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Redefining separate or integrated food waste and wastewater streams for 29 large cities

Nature Cities (2025)Cite this article

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Abstract

Cities often treat solid waste and wastewater separately, missing the opportunity for resource integration. Diverting food waste into sewage streams offers a holistic solution but lacks city-scale evaluation. Here we developed the urban biowaste flux model integrating mechanistic bioprocesses with life-cycle assessment for quantifying material flows, energy use, costs and greenhouse gas emissions based on city-specific waste composition, treatment parameters and tariffs. We validated urban biowaste flux against detailed data from Hong Kong and applied it to 28 large cities worldwide. Our results revealed a linear rise in net costs with food waste moisture and identify a moisture threshold of about 50 kg per capita per year at which sewer integration becomes cost-effective. Optimized treatment strategies could cut overall emissions for targeted cities by up to 69% versus current separate treatment systems. Overall, the urban biowaste flux model offers policymakers a practical tool for designing sustainable and locale-specific waste management strategies.

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Fig. 1: UBF model demonstration in Hong Kong.
Fig. 2: Comparative performance of separate and integrated waste management systems in targeted cities.
Fig. 3: Correlations between food waste characteristics and sustainability outcomes.
Fig. 4: Sustainability comparison of separate versus integrated waste management across four treatment strategies.
Fig. 5: Sustainability outcomes under optimized policies for the targeted cities.

Data availability

Sources of data used to perform this study are provided in the Methods, Supplementary Data 1 and 2 and Supplementary Information.

Code availability

The UBF model framework has been developed using Microsoft Excel (v. 2023) in conjunction with Visual Basic for Applications. Comprehensive Excel workbooks and Visual Basic for Applications macros facilitating data preparation, stoichiometric mass-balance simulations, treatment process modeling, and calculations of energy consumption, costs and GHG emissions are available in the UBF model repository on GitHub (https://github.com/xzouae/UBF-model). Detailed stoichiometric equations and model parameters are provided in the Supplementary Information.

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Acknowledgements

This work was supported by the National Natural Science Foundation of China (52370136, F.Z.), the Hong Kong Research Grants Council (no. T21-604/19-R, G.C.) and the Hong Kong Innovation and Technology Commission (no. ITC-CNERC14EG03, G.C.).

Author information

Authors and Affiliations

  1. Department of Civil and Environmental Engineering, Water Technology Center, Hong Kong Branch of Chinese National Engineering Research Center for Control & Treatment of Heavy Metal Pollution, The Hong Kong University of Science and Technology, Hong Kong, China

    Xu Zou, Zi Zhang, Chengyu Xiao, Hongxuan Wang, Jiayan Deng, Chukuan Jiang, Hongxiao Guo & Guanghao Chen

  2. Hubei Key Laboratory of Multi-media Pollution Cooperative Control in Yangtze Basin, School of Environmental Science and Engineering, Huazhong University of Science and Technology, Wuhan, China

    Liezhong Fan, Yifeng Feng & Feixiang Zan

Authors
  1. Xu Zou
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  2. Zi Zhang
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  3. Chengyu Xiao
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  4. Liezhong Fan
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  5. Yifeng Feng
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  6. Hongxuan Wang
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  7. Jiayan Deng
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  8. Chukuan Jiang
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  9. Feixiang Zan
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  10. Hongxiao Guo
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  11. Guanghao Chen
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Contributions

X.Z., Z.Z. and F.Z. conceived the model. H.W., L.F. and Y.F. curated the data. Z.Z. and C.X. ran the simulations and evaluated the performance. J.D. and C.J. analyzed the results. X.Z., H.G. and F.Z. drafted, reviewed and edited the manuscript. G.C., H.G. and F.Z. supervised the project and secured the funding.

Corresponding authors

Correspondence to Feixiang Zan, Hongxiao Guo or Guanghao Chen.

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Competing interests

The authors declare no competing interests.

Peer review

Peer review information

Nature Cities thanks Adriana Gómez-Sanabria, Amani Maalouf and Yixuan Wang for their contribution to the peer review of this work.

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Supplementary information

Supplementary Information

Supplementary Texts 1–3, Figs. 1–5 and Tables 1–6.

Reporting Summary

Supplementary Data 1

City-specific UBF inputs across 29 cities.

Supplementary Data 2

Food waste characteristics in Hong Kong.

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Zou, X., Zhang, Z., Xiao, C. et al. Redefining separate or integrated food waste and wastewater streams for 29 large cities. Nat Cities (2025). https://doi.org/10.1038/s44284-025-00341-8

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