Closing nutrient loops between urban sanitation and regional agriculture
John T. Trimmer and Jeremy S. Guest
The article in Nature Sustainability is here: https://www.nature.com/articles/s41893-018-0118-9
Typically, nutrients follow a linear path through society. Crops extract nutrients from the soil, farmers harvest them and bring them to market, and consumers eat them. For most adults, nutrients enter and leave the body at essentially the same rate. Upon exiting, they make their way into sanitation systems and then ultimately to rivers and oceans, landfills, or other final destinations. Instead of this once-through paradigm, what if society could recover nutrients from sanitation systems, recirculating them to fertilize cropland and replenish what is removed in the harvest while simultaneously preventing pollution of surface waters? This idea is gaining traction, with various strategies being developed in locations around the world1,2.
In a previous study3, we looked at connections between the Sustainable Development Goals and resource recovery from sanitation, estimating the potential for countries around the world to offset existing fertilizer use (likely in more developed regions) or to increase access to nutrients (in sub-Saharan Africa, for example). One piece of this earlier work explored agricultural nutrient co-location – essentially, how close people are to agricultural land that could benefit from the nutrients they excrete. We examined the issue on a country-by-country basis, estimating the fraction of a nation’s recoverable nutrients that could be used locally to meet crop demands. As one might expect, this fraction was higher in countries with more rural populations.
Distances between nutrient recovery and crop needs might play a key role in determining if agricultural nutrient reuse is realistic, or it may limit whether certain recovery strategies are feasible4,5. We found ourselves particularly interested in this idea and wanted to investigate it further. We decided to focus on large urban settings, where dense human populations generate large quantities of recoverable nutrients but may be far removed from agriculture. Our analysis eventually evolved into this article, where we estimated the quantities of nutrients that 56 of the world’s largest cities could recover from their residents’ waste, and we explored how far those nutrients must travel to fertilize (and not oversaturate) the nearest cropland. We also examined different recovery products that sanitation technologies can generate, from dilute wastewater to nutrient-dense crystal products better able to travel long distances.
Moving forward, we hope this research can contribute toward a conversation identifying where agricultural reuse of human-derived nutrients may be especially impactful and viable. Complemented by site-specific assessments, we can better understand the tradeoffs associated with various sanitation technologies and prioritize promising options. We see nutrient reuse as being especially valuable for smallholder farmers, many of whom live in Africa and Asia6. Our results suggest cities on these continents may be some of the places most favorable for nutrient recycling to nearby farms, although in the coming decades these urban areas might expand considerably7. While nutrient recovery from sanitation is not a panacea, it could contribute toward reducing urban nutrient pollution, offsetting resource-intensive fertilizer use, and increasing farmers’ crop production and economic well-being.
- Ostara. Ostara overview: Nutrient management solutions. Ostara Nutrient Recovery Technologies, Inc. http://ostara.com/nutrient-management-solutions/ (2018).
- Wald, C. The new economy of excrement. Nature News, 549(7671), 146, doi:10.1038/549146a (2018).
- Trimmer, J.T. et al. Amplifying progress toward multiple development goals through resource recovery from sanitation. Environmental Science & Technology 51(18), 10765-10776, doi:10.1021/acs.est.7b02147 (2017).
- Larsen, T.A. CO2-neutral wastewater treatment plants or robust, climate-friendly wastewater management? A systems perspective. Water Research 87, 513-521, doi:10.1016/j.watres.2015.06.006 (2015).
- Metson, G.S. et al. Feeding the Corn Belt: Opportunities for phosphorus recycling in U.S. agriculture. Science of The Total Environment, 542, Part B, 1117-1126, doi:10.1016/j.scitotenv.2015.08.047 (2016).
- Samberg, L.H. et al. Subnational distribution of average farm size and smallholder contributions to global food production. Environmental Research Letters, 11(12), 124010, doi:10.1088/1748-9326/11/12/124010 (2016).
- D’Amour, C.B. et al. Future urban land expansion and implications for global croplands. Proceedings of the National Academy of Sciences, 114(34), 8933-8944, doi:10.1073/pnas.1606036114 (2017).
Poster image: https://www.istockphoto.com/photo/green-cornfield-early-morning-light-at-sunrise-gm827711200-134526027 (photo credit: hauged); https://www.istockphoto.com/photo/chicago-by-the-lake-gm646310240-117197809 (photo credit: lhongfoto).