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

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. Human waste in slums is often collected untreated in pit latrines, which emit GHGs and have negative impacts on human health. If adopted in slums globally, off-site composting could reduce methane emissions from the sanitation sector by 13-44% while improving public health.