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Climate change mitigation potential in sanitation via off-site composting of human waste

Abstract

Approximately 4.5 billion people lack access to safely managed sanitation globally, and 1 billion live in slums, often relying on anaerobic waste containment in pit latrines. Providing access to safely managed sanitation may lead to reduced GHG emissions and thus simultaneously address both Sustainable Development Goals. Here we measure cumulative GHG emissions of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) during the off-site composting of human waste to estimate scalable emission factors. We find that CH4 emission factors are one to two orders of magnitude smaller than IPCC values for other excreta collection, treatment and disposal processes. After accounting for GHG emissions throughout the sanitation cycle, including transport, urine and compost end-use, the climate change mitigation potential is 126 kg of CO2-equivalent per capita per year for slum inhabitants. If scaled to global slum populations, composting could mitigate 3.97 Tg CH4 yr−1, representing 13-44% of sanitation sector CH4 emissions.

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Fig. 1: Competing linear and circular models for sanitation and the associated environmental interactions from local to global scales.
Fig. 2: GHG emission rate dynamics during three stages of thermophilic composting.
Fig. 3: GHG emission rates during paired-pile management experiments.
Fig. 4: Local- to global-scale climate change mitigation potentials at different levels of waste diversion to composting.

Data availability

The datasets generated and/or analysed during the current study are available from the corresponding author on reasonable request. Source Data for Figs. 2 and 3 are provided with the paper.

Code availability

The code for the figures and analysis in the current study are available from the corresponding author on reasonable request.

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Acknowledgements

This work was supported by the Department of Natural Resources and Environmental Management at the University of Hawaii at Mānoa, the Department of Life and Environmental Sciences and the Blum Center at the University of California, Merced, and a donation from J. Wick and P. Rathmann. We thank the staff and interns at Sustainable Organic Integrated Livelihoods in Cap-Haïtien, Haiti, for their assistance with coordinating travel and experimental logistics. We also thank S. Crow, J. Wells and S. Hart for access to a gas chromatograph, K. Porterfield for laboratory assistance and J. Bravo for graphic design.

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G.M. and R.R. designed the study and wrote the manuscript. J.J. assisted with the sample collection and site logistics and contributed to the manuscript writing. J.J.F. contributed to the site logistics and organized the sample collection. S.K. contributed to the study design and manuscript writing.

Corresponding author

Correspondence to Gavin McNicol.

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Peer review information Nature Climate Change thanks Mathew Reid, Guillermo Pardo and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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Supplementary Figs. 1–6 and Tables 1–3.

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McNicol, G., Jeliazovski, J., François, J.J. et al. Climate change mitigation potential in sanitation via off-site composting of human waste. Nat. Clim. Chang. 10, 545–549 (2020). https://doi.org/10.1038/s41558-020-0782-4

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