Wastewater sector is known to be a major but understudied source of N₂O emissions. Current wastewater N₂O emission inventories are built on point or sparse measurements which are insufficient to represent plant-wide, regional, or countrywide emissions. Here, we present an up-to-date metadata analysis of N₂O emission factors (EFs) for centralized wastewater treatment and show discrepancies in both magnitude and uncertainties among various measurement scales, treatment processes, geographical locations, and monitoring techniques.

We find plant-wide N₂O EFs span four orders of magnitude, from 0.003% to 31% of influent TN load, with high emissions stem from biological treatment reactors with average (lower-upper 95% CI) N₂O emission at 1.36% (0.85% - 1.86%) of influent TN load. Besides, sludge treatment (0.37 (0.07 % - 0.67%) of influent TN load) and side stream treatment (1.82% (1.27% - 2.36%) of influent TN load) also emit a large amount of N₂O, which are overlooked in current wastewater GHG inventories. For nationwide N₂O emissions from centralized, municipal wastewater treatment in the U.S., we emphasize the IPCC (2019) Tier 2 methodology overestimates N₂O emissions from the wastewater sector by around 90% compared with estimates using updated dataset. The results indicate that a better documentation and classification of N₂O EFs based on measurement scales and treatment processes will enhance the accuracy of regional and countrywide N₂O emission inventories. This study provides a foundational basis for the development of decarbonization pathways for the wastewater sector, thereby informing future N₂O mitigation efforts aimed at mitigating wastewater sector’s impact on the climate change crisis.

Methane is a major greenhouse gas that holds > 80 times the global warming potential of carbon dioxide when measured in a 20-year duration. The wastewater system represents one of the major sources of the anthropogenic methane emission. Although sewer was once estimated to contribute 33% of the ground methane emission, it has been frequently overlooked. In a closed, dynamic, and complex environment like sewer pipes, previous efforts showed it is too costly and difficult to conduct direct emission monitoring using existing sensors and other devices. Fortunately, sewer-originated methane is almost exclusively biogenic via methanogenesis, and there are plenty of metagenomic studies that analyzed world-wide microbial composition of sewer systems. Therefore, it is possible to estimate methane emissions from microbiome information. Combining the power of data mining and bioinformatics databases, we developed an approach that analyzed literature taxonomic data in conjunction with microbial function potential to unravel the sewer microbiome. Environmental and operational factors were used in concert with the calculated microbial potential to reveal the deterministic mechanism of methane emission from sewer. Organic loading and sulfide concentration were identified as the most important environmental factors in impacting sewer methane emission. K-Means clustering categorized three scenarios of methane emission mainly determined by the hydrogenotrophic and acetoclastic methanogenic potentials in the microbiome, which interestingly possessed opposite impacts on methane emission rate. These results reveal the methanogenic potential in the sewer microbiome for the first time, providing insights on possible mitigation strategies.

Treatment wetlands (TW) designed to treat wastewater efficiently can also generate nitrous oxide (N₂O), methane (CH₄), and carbon dioxide (CO₂). These greenhouse gas (GHG) emissions remain poorly quantified and characterized, especially in cold-climate systems. This study explores the GHG fluxes from a two-stage, cold-climate vertical-flow TW treating ski area wastewater at 3°C average water temperature. The system is designed similarly to a modified Ludzak-Ettinger process, with the first stage a saturated, denitrifying TW followed by an unsaturated nitrifying TW and recycling of nitrified effluent, with wastewater intermittently dosed onto each stage. Over seven years of winter operation, it has demonstrated effective COD and nitrogen removal in high-strength wastewater.

Following two closed-loop, intensive GHG winter sampling campaigns at the TW, the magnitude of N₂O flux was 2.2 times higher for denitrification than nitrification. Methane and N₂O emissions were strongly correlated with hydraulic loading, whereas CO₂ was correlated with surface temperature. GHG fluxes from each stage were related to microbial activity and outgassing of dissolved species during wastewater dosing; thus, sampling time relative to dosing strongly influenced observed emissions. These results suggest that GHG fluxes from TW may be over or underestimated if mass transfer and mechanisms of wastewater application are not considered. Emission factors for N₂O and CH₄ were 0.29% as kg N₂O-N/kg TN_removed and 0.05% kg CH₄-C/kg COD_removed, respectively_. The system had an observed seasonal global warming potential of 629.4 kg of CO₂ equivalent GHG. These results indicate a need to enhance current wastewater treatment processes to target GHG mitigation.

Constructed wetlands can sustainably contribute to water and wastewater treatment by removing contaminants such as nutrients and recalcitrant organic compounds using less energy than actively managed engineering solutions. However, biological treatment is accompanied by the release of greenhouse gases (GHGs) with the potential to counter these gains. Unit process open-water (UPOW) constructed wetlands are one such system characterized by a benthic, photosynthetic microbial mat which colonizes in a shallow (20-30 cm), macrophyte-free water column. Here, we quantify and compare the GHG emissions of this unique nature-based treatment system with more traditional vegetated wetlands operated within the same Prado Constructed Wetlands complex. Across three seasons, we contrast paired GHG flux measurements of CH₄, CO₂, and N₂O with radiative forcing calculations to estimate annual, seasonal, and diel trends. Considering all sampling campaigns, UPOW wetlands emitted more CH₄ and N₂O whereas vegetated wetlands emitted more CO₂. Accounting for global warming potentials indicated similar overall radiative forcing between the two designs, with CH₄ comprising most (~58%) of overall UPOW wetland forcing whereas vegetated wetlands were dominated by CO₂ (~77%). When parsed by season, UPOW wetlands exerted greater radiative forcing relative to vegetated wetlands in summer months (June), the two designs were comparable in fall months (October), and vegetated wetlands exerted greater radiative forcing in winter months (December). Finally, UPOW wetland CH₄ flux measurements represent a modest increase relative to typical wastewater treatment plant CH₄ emissions (~10X), with ongoing work aiming to improve these estimates by incorporating CO₂ and N₂O, as well as energy requirements.

Low-emission H₂ has emerged as a key player in the carbon-neutral future, as it holds a good potential to decarbonize the industry sectors where direct electrification is hard to achieve. Though fossil-fuel based production such as steam-methane reforming (SMR) and coal gasification remain the major source of H₂, green hydrogen that is produced from water electrolysis powered by renewable electricity is expected to grow exponentially in the coming years. However, green hydrogen production requires approximately 10 kg of H₂O for each kg of H₂, and current electrolyzers require high-purity water. Such quantity and quality requirements, along with the spatial disparity of renewable energy resources and water resources, may hinder the deployment of green H₂ projects across the nation. Previous work has demonstrated that non-traditional water source could provide abundant water access while adding marginal cost. In this work, we proposed an infrastructural symbiosis between wastewater treatment plants (WWTPs) and green H₂ producers (GH₂Ps), where the water consumption of green H₂ production is satisfied by reclaimed water, and the produced byproduct O₂ is supplied to WWTPs for more efficient aeration. We developed a framework for determining the optimal symbiotic pair based on economic constraints and geographic information, and we applied the framework to the established geodatabase of major H₂ producers in the US. Based on the optimal symbiosis scenario, we further quantified the benefits including freshwater saving and reduction in the levelized cost of H₂ (LCOH₂), demonstrating that such a symbiotic scenario could bring both environmental and economic benefits.