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Substantial ammonia emissions from sludge drying pans in wastewater treatment plants

Nature Water (2025)Cite this article

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Abstract

Wastewater treatment plants (WWTPs) are major sources of gaseous nitrogen (N) emissions. The nitrous oxide (N2O) emissions have been extensively studied, however, how ammonia (NH3) emissions contribute to N2O emissions, soil acidification and particulate matter (PM2.5) formation, have been largely overlooked. This study quantified NH3 and N2O emissions from a sludge drying pan (SDP), using inverse-dispersion modelling coupled with open-path Fourier infrared spectroscopic techniques. Here we show low N2O emissions (<0.001 g m−2 h−1) and mean NH3 emissions that are much higher in summer (0.293 g m−2 h−1) than in winter (0.060 g m−2 h−1). A mechanistic process model, correlating NH3 emissions with wind speed and temperature, predicts total NH3 emissions over the SDP cycle (634 days) at 43 t of NH3‒N. This represents 30% of total N in the SDP and 6–9% of total N in the WWTPs influent. This study highlights that the use of SDPs by WWTPs is a substantial NH3 emission source, offering new perspectives for mitigating global N emissions.

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Fig. 1: Measured ammonia emission rates and nitogen input.
Fig. 2: Diurnal variations of ammonia emission rates.
Fig. 3: Correlation of temperature and wind speed with ammonia emission rates.
Fig. 4: A calibrated model used to predict ammonia emission rates.
Fig. 5: Ammonia emissions based on temperature and wind speed data collected throughout an SDP operation.

Data availability

The data including ammonia/nitrous oxide fluxes, climate wind and operational information about SDPs are available at https://doi.org/10.48610/169e2e3. The data are freely accessible and allowed for reuse under a Data Sharing Agreement with The University of Queensland and recipient. Source data are provided with this paper.

Code availability

The code used to predict ammonia emissions over the whole sludge drying cycle is available via Github at https://github.com/uqzhiyao/SDP-ammonia-emissions.

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Acknowledgements

We thank Melbourne Water for funding and access to operational data. We acknowledge D. Milano and R. Gough at Melbourne Water for their kind assistance. We also thank R. Trouvé for kindly assisting in the field. M.B. acknowledges the support of Australian Research Council Hub Smart Fertilizer. Z.W. and L.Y. would like to acknowledge the support of Australian Research Council Research Hub (IH210100001).

Author information

Author notes

  1. These authors contributed equally: Mei Bai, Zhiyao Wang.

Authors and Affiliations

  1. School of Agriculture, Food and Ecosystem Sciences, Faculty of Science, The University of Melbourne, Parkville, Victoria, Australia

    Mei Bai & Deli Chen

  2. School of Chemical Engineering, The University of Queensland, St. Lucia, Queensland, Australia

    Zhiyao Wang & Liu Ye

  3. Australian Centre for Water and Environmental Biotechnology (ACWEB, formerly AWMC), The University of Queensland, St. Lucia, Queensland, Australia

    Zhiyao Wang

  4. Melbourne Water Corporation, Docklands, Victoria, Australia

    Dilini Seneviratne, James Lloyd & Pieter De Jong

Authors
  1. Mei Bai
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Contributions

M.B.: conceptualization (lead), methodology (lead), formal analysis (lead), writing—original draft preparation (lead), visualization (supporting), data curation (supporting), funding acquisition (equal), writing—review and editing (equal). Z.W.: conceptualization (lead), methodology (lead), formal analysis (lead), writing—original draft preparation (lead), visualization (lead), data curation (lead), writing—review and editing (equal). D.S.: conceptualization (supporting), methodology (supporting), resources (equal), writing—review and editing (equal), supervision (equal), funding acquisition (equal), project administration (lead). J.L.: conceptualization (supporting), methodology (supporting), resources (equal), writing—review and editing (equal), supervision (equal), funding acquisition (equal). P.D.J.: conceptualization (supporting), resources (equal), writing—review and editing (equal), funding acquisition (equal). L.Y.: conceptualization (supporting), writing—review and editing (equal), supervision (equal). D.C.: conceptualization (supporting), methodology (supporting), writing—review and editing (equal), supervision (equal), funding acquisition (equal), project administration (lead).

Corresponding authors

Correspondence to Mei Bai or Zhiyao Wang.

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The authors declare no competing interests.

Peer review

Peer review information

Nature Water thanks Thomas Kupper and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary information

Supplementary Information

SDP ammonia estimates in the USA, Wind data measurement, Data filtering criteria, as well as the IDM emission measurement uncertainty, with Supplementary Figs. 1 and 2.

Reporting Summary

Supplementary Data

Measured ammonia emission rate in summer campaign and sludge turning time.

Source data

Source Data Fig. 1

Ammonia emission rates measured in winter and summer campaign (a), nitrogen balance in feed sludge over the drying cycle (b).

Source Data Fig. 2

Diurnal variation of ammonia emission rate Q (μg m−2 s−1) over 3 d in winter (a–c) and summer campaign (d–f).

Source Data Fig. 3

Pearson correlation of ammonia emission rate (μg m−2 s−1) with temperature (°C) (a) and with wind speed (m s−1) (b).

Source Data Fig. 4

Measured and modelled ammonia emission rate (μg m−2 s−1) with wind speed and temperature (a). Predicted ammonia emission rates in scenarios with a range of temperature (b).

Source Data Fig. 5

Weekly mean temperature and wind speed over the cycle of the SDP (a), predicted ammonia emission rates (b).

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Bai, M., Wang, Z., Seneviratne, D. et al. Substantial ammonia emissions from sludge drying pans in wastewater treatment plants. Nat Water (2025). https://doi.org/10.1038/s44221-025-00479-8

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