In 2009, Rockstrӧm et al. argued that human actions, primarily driven by the conversion of atmospheric nitrogen species N2 to reactive nitrogen for agricultural and other use via the Haber-Bosch process, have dramatically impacted the global nitrogen cycle (Erisman, Sutton, Galloway, Klimont, & Winiwarter, 2008; Rockström et al., 2009). The radical population growth over the past century can be attributed to this process and the resulting increase in food production (Erisman et al., 2008). The global agricultural sector produces more reactive nitrogen than all terrestrial natural processes (Liu, Ma, Ciais, & Polasky, 2016). As global population continues to grow, global demand for food has driven up demand for fertilizer, which drives the production and application of synthetic fertilizer (also referred to as mineral fertilizer) for food production (Smil, 1999). The resulting disturbances to the global nitrogen cycle are comprehensive, ranging from upstream emissions of greenhouse gases related to the combustion of fossil fuels to energize the Haber-Bosch process to downstream pollution of surface waters related to the release of unmanaged human excreta. Human actions have, directly and indirectly, transformed stable atmospheric nitrogen to environmental pollutants.
Even though globally there has been a net increase in the use of synthetic fertilizers for agriculture, the use of fertilizer – and the resulting agricultural production and economic development – are unequally distributed (Wang, Cai, Hoogmoed, & Oenema, 2011). Developing countries are typically characterized by lower rates of nitrogenous fertilizer application, an input to crop production of 0 to 15 kg nitrogen per capita per year, compared with higher rates of nitrogenous fertilizer application in developed countries, from 15 to greater than 60 kg nitrogen per capita per year (Liu et al., 2016). A critical challenge of sustainable development is how to improve the food security in developing countries while mitigating the overall negative impacts of reactive nitrogen released into the environment because of agricultural activity.
Coupled with the imperative to improve food security in developing countries is the need to implement sanitation and waste treatment services to manage the reactive nitrogen content of human excreta. Like food security, access to sanitation is an equity issue: more than 60% of the global population lacks access to safely managed sanitation services, which is defined as access to a sanitation facility at the household where excreta is safely treated (Andersson et al., 2016; Andersson, Otoo, & Nolasco, 2018). This is a sanitation crisis, and it is estimated that lack of sanitation access cost the global economy an estimated $222.9 billion USD in 2015 related to premature deaths, loss of productivity related to sanitation-related diseases, healthcare to treat sanitation-related diseases, and in time lost due to lack of access (LIXIL, WaterAid, & Oxford Economics, 2016). This has significant environmental health consequences; untreated human excreta is a significant contributor to reactive nitrogen in the environment (United Nations Environment Programme & Woods Hole Research Center, 2007). But even in developed country contexts, there is discharge of reactive nitrogen to air and water. It has been suggested that all the nitrogen applied in the agricultural sector, even the portion assimilated by plants and consumed as food, is eventually released to the environment (Liu et al., 2016). A study in Sweden found that the nitrogen content of excreta corresponds to 28% of the total nitrogen in fertilizers sold in Sweden in 2010/11 (Spångberg, Tidåker, & Jönsson, 2014). Human waste dominates the discharge of nitrogen to surface water from urban areas, and global urban discharge has increased in the past century to 7.7 Tg nitrogen per year (Morée, Beusen, Bouwman, & Willems, 2013).
Resource recovery from sanitation has been proposed as a strategy to address these multiple social and environmental challenges simultaneously (Ddiba, 2016; Masi, Rizzo, & Regelsberger, 2018; Meinzinger, 2010; Trimmer, Cusick, & Guest, 2017). It is estimated that current nutrient recovery from human sanitation for agriculture is between 0-15% of nitrogen and 0-55% of phosphorus (Trimmer et al., 2017), but one potential waste treatment method -- the thermophilic composting of urine-diverted human waste -- enables a nutrient recover of 92% organic carbon, 100% phosphorous, and 86% nitrogen. This has dramatic implications for meeting agricultural nitrogen demand with nutrients recovered from human waste, and a case study of the city of Hamburg found that up to 29% of the synthetic fertilizer used could be substituted by the nutrients found in wastewater (Meinzinger, 2010). Additionally, compost is like manure in that in addition to supplying critical nutrients, it also provides soil-conditioning properties, like the input of carbon substrates into soil which may immobilize nitrogen and reduce runoff and volatilization of reactive nitrogen (Krounbi, van Es, Karanja, & Lehmann, 2018; Xia, Lam, Yan, & Chen, 2017). Recovering nutrients from sanitation could be a driver for an improved management of reactive nitrogen, with implications for improving global food security, access to sanitation, and stabilizing the nitrogen planetary boundary.
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