It was found that the maximum SNRs in the IACM tank (0.39-1.69 mgNH⁺₄-N/gMLVSSxh⁻¹ ) were considerably lower than those in the 3-Bardenpho aerobic zone (3.4-8.1 mgNH⁺₄- N/gMLVSSxh⁻¹); this indicated that low DO conditions inhibits nitrification. Further, the maximum SDNRs in the IACM tank were in a range of 0.16-1.26 mgNOx-N/gMLVSSxh⁻¹, which were also considerably lower than that in the 3-Bardenpho anoxic zone (2.5 mgNOx- N/gMLVSSxh⁻¹); this indicated that low DO conditions, compared to anoxic conditions, inhibits denitrification.

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