Phosphorus recycling from human excreta in French agroecosystems and potential for food self-sufficiency

Phosphorus (P) is an essential constituent of life but large P losses from agroecosystems and sanitation systems are a major source of eutrophication in water bodies. These losses are doubly negative as P in human excretions can be used for crop fertilization. Using a unique dataset of 20,000 French WasteWater Treatment Plants (WWTPs) operational measurements over two decades and a P mass balance of the sanitation system, we assess the fate of human excretions and their agricultural potential. Despite 75% of French WWTPs sludge being applied to crops, only 50% of the excreted P is returned to agroecosystems. This is among the highest rate in Western countries. Meanwhile, another 35% of the excreted P ends up in surface waters or the terrestrial environment through WWTP discharge, diffuses losses from individual autonomous systems, and sewers leaks. The remaining 15% is contained in sludge that is incinerated or sent to landfills. Moreover, while WWTP removal efficiency increased in the 2000s, reaching 80% on average nationally, it has been followed by a decade of stagnation in every French basin. The final removal efficiency for each basin (65% to 85%) closely correlates with how much of the basin area is defined as P-sensitive in the European directive. Our results suggest that recycling all P in excretions could help supply 7 to 34% of the P demand in French food supply, without changing the current food system. Reshaping agricultural systems (shifting to more plant-based diets, decreasing P losses and food waste) would enable to go even further on the road to food sufficiency.