This study examines limitations of fugitive GHG (mainly CH4 and N2O) emissions reported for the wastewater sector in NIRs. Direct CO2 emissions are excluded because they are predominately biogenic and do not result in long-term net carbon additions. Although the IPCC notes that 4–15% of wastewater-related CO2 emissions derive from fossil sources (for example, soaps and detergents)48,49, no consensus method exists to quantify these non-biogenic CO2 emissions; hence, CO2 emissions from wastewater are not generally reported to NIRs.

System boundary

Five main groups of wastewater treatment systems and pathways contribute to CH4 and N2O emissions and are reported under the wastewater sector according to IPCC guidelines and sanitation service delivered, including (1) latrines which provide basic/limited sanitation services for households not collected to public sewers, (2) septic systems or other decentralized systems that treat uncollected wastewater, (3) centralized WRRFs—including both water and sludge lines and onsite sludge disposal; (4) WRRF effluent and (5) untreated wastewater discharged from scattered households, overflows or leakages. Onsite sludge disposal (for example, incineration and composting) is included within the WRRF category because it is an integral component of WRRF operations, and its emissions are reported under the wastewater sector according to the IPCC 2019 Refinement. Sludge disposed offsite is not considered in this study, as these emissions are attributed to other sectors. For example, CH4 emissions from disposal of sludge in a landfill are included in the solid waste sector, and N2O emissions from land application are reported under the agriculture sector.

Beyond the five main wastewater pathways, sewer collection systems are probably a major but under-characterized emissions source. The IPCC guidelines provide CH4 EFs only for open/stagnant systems (Fig. 1b). Although IPCC acknowledges flowing sewers as potential sources of CH4 and N2O, their emissions are assumed negligible due to insufficient data. Among all the NIRs available, only Switzerland reports emissions from flowing sewers by applying a mean value of 0.00255 kg CH4 kg−1 COD (ref. 50) according to peer-reviewed measurements51,52. This study does not address this topic given such limited consideration in current accounting practices. With more data on sewer types, operational conditions and monitored emissions, flowing sewers may be incorporated into future IPCC guidelines.

IPCC GHG accounting methodologies

Given the IPCC guidelines form the basis of fugitive GHG accounting methodologies, we compiled default EFs by wastewater pathways suggested in three main IPCC documents, including the IPCC 2006 Guideline, the IPCC 2013 Supplement and the IPCC 2019 Refinement (Fig. 1b,c and Supplementary Dataset). Wastewater pathways covered and EFs suggested in the successive IPCC guidelines are evolving with more field measurements and enhanced understanding of the magnitude and variations of GHG emissions from each pathway. For example, the CH4 EFs are mainly derived by multiplying the maximum potential methane emissions (B₀) and modifying methane correction factors. In the IPCC 2006 guideline, although it covers most wastewater pathways for CH4 emissions estimations (Fig. 1b), the methane correction factors are derived from expert judgement, and it is assumed that there are no CH4 emissions from well-managed, aerobic WRRFs and discharge of treated effluent. Methods for N2O emissions are only provided for treated effluent discharge and WRRFs with controlled nitrification and denitrification, based on one study of a single plant in the USA53. In comparison, the IPCC 2019 Refinement made substantial updates. Generally, it incorporated CH4 and N2O EFs for constructed wetlands through cross-referencing the IPCC 2013 Supplement, updated CH4 and N2O EFs for centralized WRRFs and septic tank based on dozens of peer-reviewed literatures and differentiated discharge-related EFs by receiving waterbody types to enable application of high-tier approaches (Fig. 1b,c).

NIR data collection

The most recent NIRs for the 38 studied countries, submitted by 15 July 2025, were downloaded from the UNFCCC portal (https://unfccc.int/ghg-inventories-annex-i-parties/2025). NIRs in French (N = 1), Spanish (N = 2), Portugal (N = 1) and Russian (N = 2) were translated into English using open-source software (Google Translate and ChatGPT). We reviewed the domestic wastewater treatment and discharge section in detail and collected data of interest, including wastewater pathways considered, methodologies (for example, EFs, population served by each wastewater pathway, organic inputs in wastewater and other activity data), reported CH4 and N2O emissions in kiloton (kt), emissions contribution of each pathway if data are available, among others. For comparison, EF units for CH4 and N2O are converted to kg−1 CH4 kg−1 BOD and kg−1 N2O-N kg−1 TN, respectively.

For China, its NIRs do not provide granular data on emissions from domestic wastewater treatments, the specific pathways included and detailed methodologies. We therefore supplement these data with national estimates from peer-reviewed literature. Given substantial differences in system boundaries, input data, EFs and methodological assumptions across studies, we solely compiled emissions data by wastewater pathways rather than comparing input data and EF discrepancies or uncertainties among these studies. Specifically, latrine and septic tank emissions are obtained from Cheng et al.45; WRRFs and effluent emissions from Wang et al.47 and wastewater collection system emissions from Gao et al.46. Data extracted were cross checked by team members to validate and enhance the accuracy of the information extracted. The collected data points were documented into a structured dataset as provided in the Supplementary Dataset.

In addition to the 38 countries analysed in this study, many other countries also submit NIRs to UNFCCC that probably contain methodological details relevant to wastewater GHG accounting. However, our objective is not exhaustively reviewing all NIRs but to identify their discrepancies in coverage of wastewater pathways and accounting methodologies among countries. The 38 selective countries, including 30 Annex I and eight non-Annex I countries, are considered representative that collectively contribute to approximately 36–67% of reported emissions for the wastewater sector. They span five continents and cover a wide range of economic contexts: developing and emerging economies (n = 12; eight non-Annex I countries plus four Annex I countries including Belarus, Russia, Turkey and Ukraine), the G7 developed economies (n = 7; all Annex I countries: Canada, France, Germany, Italy, Japan, UK and USA) and non-G7 developed economies (n = 19; the remaining Annex I).

Ratio of population served by various wastewater treatment pathways

The ratio of the population served by each pathway was obtained from the World Health Organization and United Nations Children’s Fund (WHO/UNICEF) Joint Monitoring Programme (JMP), which tracks the global percentage of the population using safely managed sanitation services over time (2000–2022; accessible from https://data.unicef.org/resources/dataset/drinking-water-sanitation-hygiene-database/). Household surveys and censuses are the primary source of JMP on the different types of facility used by the population. Given the rapid development of wastewater industry especially in developing countries, the latest JMP data in 2022 were used for the following analysis to capture the updated wastewater pathways in each country.

The WHO/UNICEF JMP classified sanitation services into four main groups: (1) improved sanitation facilities (that is, latrines and other, septic tanks, sewer connections, wastewater treated); (2) basic (improved facilities that are not shared with other households); (3) limited (improved facilities shared with other households) and (4) unimproved (use of pit latrines without a slab or platform) and open defecation. Although groups of these sanitation services differ from wastewater pathways defined in this study, underlining linkages exist and the ratio of population served by each pathway is calculated as follows.

$${r}_{\mathrm{latrine}}={\mathrm{JMP}}_{\mathrm{latrines}}$$

(1)

$${r}_{\mathrm{septic}}={\mathrm{JMP}}_{\mathrm{septic}\,\mathrm{tanks}}$$

(2)

$${r}_{\mathrm{WRRF}}={\mathrm{JMP}}_{\mathrm{wastewater}\,\mathrm{treated}}$$

(3)

$${r}_{\mathrm{effluent}}={\mathrm{JMP}}_{\mathrm{wastewater}\,\mathrm{treated}}$$

(4)

$${r}_{\mathrm{untreated}}={\mathrm{JMP}}_{\mathrm{sewer}\,\mathrm{connections}}-{\mathrm{JMP}}_{\mathrm{wastewater}\,\mathrm{treated}}+{\mathrm{JMP}}_{\mathrm{open}\,\mathrm{defecation}}$$

(5)

This calculation and categorization were double checked and validated through the following three steps. First, we summed the ratio of population served by each pathway. Most countries reached a total of 100%, while a few slightly lower but still exceeded 99.1%. Second, we compared JMP data with those reported in NIRs and identified several inconsistencies. For example, the JMP dataset reports no population using decentralized systems in the Netherlands and Switzerland, which contradicts the NIRs. To resolve this, we aligned our data with NIRs where possible. Third, we identified 11 countries that reported CH4 or N2O emissions from latrines or untreated wastewater in their NIRs, but ten of them had no corresponding population recorded for these pathways in the JMP dataset. In such cases, we manually adjusted the population share for latrines and untreated wastewater to match data reported in NIR if data were available; otherwise, we estimated them as the remainder after subtracting the sum of all other pathway shares from 100%.

We also cross checked the amount of treated wastewater by comparing the WHO/UNICEF dataset (calculated as treated wastewater per population equivalent (PE) × total PE × ratio of population served by WRRF) with HydroWASTE33 and Jones et al.’s dataset34, resulting in ± 40% differences primarily because of high uncertainties of rough estimates using treated wastewater per PE and percentage of population using WRRFs. Despite huge inconsistencies, we adopted the WHO/UNICEF JMP dataset for the following assessment as it is the most comprehensive wastewater dataset with granular data of all wastewater pathways. Further validation and uncertainty analysis are needed once a more accurate wastewater dataset becomes available.

Estimation of emissions gaps

Emissions gaps caused by both omitted pathways and underestimated CH4 and N2O emissions from WRRFs by following the IPCC 2006 guidelines were estimated in this study. These emissions gaps were estimated using updated EFs for septic tanks and WRRFs based on large amount of field measurements published in literature and IPCC 2019 default EFs for other pathways (Table 1), which reflect the latest understanding on emissions from each wastewater pathway. Given a lack of detailed activity data for each country such as treatment technologies and waterbodies that untreated and treated wastewater discharged, we did not consider these variations, and corresponding assumptions are listed in Table 1.

Field measurements have been widely shown that the IPCC 2006 default CH4 EF (0 kg CH4 kg−1 BOD) and N2O EF (3.2 g N2O PE−1 year−1) for well-managed aerobic WRRFs underestimate actual WRRF emissions. This suggests a systematic underestimation in national inventories that rely on these default values. We identified several countries using the above default CH4 EFs for aerobic WRRFs, including Austria, France, Spain, Turkey, Ukraine; and others applying the above default N2O EFs, including Belgium, Italy, South Africa, Turkey, Ukraine. For these countries, beyond omitted wastewater pathways, we also estimated the net emissions change resulting from updating WRRF EFs.

The annual CH4 and N2O emissions (in kg) from latrine, septic tank, WRRF and untreated wastewater discharge, for any given country are calculated using equations (6) and (7), respectively

$${\mathrm{CH}}_{4,i,j}={P}_{i}\times {r}_{i,j}\times {\mathrm{BOD}}_{i}\times 0.001\times 365\times {\mathrm{EF}}_{\mathrm{CH}4,j}$$

(6)

$${{\rm{N}}}_{2}{{\rm{O}}}_{i,j}={P}_{i}\times {r}_{i,j}\times {\mathrm{TN}}_{i}\times {\mathrm{EF}}_{{\rm{N}}2{\rm{O}},j}$$

(7)

where i and j indicate country i and wastewater pathway j, respectively; Pi is population of country i; ri,j is percentage of population severed by pathway j in country i; BODi is country-specific per capita biochemical oxygen demand, g BOD PE−1 day−1; TNi is country-specific total nitrogen in wastewater, kg TN PE−1 year−1; EFCH4,j is and EFN2O,j are CH4 and N2O EFs for wastewater pathway j, in kg CH4 kg−1 BOD and kg N2O-N kg−1 TN, respectively.

The annual CH4 and N2O emissions (in kg) from discharge of treated wastewater are further considered BOD and TN removal rate in WRRFs as listed in equations (8) and (9), respectively

$${\mathrm{CH}}_{4,i,j}={P}_{i}\times {r}_{i,j}\times {\mathrm{BOD}}_{i}\times {(1-R}_{i})\times 0.001\times 365\times {\mathrm{EF}}_{\mathrm{CH}4,j}$$

(8)

$${{\rm{N}}}_{2}{{\rm{O}}}_{i,j}={P}_{i}\times {r}_{i,j}\times {\mathrm{TN}}_{i}\times {(1-R}_{i})\times {\mathrm{EF}}_{i,j}$$

(9)

where Ri represents country-specific BOD removal rate in WRRFs, unitless.

TNi is calculated by multiplying country-specific protein consumption per capita per year and a list of constant parameters without considering country-specific variations, including fraction of nitrogen to protein (0.16), factor for additional nitrogen from household products (1.1), factor for non-consumed protein (1.1) and factor for industrial and commercial co-discharged protein (1.25). Country-specific protein consumption and BOD per capita were obtained from NIRs and supplemented with data from the Food and Agriculture Organization (FAO) Statistics Division (https://www.fao.org/faostat/en/#data/FBS/visualize).

CH4 and N2O emissions (in kg) were further converted to million metric tons (MMT) of CO2e by multiplying global warming potential of 27 and 273, respectively. Detailed data are available in Supplementary Dataset.