The Confounding Effect of Nitrite on N₂O Production by an Enriched Ammonia-Oxidizing Culture

The effect of nitrite (NO₂^–) on the nitrous oxide (N₂O) production rate of an enriched ammonia-oxidizing bacteria (AOB) culture was characterized over a concentration range of 0–1000 mg N/L. The AOB culture was enriched in a nitritation system fed with synthetic anaerobic digester liquor. The N₂O production rate was highest at NO₂^– concentrations of less than 50 mg N/L. At dissolved oxygen (DO) concentration of 0.55 mg O₂/L, further increases in NO₂^– concentration from 50 to 500 mg N/L resulted in a gradual decrease in N₂O production rate, which maintained at its lowest level of 0.20 mg N₂O–N/h/g VSS in the NO₂^– concentration range of 500–1000 mg N/L. The observed NO₂^–-induced decrease in N₂O production was even more apparent at increased DO concentration. At DO concentrations of 1.30 and 2.30 mg O₂/L, the lowest N₂O production rate (0.25 mg N₂O–N/h/g VSS) was attained at a lower NO₂^– concentration of 200–250 mg N/L. These observations suggest that N₂O production by the culture is diminished by both high NO₂^– and high DO concentrations. Collectively, the findings show that exceedingly high NO₂^– concentrations in nitritation systems could lead to decreased N₂O production. Further studies are required to determine the extent to which the same response to NO₂^– is observed across different AOB cultures

Mathematical Modeling of Nitrous Oxide (N₂O) Emissions from Full-Scale Wastewater Treatment Plants

Mathematical modeling of N₂O emissions is of great importance toward understanding the whole environmental impact of wastewater treatment systems. However, information on modeling of N₂O emissions from full-scale wastewater treatment plants (WWTP) is still sparse. In this work, a mathematical model based on currently known or hypothesized metabolic pathways for N₂O productions by heterotrophic denitrifiers and ammonia-oxidizing bacteria (AOB) is developed and calibrated to describe the N₂O emissions from full-scale WWTPs. The model described well the dynamic ammonium, nitrite, nitrate, dissolved oxygen (DO) and N₂O data collected from both an open oxidation ditch (OD) system with surface aerators and a sequencing batch reactor (SBR) system with bubbling aeration. The obtained kinetic parameters for N₂O production are found to be reasonable as the 95% confidence regions of the estimates are all small with mean values approximately at the center. The model is further validated with independent data sets collected from the same two WWTPs. This is the first time that mathematical modeling of N₂O emissions is conducted successfully for full-scale WWTPs. While clearly showing that the NH₂OH related pathways could well explain N₂O production and emission in the two full-scale plants studied, the modeling results do not prove the dominance of the NH₂OH pathways in these plants, nor rule out the possibility of AOB denitrification being a potentially dominating pathway in other WWTPs that are designed or operated differently

Modeling of Nitrous Oxide Production by Autotrophic Ammonia-Oxidizing Bacteria with Multiple Production Pathways

Autotrophic ammonia oxidizing bacteria (AOB) have been recognized as a major contributor to N₂O production in wastewater treatment systems. However, so far N₂O models have been proposed based on a single N₂O production pathway by AOB, and there is still a lack of effective approach for the integration of these models. In this work, an integrated mathematical model that considers multiple production pathways is developed to describe N₂O production by AOB. The pathways considered include the nitrifier denitrification pathway (N₂O as the final product of AOB denitrification with NO₂^– as the terminal electron acceptor) and the hydroxylamine (NH₂OH) pathway (N₂O as a byproduct of incomplete oxidation of NH₂OH to NO₂^–). In this model, the oxidation and reduction processes are modeled separately, with intracellular electron carriers introduced to link the two types of processes. The model is calibrated and validated using experimental data obtained with two independent nitrifying cultures. The model satisfactorily describes the N₂O data from both systems. The model also predicts shifts of the dominating pathway at various dissolved oxygen (DO) and nitrite levels, consistent with previous hypotheses. This unified model is expected to enhance our ability to predict N₂O production by AOB in wastewater treatment systems under varying operational conditions