Circular management of human excreta as a pillar of sustainable cities and food systems

Human excreta (urine and faeces) is a major nutrient deposit; in the context of sustainable agri-food systems, it would in fact constitute the largest source of renewable phosphorus. However, current management through sewer-based sanitation leads to significant degradation of this resource as well as to the loss of the majority of the nutrients to the environment. This paper details the flows of nitrogen throughout the French sanitation system and discusses the levers and obstacles to nutrient recovery in sewer-based sanitation systems.

Using monitoring data from wastewater treatment plants (WWTPs) as well as records on population and dietary intakes, we assessed the flows of nutrients from food, through sanitation systems, and to the environment or environmental parcels. The mass flow diagram showed that 90% of the nitrogen and 50% of the phosphorus entering the system were not returned to agricultural fields but lost to the environment, thus inducing an intrinsic dependence on external nutrient imports.

Though legislation has contributed to reducing water pollution by limiting the amount of nutrients discharged to water, they have had little effect on recycling, especially for nitrogen. Further regulations on WWTPs are unlikely to enable the high nutrient recycling rates that could help secure domestic fertiliser supply chains.

Because of this and given the vulnerability of sewer-based sanitation in the face of climate change and increasing water scarcity, we investigate the potential of excreta source-separation to provide high-quality resources for bio-based fertiliser production. We show that, if secured to fertilise crop production for human consumption, the nutrients in French excreta could cover the nutrient requirements for a third of the domestic food consumption.