Variable contribution of wastewater treatment plant effluents to nitrous oxide emission

Tang, Weiyi; Talbott, Jeff; Jones, Timothy; Ward, Bess B.

doi:https://doi.org/10.5194/egusphere-2024-638

Preprints

https://doi.org/10.5194/egusphere-2024-638

Preprints

12 Mar 2024

| 12 Mar 2024

Variable contribution of wastewater treatment plant effluents to nitrous oxide emission

Weiyi Tang, Jeff Talbott, Timothy Jones, and Bess B. Ward

Abstract. Nitrous oxide (N₂O), a potent greenhouse gas and ozone-destroying agent, is produced during nitrogen transformations in both natural and human-constructed environments. Wastewater treatment plants (WWTPs) produce and emit N₂O into the atmosphere during the nitrogen removal process. However, the impact of WWTPs on N₂O emissions in downstream aquatic systems remains poorly constrained. By measuring N₂O concentrations at a monthly resolution over a year in the Potomac River Estuary, a tributary of Chesapeake Bay in the eastern United States, we found a strong seasonal variation in N₂O concentrations and fluxes: N₂O concentrations were larger in fall and winter but the flux was larger in summer and fall. Observations at multiple stations across the Potomac River Estuary revealed hotspots of N₂O emissions downstream of WWTPs. N₂O concentrations were higher at stations downstream of WWTPs compared to other stations (median: 21.2 nM vs 16.2 nM) despite the similar concentration of dissolved inorganic nitrogen, suggesting the direct discharge of N₂O from WWTPs into the aquatic system or a higher N₂O production yield in waters influenced by WWTPs. Since wastewater production has increased substantially with the growing population and is projected to continue to rise, accurately accounting for N₂O emissions downstream of the WWTPs would better constrain the global N₂O emissions. Efficient N₂O removal, in addition to dissolved nitrogen removal, should be an essential part of water quality control in WWTPs.

Received: 07 Mar 2024 – Discussion started: 12 Mar 2024

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.

Download & links

Preprint (PDF, 2532 KB)

Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
Preprint (2532 KB)

Supplement (1808 KB)

Download & links

The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.

Journal article(s) based on this preprint

17 Jul 2024

Variable contribution of wastewater treatment plant effluents to downstream nitrous oxide concentrations and emissions

Weiyi Tang, Jeff Talbott, Timothy Jones, and Bess B. Ward

Biogeosciences, 21, 3239–3250, https://doi.org/10.5194/bg-21-3239-2024,https://doi.org/10.5194/bg-21-3239-2024, 2024

Short summary

Weiyi Tang, Jeff Talbott, Timothy Jones, and Bess B. Ward

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2024-638', Anonymous Referee #1, 02 Apr 2024

GENERAL COMMENTS
Currently, there is considerable interest in understanding the production and emissions of greenhouse gases (GHGs) from wastewater treatment plants (WWTPs). Indeed, emissions of these gases have become a major concern in efforts to mitigate climate change and reduce global emissions. This renders the article by Tang et al. particularly significant as it investigates the impact of WWTPs on N₂O emissions in aquatic systems downstream of the Potomac River estuary by measuring nitrogenous nutrients and N₂O concentrations on a monthly resolution over the course of a year. The authors have identified spatially and temporally variable concentrations of N₂O and fluxes of N₂O, generally higher downstream of the WWTPs, highlighting the necessity for effective N₂O removal alongside nitrogen treatment at WWTPs.
The data are well presented and the discussion of the dataset is comprehensive and conclusive. However, from my point of view, I have some suggestions to render the work more attractive to readers. Therefore, I suggest its publication after major revisions.
It would be valuable for the study to clarify whether the three treatment plants (Noman Cole, Mooney, and Aquia) utilize identical wastewater treatment processes and treat similar volumes of water. Additionally, assessing whether the receiving channels into which the WWTPs discharge exhibit comparable water volumes is crucial for ensuring a consistent dilution effect of the gas in the water column. Moreover, understanding the depth of the water column is essential; in cases of shallow depths, the influence of gas emission from the sediment to the water column could be substantial.
It would be interesting for the study to elucidate whether the three treatment plants (Noman Cole, Mooney, and Aquia) employ the same wastewater treatment processes and the volume of water they treat. It is also important to determine if the receiving channels where the WWTPs discharge have similar water volumes, so that the dilution effect of the gas in the water column is similar. Similarly, it would be interesting to know the depth of the water column; if it is shallow, the influence of gas emission from the sediment to the water column could be significant.
The bibliographical references cited do not always follow the same criteria (chronological order or alphabetical order).

SPECIFIC COMMENTS
Ln 49. In a more recent article than those cited, Rosentreter et al., 2023 there are compiled N₂O emissions data from various estuaries, providing a wider range of emissions variation (0.2 – 5.7 Tg N₂O yr^-1). Specifically, the paper states: “Global estimates of estuarine N₂O emissions are highly uncertain, with large discrepancies for both observation-based (220–5,710 GgN₂O yr⁻¹) and modelling approaches (94–1,084 GgN₂O yr⁻¹).
Ln 65-66. References should be listed in ascending chronological order, consistent with the rest of the paper.
Ln 87. It should be indicated what type of treatment is given in the WWTPs (primary, secondary, tertiary, etc.) in order to understand if the nitrogen removal capacity of the three wastewater treatment plants is the same. At what distance from the WWTPs were the samples taken? Were the samples taken at approximately the same distance from the discharge point at all three WWTPs? Were the channels where the samples were taken similar? Did they have approximately the same water volume? An important factor when comparing the amount of N₂O in the receiving channels is dilution.
Ln 99. Were the samples collected from a vessel? Please specify.
Ln 110-11. How was a 3 mL air headspace created in the 60 mL serum bottles? Did all samples have exactly the same volume of air headspace? Could this 3 mL of air in contact with the sample potentially interfere with the measurement? Was the N₂O content in the air also measured? Were the samples taken in duplicate?
Ln 112. Leave a space between the 10 and the M.
Ln 124-128. Figure 4a, depicting the sampling points of the four streams/rivers (Neabsco Creek (5 stations), Occoquan River (3 stations), Pohick Creek (4 stations), and Accotink Creek), should be included in the Materials and Methods section.
Ln 128. Where have the data on water discharge and nitrogen (kg) per day from the wastewater treatment plants been obtained? It would be interesting to include this information in the manuscript.
Ln 149. It is not reflected in the text how the 3 mL air headspace is taken from the serum bottles to estimate the amount of N₂O in the sample.
Ln 167. “The equilibrium N₂O concentration was calculated based on the solubility of N2O (Weiss and Price, 1980)…” Where did you obtain the value of N₂O in the atmosphere for the calculations? Which value did you consider, the daily, monthly...?
Ln 170: What do the initials NCEP stand for? It would be more comprehensive to include the website from which the value of U was taken.
Ln 171. You should cite in the paper the expression from which Sc has been estimated, possibly from the proposed expression by Wanninkhof (2014). You should indicate whether for the calculation of Sc, you have considered the expression for salinity equal to zero, or if, on the contrary, the N₂O Schmidt number for each point has been scaled to the values proposed by Wanninkhof (2014) for salinities between 0 and 35, assuming that Sc varies linearly with salinity.
Ln 173. References should be listed in ascending chronological order, consistent with the rest of the paper.
Ln 170-176. I don't understand why they are using a gas transfer velocity parameterization (k) proposed for the ocean, such as the expression by Wanninkhof (2014), rather than a k for a coastal system. If they didn't have data on current velocity and depth of the system necessary to use the k by Borges et al. (2004) and Rosentreter et al. (2021), they could have used the expression proposed by Raymond and Cole (2001), which is based on a compilation of k proposed for different coastal systems, or that by Jiang et al. (2008), based on the compilation of Raymond and Cole (2001) as well as other studies conducted in estuaries. Furthermore, given the uncertainty associated with k, to minimize this, they could have estimated water-atmosphere fluxes considering two expressions of k (Raymond and Cole, 2001; Jiang et al., 2008; Wanninkhof, 2014), and taken the average value of the three fluxes obtained, as many other authors do in coastal systems (e.g., Call et al., 2015; Sánchez-Rodriguez et al., 2024)
Ln 233. I suggest wording it like this: ….vs 6‰ for stations of the central channel and without the influence by WWTPs
Ln 242. In general, denitrification typically occurs in environments with low oxygen concentrations (DO ≤ 5 μmol L−1, Codispoti et al., 2001). As illustrated in Supplementary Figure 2, oxygen concentrations at the stations never reached low values. In fact, downstream stations of wastewater treatment plants exhibited dissolved oxygen levels ranging between 139.38 (25/07/2022) – 430.94 μM (7/02/2023). It is recognized that denitrification can also take place within oxygenated water columns containing suspended organic matter particles (Bange, 2008). Is there a substantial amount of suspended material in the studied system that could induce denitrification in oxygenated water? On the other hand, it is well-established that coastal sediments provide optimal environments for denitrification due to continuous inputs of nutrients and organic matter from land. Could it be that some of the measured N₂O in the water originates from the sediment?
Ln 252. Correlations of 0.62 (r²=0.38) and 0.51 (r²=0.26), I do not consider them strong correlations, remove the word strong.
Ln 252-256 and 263-264. In stations unaffected by WWTPs, there appears to be a good positive correlation between N₂O and DO and NOx, which could indicate that nitrification is an important process in these N₂O production stations.
Ln 262. References should be listed in ascending chronological order, consistent with the rest of the paper.
Ln 278-279. References should be listed in ascending chronological order, consistent with the rest of the paper.
Ln 292-294. Why does it not also present the predictive model of N₂O concentration based on total nitrogen and temperature for stations in the central channel of the Potomac Estuary? It could be interesting to have it to extrapolate to other areas of the estuary located in the channel. Perhaps you have included the data measured in the channel in the samples without wastewater treatment plants (WWTPs). If so, please indicate it. I believe you should have stated the number of stations/data considered in each prediction.
Ln 298-300. Did you use the prediction model for stations without WWTPs? Please indicate it in the paper.

FIGURES
Figure 2 and Supplementary Figure 1. What does the "01" after the slash indicate on the x-axis of the central graphs? Wouldn't it be more intuitive for the reader to use "22" or "23" instead of "01," depending on the year the sampling was conducted?
Supplementary Figure 2. In the figure caption and in Figure a, a negative sign as a subscript is missing on NOx^- on the x-axis. In Figure b, on the x-axis, remove the space between N₂ and O.

REFERENCES:
Bange, H. W., 2008. Gaseous Nitrogen Compounds (NO, N₂O, N₂, NH₃) in the Ocean. In Nitrogen in the Marine Environment (pp. 51–94). Elsevier Inc. https://doi.org/10.1016/B978-0-12-372522-6.00002-5
Call, M., Maher, D. T., Santos, I. R., Ruiz-Halpern, S., Mangion, P., Sanders, C. J., ... & Eyre, B. D., 2015. Spatial and temporal variability of carbon dioxide and methane fluxes over semi-diurnal and spring–neap–spring timescales in a mangrove creek. Geochimica et Cosmochimica Acta, 150, 211-225.
Codispoti, L.A., Brandes, J.A., Christensen, J.P., Devol, A.H., Naqvi, S.W.A., Paerl, H.W., Yoshinari, T., 2001. The oceanic fixed nitrogen and nitrous oxide budgets: moving targets as we enter the anthropocene? Sci. Mar. 65 (S2), 85–105.
Jiang, L.Q., Cai, W.J., & Wang, Y. (2008). A comparative study of carbon dioxide degassing in river- and marine-dominated estuaries. Limnology and Oceanography, 53(6), 2603–2615.
Rosentreter, J. A., Laruelle, G. G., Bange, H. W., Bianchi, T. S., Busecke, J. J. M., Cai, W. J., Eyre, B. D., Forbrich, I., Kwon, E. Y., Maavara, T., Moosdorf, N., Najjar, R. G., Sarma, V. V. S. S., Van Dam, B., & Regnier, P., 2023. Coastal vegetation and estuaries are collectively a greenhouse gas sink. Nature Climate Change, 13(6), 579–587. https://doi.org/10.1038/s41558-023-01682-9
Sánchez-Rodríguez, J., Ortega, T., Sierra, A., Mestre, M., Ponce, R., Fernández-Puga, M. D. C., & Forja, J., 2024. Distribution, reactivity and vertical fluxes of methane in the Guadalquivir Estuary (SW Spain). Science of The Total Environment, 907, 167758.

Citation: https://doi.org/10.5194/egusphere-2024-638-RC1
- AC1: 'Reply on RC1', Weiyi Tang, 05 May 2024
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-638/egusphere-2024-638-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-638-AC1
RC2:
'Comment on egusphere-2024-638', Anonymous Referee #2, 04 Apr 2024

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-638/egusphere-2024-638-RC2-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2024-638-RC2
- AC2: 'Reply on RC2', Weiyi Tang, 05 May 2024
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-638/egusphere-2024-638-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-638-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2024-638', Anonymous Referee #1, 02 Apr 2024

GENERAL COMMENTS
Currently, there is considerable interest in understanding the production and emissions of greenhouse gases (GHGs) from wastewater treatment plants (WWTPs). Indeed, emissions of these gases have become a major concern in efforts to mitigate climate change and reduce global emissions. This renders the article by Tang et al. particularly significant as it investigates the impact of WWTPs on N₂O emissions in aquatic systems downstream of the Potomac River estuary by measuring nitrogenous nutrients and N₂O concentrations on a monthly resolution over the course of a year. The authors have identified spatially and temporally variable concentrations of N₂O and fluxes of N₂O, generally higher downstream of the WWTPs, highlighting the necessity for effective N₂O removal alongside nitrogen treatment at WWTPs.
The data are well presented and the discussion of the dataset is comprehensive and conclusive. However, from my point of view, I have some suggestions to render the work more attractive to readers. Therefore, I suggest its publication after major revisions.
It would be valuable for the study to clarify whether the three treatment plants (Noman Cole, Mooney, and Aquia) utilize identical wastewater treatment processes and treat similar volumes of water. Additionally, assessing whether the receiving channels into which the WWTPs discharge exhibit comparable water volumes is crucial for ensuring a consistent dilution effect of the gas in the water column. Moreover, understanding the depth of the water column is essential; in cases of shallow depths, the influence of gas emission from the sediment to the water column could be substantial.
It would be interesting for the study to elucidate whether the three treatment plants (Noman Cole, Mooney, and Aquia) employ the same wastewater treatment processes and the volume of water they treat. It is also important to determine if the receiving channels where the WWTPs discharge have similar water volumes, so that the dilution effect of the gas in the water column is similar. Similarly, it would be interesting to know the depth of the water column; if it is shallow, the influence of gas emission from the sediment to the water column could be significant.
The bibliographical references cited do not always follow the same criteria (chronological order or alphabetical order).

SPECIFIC COMMENTS
Ln 49. In a more recent article than those cited, Rosentreter et al., 2023 there are compiled N₂O emissions data from various estuaries, providing a wider range of emissions variation (0.2 – 5.7 Tg N₂O yr^-1). Specifically, the paper states: “Global estimates of estuarine N₂O emissions are highly uncertain, with large discrepancies for both observation-based (220–5,710 GgN₂O yr⁻¹) and modelling approaches (94–1,084 GgN₂O yr⁻¹).
Ln 65-66. References should be listed in ascending chronological order, consistent with the rest of the paper.
Ln 87. It should be indicated what type of treatment is given in the WWTPs (primary, secondary, tertiary, etc.) in order to understand if the nitrogen removal capacity of the three wastewater treatment plants is the same. At what distance from the WWTPs were the samples taken? Were the samples taken at approximately the same distance from the discharge point at all three WWTPs? Were the channels where the samples were taken similar? Did they have approximately the same water volume? An important factor when comparing the amount of N₂O in the receiving channels is dilution.
Ln 99. Were the samples collected from a vessel? Please specify.
Ln 110-11. How was a 3 mL air headspace created in the 60 mL serum bottles? Did all samples have exactly the same volume of air headspace? Could this 3 mL of air in contact with the sample potentially interfere with the measurement? Was the N₂O content in the air also measured? Were the samples taken in duplicate?
Ln 112. Leave a space between the 10 and the M.
Ln 124-128. Figure 4a, depicting the sampling points of the four streams/rivers (Neabsco Creek (5 stations), Occoquan River (3 stations), Pohick Creek (4 stations), and Accotink Creek), should be included in the Materials and Methods section.
Ln 128. Where have the data on water discharge and nitrogen (kg) per day from the wastewater treatment plants been obtained? It would be interesting to include this information in the manuscript.
Ln 149. It is not reflected in the text how the 3 mL air headspace is taken from the serum bottles to estimate the amount of N₂O in the sample.
Ln 167. “The equilibrium N₂O concentration was calculated based on the solubility of N2O (Weiss and Price, 1980)…” Where did you obtain the value of N₂O in the atmosphere for the calculations? Which value did you consider, the daily, monthly...?
Ln 170: What do the initials NCEP stand for? It would be more comprehensive to include the website from which the value of U was taken.
Ln 171. You should cite in the paper the expression from which Sc has been estimated, possibly from the proposed expression by Wanninkhof (2014). You should indicate whether for the calculation of Sc, you have considered the expression for salinity equal to zero, or if, on the contrary, the N₂O Schmidt number for each point has been scaled to the values proposed by Wanninkhof (2014) for salinities between 0 and 35, assuming that Sc varies linearly with salinity.
Ln 173. References should be listed in ascending chronological order, consistent with the rest of the paper.
Ln 170-176. I don't understand why they are using a gas transfer velocity parameterization (k) proposed for the ocean, such as the expression by Wanninkhof (2014), rather than a k for a coastal system. If they didn't have data on current velocity and depth of the system necessary to use the k by Borges et al. (2004) and Rosentreter et al. (2021), they could have used the expression proposed by Raymond and Cole (2001), which is based on a compilation of k proposed for different coastal systems, or that by Jiang et al. (2008), based on the compilation of Raymond and Cole (2001) as well as other studies conducted in estuaries. Furthermore, given the uncertainty associated with k, to minimize this, they could have estimated water-atmosphere fluxes considering two expressions of k (Raymond and Cole, 2001; Jiang et al., 2008; Wanninkhof, 2014), and taken the average value of the three fluxes obtained, as many other authors do in coastal systems (e.g., Call et al., 2015; Sánchez-Rodriguez et al., 2024)
Ln 233. I suggest wording it like this: ….vs 6‰ for stations of the central channel and without the influence by WWTPs
Ln 242. In general, denitrification typically occurs in environments with low oxygen concentrations (DO ≤ 5 μmol L−1, Codispoti et al., 2001). As illustrated in Supplementary Figure 2, oxygen concentrations at the stations never reached low values. In fact, downstream stations of wastewater treatment plants exhibited dissolved oxygen levels ranging between 139.38 (25/07/2022) – 430.94 μM (7/02/2023). It is recognized that denitrification can also take place within oxygenated water columns containing suspended organic matter particles (Bange, 2008). Is there a substantial amount of suspended material in the studied system that could induce denitrification in oxygenated water? On the other hand, it is well-established that coastal sediments provide optimal environments for denitrification due to continuous inputs of nutrients and organic matter from land. Could it be that some of the measured N₂O in the water originates from the sediment?
Ln 252. Correlations of 0.62 (r²=0.38) and 0.51 (r²=0.26), I do not consider them strong correlations, remove the word strong.
Ln 252-256 and 263-264. In stations unaffected by WWTPs, there appears to be a good positive correlation between N₂O and DO and NOx, which could indicate that nitrification is an important process in these N₂O production stations.
Ln 262. References should be listed in ascending chronological order, consistent with the rest of the paper.
Ln 278-279. References should be listed in ascending chronological order, consistent with the rest of the paper.
Ln 292-294. Why does it not also present the predictive model of N₂O concentration based on total nitrogen and temperature for stations in the central channel of the Potomac Estuary? It could be interesting to have it to extrapolate to other areas of the estuary located in the channel. Perhaps you have included the data measured in the channel in the samples without wastewater treatment plants (WWTPs). If so, please indicate it. I believe you should have stated the number of stations/data considered in each prediction.
Ln 298-300. Did you use the prediction model for stations without WWTPs? Please indicate it in the paper.

FIGURES
Figure 2 and Supplementary Figure 1. What does the "01" after the slash indicate on the x-axis of the central graphs? Wouldn't it be more intuitive for the reader to use "22" or "23" instead of "01," depending on the year the sampling was conducted?
Supplementary Figure 2. In the figure caption and in Figure a, a negative sign as a subscript is missing on NOx^- on the x-axis. In Figure b, on the x-axis, remove the space between N₂ and O.

REFERENCES:
Bange, H. W., 2008. Gaseous Nitrogen Compounds (NO, N₂O, N₂, NH₃) in the Ocean. In Nitrogen in the Marine Environment (pp. 51–94). Elsevier Inc. https://doi.org/10.1016/B978-0-12-372522-6.00002-5
Call, M., Maher, D. T., Santos, I. R., Ruiz-Halpern, S., Mangion, P., Sanders, C. J., ... & Eyre, B. D., 2015. Spatial and temporal variability of carbon dioxide and methane fluxes over semi-diurnal and spring–neap–spring timescales in a mangrove creek. Geochimica et Cosmochimica Acta, 150, 211-225.
Codispoti, L.A., Brandes, J.A., Christensen, J.P., Devol, A.H., Naqvi, S.W.A., Paerl, H.W., Yoshinari, T., 2001. The oceanic fixed nitrogen and nitrous oxide budgets: moving targets as we enter the anthropocene? Sci. Mar. 65 (S2), 85–105.
Jiang, L.Q., Cai, W.J., & Wang, Y. (2008). A comparative study of carbon dioxide degassing in river- and marine-dominated estuaries. Limnology and Oceanography, 53(6), 2603–2615.
Rosentreter, J. A., Laruelle, G. G., Bange, H. W., Bianchi, T. S., Busecke, J. J. M., Cai, W. J., Eyre, B. D., Forbrich, I., Kwon, E. Y., Maavara, T., Moosdorf, N., Najjar, R. G., Sarma, V. V. S. S., Van Dam, B., & Regnier, P., 2023. Coastal vegetation and estuaries are collectively a greenhouse gas sink. Nature Climate Change, 13(6), 579–587. https://doi.org/10.1038/s41558-023-01682-9
Sánchez-Rodríguez, J., Ortega, T., Sierra, A., Mestre, M., Ponce, R., Fernández-Puga, M. D. C., & Forja, J., 2024. Distribution, reactivity and vertical fluxes of methane in the Guadalquivir Estuary (SW Spain). Science of The Total Environment, 907, 167758.

Citation: https://doi.org/10.5194/egusphere-2024-638-RC1
- AC1: 'Reply on RC1', Weiyi Tang, 05 May 2024
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-638/egusphere-2024-638-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-638-AC1
RC2:
'Comment on egusphere-2024-638', Anonymous Referee #2, 04 Apr 2024

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-638/egusphere-2024-638-RC2-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2024-638-RC2
- AC2: 'Reply on RC2', Weiyi Tang, 05 May 2024
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-638/egusphere-2024-638-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-638-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

ED: Reconsider after major revisions (08 May 2024) by Hermann Bange

AR by Weiyi Tang on behalf of the Authors (08 May 2024) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (17 May 2024) by Hermann Bange

RR by Anonymous Referee #1 (29 May 2024)

Suggestions for revision or reasons for rejection

The paper by Tang et al., with the title: “Variable contribution of wastewater treatment plant effluents to N2O emission " has greatly improved in clarity and quality with the new changes. Given that there are very few studies worldwide on the impact of WWTPs on N2O emissions in aquatic systems, as shown in the Supplementary figure 8 and in the Table 1 of the paper, I consider the article to be of great interest and that it will be widely disseminated. However, I have some suggestions:

General comments:
The title of the paper refers to the contribution of WWTPs to N2O emissions. However, the manuscript hardly discusses water-atmosphere fluxes of N2O, nor the contribution of WWTPs to water-atmosphere fluxes of N2O in the Potomac River estuary in detail. So my suggestion is that just as the contribution of WWTPs to N2O concentration is discussed, there should be more discussion of the effect of WWTPs on N2O fluxes to the atmosphere in the system. Another option would be to change the title of the paper.

Material and methods:
Ln 158: There is no mention of phosphate in the manuscript, so you should delete it. Were the samples taken in triplicate like the N2O samples? Please indicate.
Ln 167-168: Were obtained N2O concentrations in the water from the measurements made in the headspace using the solubility proposed by Weiss and Price (1980)?
Ln 168-170: The text: “Specifically, the monthly atmospheric N2O concentrations were obtained from the nearby atmospheric station in Brentwood, Maryland (https://gml.noaa.gov/) (Andrews et al., 2023).” should be included in the N2O flux calculation section, it could go on line 191 after (Weiss and Price, 1980).
Ln 173: It should be included how you have calculated the saturation percentage. This parameter is discussed in the text and presented in figure 2a.
Ln 186: It should be indicated how the total N cited in lines 158-160 has been measured.
Ln 191-194. The three gas transfer velocity (k) equations should be written in the same format:
- Or write the k proposed by Wanninkhof (2014) as k660 as has been done for the other two parameterisations.:
k_660=0.251 x U^2
- Or write:
Raymond and Cole (2001): k=1.91 x e^0.35xU x (Sc/600)^(-0.5)
Jiang et al. (2008): k=0.314 x U^2-0.436 x U+3.99 x (Sc/600)^(-0.5)
Wanninkhof (2014): k=0.251 x U^2 x (Sc/660)^(-0.5)

Results and discussion
Ln 219-228. More should be commented on the water-atmosphere N2O fluxes, practically only their range of variation in the whole system is presented. As with the N2O concentrations, the water-atmosphere fluxes present seasonal variations (this if is commented in the abstract) and surely present spatial variations (you should comment on this). However, it is mentioned in the paper that the saturation percentage of N2O is always higher than 100%, so the system behaves as a source of this gas, and that there is seasonal variation, but little is said about the fluxes to the atmosphere (Ln 218-220).
Ln 226-228: I do not believe that a maximum flux of N2O to the atmosphere of 31.7µmol m-2 d-1 in the Potomac River Estuary can be considered as an intense source of N2O to the atmosphere, as there are other estuaries with much more intense emissions. Perhaps it would be more accurate to put: Therefore, tributaries to the Chesapeake Bay (i.e., the Potomac River) are more intense sources of N2O to the atmosphere than the Bay.
Ln 285-286: For better clarity and interpretation of the text, the values of your observed 𝛿15N of N2O downstream of WWTPs and in the urban WWTPs should be included.
Ln 288: Do you know of any work where there is evidence of denitrification in WWTPs, in downstream creeks, or in sediments? If so, could you please cite it.
Ln 343-345: Text in brackets is not in Times New Roman 12. Why is the number of data considered for the predictions so small? Especially for the stations without WWTPs, in the complete study there are 8 sampling x 8 stations (4 stations without WWTPs + 4 stations central channel) = 64 data compared to the 23 considered in the prediction.
Ln 365. In the section: “Impact of wastewater treatment plants on N2O concentrations and emissions” very little is mentioned about how N2O fluxes to the atmosphere vary in the stations upstream and downstream of the WWTPs. However, there is much discussion of the effect of the WWTPs on N2O concentrations. More should be said about these emissions, as the title of the paper says "Variable contribution of wastewater treatment plant effluents to N2O emission". Furthermore, table 1 could present the N2O fluxes as well as the concentrations.
Ln 366 - 369: Figure 4a should be mentioned, where the sampling stations considered in this study are shown in detail.

Figures:
Figures 2 and 3 and Supplemetary figures 3 and 5. It is not necessary to write the word "concentration" on the axes of the figures when referring to N2O concentration (nM), just as you do not write NOx- concentration.
References
Ln 604-613: Rosentreter references should be put in chronological order.

Referee Report: PDF

Hide

RR by Anonymous Referee #2 (30 May 2024)

ED: Publish subject to minor revisions (review by editor) (31 May 2024) by Hermann Bange

AR by Weiyi Tang on behalf of the Authors (04 Jun 2024) Author's response Author's tracked changes Manuscript

ED: Publish as is (05 Jun 2024) by Hermann Bange

AR by Weiyi Tang on behalf of the Authors (05 Jun 2024) Manuscript

Journal article(s) based on this preprint

17 Jul 2024

Variable contribution of wastewater treatment plant effluents to downstream nitrous oxide concentrations and emissions

Weiyi Tang, Jeff Talbott, Timothy Jones, and Bess B. Ward

Biogeosciences, 21, 3239–3250, https://doi.org/10.5194/bg-21-3239-2024,https://doi.org/10.5194/bg-21-3239-2024, 2024

Short summary

Weiyi Tang, Jeff Talbott, Timothy Jones, and Bess B. Ward

Supplement

https://doi.org/10.5194/egusphere-2024-638-supplement

Weiyi Tang, Jeff Talbott, Timothy Jones, and Bess B. Ward

Viewed

Total article views: 367 (including HTML, PDF, and XML)

HTML	PDF	XML	Total	Supplement	BibTeX	EndNote
271	68	28	367	33	15	18

HTML: 271
PDF: 68
XML: 28
Total: 367
Supplement: 33
BibTeX: 15
EndNote: 18

Views and downloads (calculated since 12 Mar 2024)

Month	HTML	PDF	XML	Total
Mar 2024	114	26	6	146
Apr 2024	67	14	10	91
May 2024	57	14	6	77
Jun 2024	25	9	5	39
Jul 2024	8	5	1	14
Aug 2024	0

Cumulative views and downloads (calculated since 12 Mar 2024)

Month	HTML	PDF	XML	Total
Mar 2024	114	26	6	146
Apr 2024	67	14	10	91
May 2024	57	14	6	77
Jun 2024	25	9	5	39
Jul 2024	8	5	1	14
Aug 2024	0

Viewed (geographical distribution)

Total article views: 371 (including HTML, PDF, and XML) Thereof 371 with geography defined and 0 with unknown origin.

Country	#	Views	%

Latest update: 01 Sep 2024

Download

The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.

Preprint (2532 KB)
Metadata XML

Short summary

Wastewater treatment plants (WWTPs) are known to be hotspots of greenhouse gas emissions. However, the impact of WWTPs on the emission of the greenhouse gas N₂O in downstream aquatic environments is less constrained. We found spatially and temporally variable but overall higher N₂O concentrations and fluxes in waters downstream of WWTPs, pointing to the need for efficient N₂O removal in addition to treating nitrogen in WWTPs.


Total:	0
HTML:	0
PDF:	0
XML:	0