Effect of H₂S on N₂O Reduction and Accumulation during Denitrification by Methanol Utilizing Denitrifiers

Sulfide is produced in sewer networks, and previous studies suggest that sulfide in sewage could alter the activity of heterotrophic denitrification and lead to N₂O accumulation during biological wastewater treatment. However, the details of this phenomenon are poorly understood. In this study, the potential inhibitory effects of sulfide on nitrate, nitrite, and N₂O reduction were assessed with a methanol-utilizing denitrifying culture both prior to and after its exposure and adaptation to sulfide. Hydrogen sulfide was found to be strongly inhibitory to N₂O reduction, with 50% inhibition observed at H₂S concentrations of 0.04 mg H₂S–S/L and 0.1 mg H₂S–S/L for the unadapted and adapted cultures, respectively. In comparison, both nitrate and nitrite reduction was more tolerant to H₂S. A 50% inhibition of nitrite reduction was observed at approximately 2.0 mg H₂S–S/L for both unadapted and adapted cultures, while no inhibition of nitrate reduction occurred at the highest H₂S concentrations applied (2.0 mg H₂S–S/L) to either culture. N₂O accumulation was observed during nitrate and nitrite reduction by the adapted culture when H₂S concentrations were above 0.5 and 0.2 mg H₂S–S/L, respectively. Additionally, we reveal that hydrogen sulfide (H₂S), rather than sulfide, was likely the true inhibitor of N₂O reduction, and the inhibitory effect was reversible. These findings suggest that sulfide management in sewers could potentially have a significant impact on N₂O emission from wastewater treatment plants

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

Stereoselective Radical Cyclization Cascades Triggered by Addition of Diverse Radicals to Alkynes To Construct 6(5)–6–5 Fused Rings

Cascade radical cyclization of alkynyl ketones with various carbon- and heteroatom-centered radical precursors via a sequential radical addition/1,5-H radical shift/5-<i>exo-trig</i>/radical cyclization process was realized for the first time. This method provides a strategically novel and step-economical protocol for diversity-oriented synthesis of a wide range of carbocyclic and heterocyclic 6(5)–6–5 fused ring systems with three contiguous stereocenters, including a quaternary carbon in high yields with excellent chemo- and diastereoselectivity

Free Nitrous Acid (FNA)-Based Pretreatment Enhances Methane Production from Waste Activated Sludge

Anaerobic digestion of waste activated sludge (WAS) is currently enjoying renewed interest due to the potential for methane production. However, methane production is often limited by the slow hydrolysis rate and/or poor methane potential of WAS. This study presents a novel pretreatment strategy based on free nitrous acid (FNA or HNO₂) to enhance methane production from WAS. Pretreatment of WAS for 24 h at FNA concentrations up to 2.13 mg N/L substantially enhanced WAS solubilization, with the highest solubilization (0.16 mg chemical oxygen demand (COD)/mg volatile solids (VS), at 2.13 mg HNO₂–N/L) being six times that without FNA pretreatment (0.025 mg COD/mg VS, at 0 mg HNO₂–N/L). Biochemical methane potential tests demonstrated methane production increased with increased FNA concentration used in the pretreatment step. Model-based analysis indicated FNA pretreatment improved both hydrolysis rate and methane potential, with the highest improvement being approximately 50% (from 0.16 to 0.25 d^–1) and 27% (from 201 to 255 L CH₄/kg VS added), respectively, achieved at 1.78–2.13 mg HNO₂–N/L. Further analysis indicated that increased hydrolysis rate and methane potential were related to an increase in rapidly biodegradable substrates, which increased with increased FNA dose, while the slowly biodegradable substrates remained relatively static