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

Modeling of Simultaneous Anaerobic Methane and Ammonium Oxidation in a Membrane Biofilm Reactor

Nitrogen removal by using the synergy of denitrifying anaerobic methane oxidation (DAMO) and anaerobic ammonium oxidation (Anammox) microorganisms in a membrane biofilm reactor (MBfR) has previously been demonstrated experimentally. In this work, a mathematical model is developed to describe the simultaneous anaerobic methane and ammonium oxidation by DAMO and Anammox microorganisms in an MBfR for the first time. In this model, DAMO archaea convert nitrate, both externally fed and/or produced by Anammox, to nitrite, with methane as the electron donor. Anammox and DAMO bacteria jointly remove the nitrite fed/produced, with ammonium and methane as the electron donor, respectively. The model is successfully calibrated and validated using the long-term (over 400 days) dynamic experimental data from the MBfR, as well as two independent batch tests at different operational stages of the MBfR. The model satisfactorily describes the methane oxidation and nitrogen conversion data from the system. Modeling results show the concentration gradients of methane and nitrogen would cause stratification of the biofilm, where Anammox bacteria mainly grow in the biofilm layer close to the bulk liquid and DAMO organisms attach close to the membrane surface. The low surface methane loadings result in a low fraction of DAMO microorganisms, but the high surface methane loadings would lead to overgrowth of DAMO bacteria, which would compete with Anammox for nitrite and decrease the fraction of Anammox bacteria. The results suggest an optimal methane supply under the given condition should be applied not only to benefit the nitrogen removal but also to avoid potential methane emissions

Efficient Chloroquine Removal by Electro-Fenton with FeS₂‑Modified Cathode: Performance, Influencing Factors, Pathway Contributions, and Degradation Mechanisms

The application of chloroquine (CLQ) due to its antibacterial/antiviral nature and high potential of being persistent and bioaccumulative poses a significant environmental threat. In this study, the electro-Fenton (EF) process with pyrite (FeS2)-modified graphite felt (FeS2/GF) as the cathode (EF-FeS2/GF), capable of providing a stable acidic environment with a solution pH of 3.0 was constructed and found to (i) achieve 83.3 ± 0.4% 60 min CLQ removal and (ii) maintain about 60.0% CLQ removal during consecutive batch tests. FeS2 loading amount, current density applied, and spacing between electrodes all influenced the efficacy of EF-FeS2/GF, with the optimum CLQ removal obtained at 10 mg, 150 mA, and 2.0 cm, respectively. Adsorption and electrocatalysis were both observed to contribute to the CLQ removal while the EF process with the verified functioning of ·OH played a dominant role. Based on the detected intermediates with identified ecotoxicities, two main paths were postulated to describe the degradation processes which led to the mineralization of CLQ. These findings supported that the EF-FeS2/GF could be an efficient technology to treat wastewater contaminated with CLQ

Synthesis of Core–Shell Magnetic Nanocomposite Fe₃O₄@ Microbial Extracellular Polymeric Substances for Simultaneous Redox Sorption and Recovery of Silver Ions as Silver Nanoparticles

Microbial extracellular polymeric substance (EPS) is a complex high molecular weight compound secreted from many organisms. In this work, magnetic nanocomposite Fe₃O₄@EPS of <i>Klebsiella</i> sp. J1 were first synthesized for silver ions (Ag⁺) wastewater remediation, which synergistically combined the advantages of the easy separation property of magnetic Fe₃O₄ nanoparticles and the superior adsorption capacity of EPS of <i>Klebsiella</i> sp. J1. The physical and chemical properties of Fe₃O₄@EPS were analyzed comprehensively. Fe₃O₄@EPS exhibited the well-defined core–shell structure (size 50 nm) with high magnetic (79.01 emu g^–1). Batch adsorption experiments revealed that Fe₃O₄@EPS achieved high Ag⁺ adsorption capacity (48 mg g^–1), which was also much higher than many reported adsorbents. The optimal solution pH for Ag⁺ adsorption was around 6.0, with the sorption process followed pseudo-second-order kinetics. Ag⁺ adsorption on Fe₃O₄@EPS was mainly attributed to the reduction of Ag⁺ to silver nanoparticles (AgNPs) by benzenoid amine (−NH−), accompanied by the chelation between Ag⁺ and hydroxyl groups, ion exchange between Ag⁺ and Mg²⁺ and K⁺, and physical electrostatic sorption. The repeated adsorption–desorption experiments showed a good recycle performance of Fe₃O₄@EPS. This study has great importance for demonstrating magnetic Fe₃O₄@EPS as potential adsorbent to remove Ag⁺ from contaminated aquatic systems