The refinement of the N₂O emission factor originating from treated sewage water in rivers with different water qualities Open Access

In recent years, there has been a strong demand to reduce greenhouse gases (GHGs) in response to global warming, and the annual report of the World Meteorological Organization stated that the three major GHGs (carbon dioxide, methane, and nitrous oxide (N₂O)) continued to rise in 2022 (World Meteorological Organization 2022). Countries around the world have announced their intention to implement measures aimed at achieving carbon neutrality as part of long-term strategies (Ministry of the Environment 2021; European Climate Foundation 2024; Ministry of Foreign Affairs 2024), and research is being conducted to reduce GHG emissions, particularly in the waste sector (Dong et al. 2002; Baonan et al. 2018; Hu et al. 2018; Zhao et al. 2019). In particular, N₂O has a global warming potential 273 times higher than that of CO₂, has a long lifetime in the atmosphere of 109 years, and is the most important factor in stratospheric ozone depletion. In addition, it is believed that climate change will indirectly affect human health, such as by increasing the occurrence of skin cancer, making it an important target for reduction (de Vries 2021; IPCC 2023).

The National Greenhouse Gas Inventory Report of Japan reported that N₂O emissions associated with the treatment and discharge of wastewater, such as domestic and industrial wastewater, account for 10% of the total GHGs in the waste sector (National Institute for Environmental Studies 2024). Among them, Japan-specific emission factors are applied in the field of domestic wastewater treatment (Ministry of the Environment 2024). The calculation of N₂O emissions was carried out through research and studies conducted in Japan for each treatment facility and process. This led to the establishment of country-specific emission factors and calculation methods, resulting in precise values being derived. However, while there are cases of studies conducted worldwide on N₂O emissions resulting from natural decomposition of treated wastewater discharged into rivers and untreated domestic wastewater (Rosamond et al. 2012; Xia et al. 2013; Burgos et al. 2015; Wang et al. 2015), such studies are scarce within Japan (Hasegawa & Hanaki 2006; Otomo et al. 2017; Yamazaki et al. 2023). Therefore, Japan has not developed its own emission factors and calculation methods and has used those provided in the 2006 IPCC Guidelines for National Greenhouse Gas Inventories (IPCC 2006). However, since the default values are intended for use by countries worldwide, including developing nations, it is recommended that individual countries develop and utilize their own emission factors through independent research and studies (IPCC 2019).

The production of N₂O has been reported to occur through hydroxylamine oxidation and nitrification–denitrification by ammonia-oxidizing microorganisms, as well as through denitrification reactions by heterotrophic bacteria (Masuda & Nishimura 2010). Furthermore, in the wastewater treatment process, factors such as dissolved oxygen (DO), pH, chemical oxygen demand/nitrogen (N) ratio, and water temperature have been shown to influence redox reactions, leading to variations in N₂O production (Itokawa et al. 2001; Law et al. 2012). It is believed that N₂O emissions in rivers, originating from N compounds such as ammonium (NH₄)-N and combined nitrite and nitrate (NO₂₊₃)-N found in treated sewage water, may vary depending on the environmental conditions of the river (Cole & Caraco 2001; Calijuri et al. 2008). Additionally, treated sewage water contains not only N compounds such as NH₄⁺, nitrite, and nitrate but also dissolved N₂O (hereinafter referred to as ‘D-N₂O’) (Toyoda et al. 2009; Beaulieu et al. 2010; Otomo et al. 2017). Regarding the N₂O generated in rivers, the effects of the nitrification reaction originating from NH₄-N in wastewater after sewage treatment, as well as the presence of D-N₂O remaining in wastewater after sewage treatment following discharge, have not been clarified. The potential for the gasification of D-N₂O at the discharge site exists and should be taken into account (Masuda et al. 2012; Yamazaki et al. 2023).

Additionally, each river in Japan had its own environmental reference parameters for preserving the living environment, including those for pH, biological oxygen demand (BOD), suspended solids (SS), DO, and coliform bacteria. Details of these environmental reference values are provided in the Supplementary data. Based on these reference values, six environmental designation types (hereinafter referred to as ‘designation types’) – AA, A, B, C, D, and E – have been established for rivers (Ministry of the Environment 2024). Even when pollutants flow into clear rivers classified as designation type AA or A, it is believed that the high levels of DO allow for rapid purification of the pollutants. However, in rivers classified as designation type C or lower, water quality is somewhat polluted due to higher BOD and N components compared to those in types designated AA and A. This pollution is expected to reduce the self-purification capacity of these rivers and inhibit the nitrification process, thereby promoting N₂O production compared to clearer rivers. Therefore, since N₂O emissions are assumed to differ depending on the designation types, it is considered necessary to refine the N₂O emission factor based on these differences. Previous reports have documented N₂O emissions based on surveys of urban and agricultural rivers but have not considered the pollution load from sources such as wastewater treatment plants (WWTPs) (Hasegawa & Hanaki 2006). Similarly, while our research group has conducted surveys of rivers classified as designation types A and B (Yamazaki et al. 2023), we have not investigated rivers classified as designation type C or lower, where water contaminant concentrations are relatively high.

This study aimed to refine the N₂O emission factor originating from treated sewage water by conducting a year-round study of D-N₂O production due to the inflow of treated sewage water into rivers with different organic matter, N concentrations, and DO levels. A year-round survey was conducted on a river classified as designation type C, where the self-purification capacity is low and water quality concentrations are relatively high, followed by a comparative analysis with previous studies.

To clarify the differences in D-N₂O concentrations in rivers with different water qualities, surveys were conducted in the summer and winter of FY2020 and FY2021 at 20 environmental reference points in Saitama Prefecture, Japan, encompassing rivers classified into the designation types of AA to C. The field survey included DO, pH, oxidation–reduction potential, water temperature, and flow rate measurements. The collected samples were immediately taken to the laboratory for water quality analysis.

The collected water samples were immediately brought back to the laboratory and the following water quality analysis was performed. The SS and BOD were analyzed in accordance with JISK0102 of the Japanese Industrial Standards. The T-N (total N), NO₂-N, and NO₃-N were analyzed using ion chromatography (Thermo Fisher Scientific Co., Ltd). NH₄-N was analyzed using a colorimetric ammonia meter (Quick Ammonia AT-2000, Central Scientific Co., Ltd). The following items were measured in the river: DO (HORIBA, Ltd), pH (HORIBA, Ltd), oxidation–reduction potential (Lutron Electronic Enterprise Co. Ltd), water temperature (HORIBA, Ltd), and flow rate (Nippon Hicon Co. Ltd). Statistical analysis software R (4.1.3) was used to test for significant differences in the results, with a significance level of P < 0.05.

Table 1 shows the results of the average water quality values and D-N₂O-N measurements collected at environmental reference points corresponding to AA to C designation types in rivers with different water qualities.

Table 1 shows that all parameters (excluding the number of coliform bacteria) (Ministry of the Environment 2024) met the environmental reference values, and in many cases, the BOD and SS values were significantly lower than the reference values. The highest NH₄-N concentration was 3.7 mg L⁻¹ at one point in the designation type AA; however, all N components, except NH₄-N and BOD, tended to increase as the designation progressed from type B to type C. Furthermore, D-N₂O-N concentrations tended to increase as designation progressed from type AA to C, with average D-N₂O-N in designation type C being the highest at 0.58 μg L⁻¹. Therefore, it was inferred that the high pollution load affected the D-N₂O-N concentration.

Based on the above, the concentrations of D-N₂O likely vary according to river designation type. In rivers of designation type C, where N levels are high and D-N₂O-N concentrations increase with the decrease in DO, it is necessary to study N₂O emissions in such highly polluted rivers.

Table 2 shows the quality of the treated water from WWTP C and the quality of the water in river γ based on a year-round survey conducted in the river into which treated sewage water is released. For comparison, the quality of water treated at WWTPs A and B, as well as the water quality of rivers α (designation type B) and β (type A), which were previously reported by our group (Yamazaki et al. 2023), is also shown. At WWTP A, due to the advanced treatment using the oxidation ditch method, almost no N compounds, such as NH₄-N or NO₂₊₃-N, or organic matter remained (Table 2). However, at WWTP C, the standard activated sludge method resulted in a higher abundance of N compounds and organic matter. The T-N concentration was 11.3 times greater than that of WWTP A, while the NH₄-N concentration was 9.3 times higher.

The amount of NH₄-N increased due to the inflow of treated sewage water into river γ; however, as nitrification reactions occurred downstream, the concentrations of NH₄-N tended to decrease, while the level of D-N₂O-N remained high (Figure 3). The average DO, pH, and oxidation–reduction potential for river γ obtained in this study, along with those for rivers α and β previously reported (Yamazaki et al. 2023), are included in the Supplementary data. The average DOs in rivers α and β were 13 and 11 mg L⁻¹, respectively, while that of river γ was lower at 8.0 mg/L (P < 0.05). Although DO in river γ appears to be high, as mentioned above, since this study sampled the middle layer of the rivers, DO was likely even lower in the bottom layer where biofilms are present. When comparing average NH₄-N concentrations at the points where treated sewage water merges with rivers α, β, and γ, the concentrations were 0.8, 5.1, and 3.4 mg L⁻¹, respectively. Therefore, in river γ, the N concentration was higher compared to river α (P < 0.05), the river's low DO and reduced self-purification capacity likely contributed to the high D-N₂O-N concentrations in the water. Previous studies reported that DO, NH₄-N, and nitrite-N are the main limiting factors affecting N₂O production in rivers receiving treated sewage water (Cébron et al. 2003; Beaulieu et al. 2011; Zhou et al. 2022). Previous studies found that N₂O emissions from rivers are higher in urban areas, and it has been suggested that rivers with relatively high N concentrations can also become sources of GHG emissions (He et al. 2017; Wang et al. 2020). On the other hand, it has been reported that substrates such as organic matter and NO₂₊₃-N and anaerobic conditions are required for denitrification reactions to naturally occur (Wada & Uehara 1977) and that denitrification reactions do not generate N₂O production from treated sewage water in the riverbed but rather by nitrification reactions (Masuda et al. 2021). In our study, the BOD in river γ was low at 3.4 mg L⁻¹, and after the inflow of treated sewage water, the amount of NH₄-N tended to decrease downstream, while the decrease of NO₂₊₃-N was non-significant. Therefore, the main cause of N₂O production in this study was presumably nitrification reactions rather than denitrification.

The calculation of N₂O emissions from river γ (designation type C) was performed using Equation (1) reported from the previous study (Yamazaki et al. 2023). Furthermore, estimated N₂O emissions included the supersaturated D-N₂O in the river and the degassed D-N₂O quantity.

Based on this, the emission factor was calculated from the D-N₂O generated by the nitrification reactions of the residual NH₄-N in the treated sewage water. To calculate the emission factor in the same way as in previous reports, Equation (2) was used (Yamazaki et al. 2023).

These results suggest that the inflow of treated sewage water containing NH₄-N and D-N₂O affects N₂O emissions in rivers with relatively high water quality concentrations. This is due to the inhibition of nitrification reactions in the river caused by water pollution and the decrease in DO. Therefore, it was thought that the N₂O emission coefficients derived from the NH₄-N nitrification reaction would differ when post-sewage treatment wastewater flows into rivers with different water qualities. However, since there was no significant difference in these N₂O emission coefficients (P > 0.05), it was thought that the degree of water quality changes classified by designation types of Japanese rivers would not have a significant impact on the overall N₂O emissions from rivers and would not be enough to require the establishment of an N₂O emission coefficient for each river designation type. We conclude that the comparison of D-N₂O levels by river designation type in Japan (Figure 2) is more influenced by N pollution load in the treated sewage water released into rivers than by the influence of river water quality.

Based on the above results, the calculated average N₂O emission factor for rivers α, β, and γ, as shown in Figure 5, was 0.0026 kgN₂O-N kgNH₄-N⁻¹. This value further refines the N₂O emission factor derived from nitrification reactions originating from treated sewage water in rivers with different water qualities, designated types A to C.

This study was conducted to refine the Japan-specific N₂O emission factor for treated sewage water in rivers with different water qualities. The results are as follows:

• (1) In rivers designated as AA through C, there was a tendency for the concentrations of N components, except NH₄-N, to increase as the designation type decreased to B and then C. In addition, D-N₂O-N concentrations tended to be higher due to a decrease in DO.

• (2) In the investigation of river γ (designation type C), the inflow of treated sewage water increased both NH₄-N and D-N₂O-N levels. This suggested that the high D-N₂O concentration in the river was due to the high N levels, the low DO, and the river's low self-purification capacity.

• (3) The year-round study of river γ suggested that the N pollution load from the treated sewage water influenced the river, producing N₂O through the nitrification reaction of NH₄-N. The calculated N₂O emission factor for river γ, which had relatively high water quality concentrations, was 0.0028 kgN₂O-N kgNH₄-N⁻¹. The average N₂O emission factor from treated sewage water was 0.0026 kgN₂O-N kgNH₄-N⁻¹.

This study was supported by a grant from the Environment Research and Technology Development Fund (JPMEERF20192002) of the Environmental Restoration and Conservation Agency of Japan. Students from Toyo University also cooperated in the study.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.