Quantifying and attributing dissolved nitrous oxide discharged in sewage treatment plant effluent, and incorporating it into nitrous oxide emission inventories

Nitrous oxide ($\mathrm{N_2O}$) emissions from sewage treatment plants (STPs) are conventionally accounted for as direct gaseous release from treatment reactors plus indirect $\mathrm{N_2O}$ produced in receiving waters from effluent nitrogen under IPCC guidelines. Masuda et al. (2018) showed that a third pathway — dissolved $\mathrm{N_2O}$ already supersaturated in the effluent and discharged with it — can rival the indirect term (at their plant the winter dissolved discharge was almost equal to the IPCC-factor indirect emission and equalled about 53% of the plant's total $\mathrm{N_2O}$ emission, that plant-side total being direct gaseous release plus dissolved discharge, versus 12.6% in summer) yet is neither systematically quantified across plants, climates, and seasons, nor represented in inventory methodology. The open problem is to close the STP $\mathrm{N_2O}$ budget by establishing this effluent dissolved-$\mathrm{N_2O}$ pathway: measuring its magnitude across treatment configurations and operational regimes, attributing downstream $\mathrm{N_2O}$ emissions between near-field gasification of the discharged dissolved pool and in-stream biological production, testing the hypothesized operational drivers (seasonal solids retention time and mixed liquor adjustments, nitrite accumulation), and developing measurement and accounting methods that represent pre-formed dissolved effluent $\mathrm{N_2O}$ in greenhouse-gas inventories. Later literature through 2026 has consolidated the direct plant-emission pathway (Song et al. 2024) and downstream concentration hotspots (Chun et al. 2020; Wang et al. 2021; Tang et al. 2024), but the attribution and inventory-integration core remains open: Tang et al. (2024) state that downstream $\mathrm{N_2O}$ remains poorly constrained and that production pathways must still be disentangled, and the IPCC 2019 Refinement provides no method for pre-formed dissolved $\mathrm{N_2O}$ in effluent.

Background

Nitrous oxide ($\mathrm{N_2O}$) is a potent greenhouse gas and the dominant stratospheric ozone-depleting emission, and waste and wastewater release roughly $0.3\ \mathrm{Tg\ N\ yr^{-1}}$ of it globally. In biological sewage treatment plants (STPs, also called wastewater treatment plants, WWTPs), $\mathrm{N_2O}$ is produced as a by-product of nitrification (microbial oxidation of ammonia to nitrate) and as an intermediate of denitrification (microbial reduction of nitrate to nitrogen gas), and is emitted in gas form from aerated reactors. Conventional greenhouse-gas accounting treats STP $\mathrm{N_2O}$ as two pathways: direct emissions from the treatment plant, and indirect emissions — $\mathrm{N_2O}$ produced within receiving waters from the reactive nitrogen (ammonium, nitrite, nitrate) that the effluent carries, estimated in the guidelines of the IPCC (Intergovernmental Panel on Climate Change) — the 2006 Guidelines and their 2019 Refinement (Volume 5, Chapter 6, Equation 6.7) — as effluent nitrogen times an emission factor (default $0.005\ \mathrm{kg\ N_2O{-}N\ per\ kg\ N}$, and $0.019$ for nutrient-impacted or hypoxic waters; these are the generic Tier 1 and the higher-accuracy Tier 3 values, where IPCC method tiers escalate in detail and accuracy). A third pathway has long sat outside this structure: treated effluent can leave the plant strongly supersaturated with dissolved $\mathrm{N_2O}$ (concentration above the atmospheric equilibrium value), which is carried into the receiving river and escapes to the atmosphere by gas exchange near the discharge point or farther downstream. Treated effluent can also fuel in-stream $\mathrm{N_2O}$ production in the water column and sediments; field studies on rivers receiving wastewater (Rosamond et al. 2012 on the Grand River; Wang et al. 2021 on Beijing urban waters) show highly supersaturated, emission-hotspot reaches downstream of plants, with saturations reaching orders of magnitude. Masuda, Otomo, Maruo, and Nishimura (2018) made the effluent dissolved-$\mathrm{N_2O}$ pathway quantitative at one full-scale nitrogen-removal STP in Miyagi, Japan: effluent dissolved $\mathrm{N_2O}$ averaged $2.46\ \mu\mathrm{g\ N_2O{-}N\ L^{-1}}$ in summer but $51.82\ \mu\mathrm{g\ N_2O{-}N\ L^{-1}}$ in winter, when nitrite accumulated in the reaction tank; between the discharge point and a station 93 m downstream, 26% (summer) to 59% (winter) of the dissolved pool was lost by gas exchange; the dissolved-$\mathrm{N_2O}$ discharge equalled 12.6% (summer) and 53.5% (winter) of the plant's total estimated $\mathrm{N_2O}$ emission — that is, of the plant-side total of direct gaseous emission plus dissolved discharge, the IPCC-factor indirect emission from the receiving river being accounted separately — and in winter the dissolved discharge was almost equal to that indirect emission; and downstream river dissolved $\mathrm{N_2O}$ rose about 6.7-fold above upstream in winter. They attributed the winter peak plausibly — but as a hypothesis, without microbial or process-rate measurements — to seasonal operational adjustments: raising solids retention time (SRT, the average time activated-sludge biomass is kept in the system, extended from roughly 16 to 20 days in winter) and mixed liquor suspended solids (MLSS, the biomass concentration in the reactors, raised by 20–30%), which promoted imbalance between ammonia-oxidizing and nitrite-oxidizing microbes, nitrite accumulation, and elevated $\mathrm{N_2O}$ production. Their study was, however, a single plant and receiving river surveyed intensively in two seasons, leaving open how general the pattern is. Subsequent work has sharpened rather than settled the question: Chun et al. (2020), combining two years of monitoring, compartment-resolved sediment and water incubations, and isotope-based source partitioning (intramolecular site-preference analysis of $\mathrm{N_2O}$, which distinguishes production pathways by the distribution of $^{15}\mathrm{N}$ within the molecule) in the urbanized Han River below four Seoul WWTPs, found that hypoxic benthic denitrification is a major winter $\mathrm{N_2O}$ source supplementing effluent-borne $\mathrm{N_2O}$; and Song et al. (2024) compiled 376 direct-emission-factor observations from about 200 plants worldwide but explicitly excluded effluent emissions for lack of data. On the accounting side, the IPCC 2019 Refinement acknowledges that dissolved $\mathrm{CH_4}$ and $\mathrm{N_2O}$ present in untreated or treated discharges 'has the potential to be released', yet provides no equation or emission factor for it: pre-formed dissolved $\mathrm{N_2O}$ discharged in effluent is not a term in any current IPCC wastewater equation.

Problem Statement

Determine, with quantified uncertainty, the role of dissolved $\mathrm{N_2O}$ discharged in sewage treatment plant effluent in the total $\mathrm{N_2O}$ emissions attributable to sewage treatment, and put that pathway on the same quantitative footing as the direct (in-plant) and indirect (in-stream production from effluent nitrogen) pathways. Concretely, this question spans four linked facets: (1) measurements of effluent dissolved-$\mathrm{N_2O}$ discharge at full-scale plants spanning contrasting treatment configurations, climates, seasons, and operational regimes, establishing when this pathway is a minor versus a dominant contributor to plant-associated $\mathrm{N_2O}$ emission (at the source plant the dissolved discharge was about 13% of the plant-side total in summer and about 53% in winter, when it was almost equal to the IPCC-factor indirect emission); (2) an attribution of the downstream $\mathrm{N_2O}$ signal between near-field gasification of the discharged dissolved pool, water-column production, and benthic production, across discharge hydraulics (drop structures, turbulence) and seasonal supersaturation conditions; (3) process-level evidence for or against the hypothesized causal chain in which seasonal operational adjustments (solids retention time, mixed liquor suspended solids) produce nitrifier imbalance, nitrite accumulation, and elevated effluent dissolved $\mathrm{N_2O}$; and (4) measurement and accounting approaches by which pre-formed dissolved effluent $\mathrm{N_2O}$ is explicitly represented in greenhouse-gas inventories alongside IPCC Equation 6.7 (indirect) and Equation 6.9 (plant) estimates, for which no current IPCC method exists. These facets jointly address one objective, the closure of the STP $\mathrm{N_2O}$ budget; they constitute a single linked deliverable rather than four separately mandatory tasks, so a partial or single-facet result does not by itself resolve the question. The question concerns the pathway itself and not the conduct of further field measurement.

References

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