Closing the nutrient loop between urban and rural areas: a case study analyzing the technical potential of recycling fertilizers from urban biorefineries

The world’s population is expected to grow rapidly, with the vast majority of people living in urban areas. Closing nutrient loops is therefore increasingly important for food security and environmental stability. In Germany however, the current treatment of organic and green waste and sewage sludge (derived from wastewater) includes composting and co-incineration, resulting in significant nutrient losses. This paper analyses the technical potential of novel urban biorefinery concepts for the recycling of plant nutrition minerals from waste streams and waste water in metropolitan regions. Using the Stuttgart region as a case study, four modular biorefinery concepts were assessed, including phosphorus precipitation, membrane gas absorption, and insect-based bioconversion. These technologies produce valuable recycling fertilizers – such as ammonium sulfate, struvite, and organic NPK fertilizers – which meet legal requirements but differ in nutrient concentrations and availability to plants. The implementation of the evaluated biorefinery technologies has the potential to replace almost all conventional fertilizers used in the Stuttgart region with regionally produced recycling fertilizers, while mitigating environmental impacts and creating economic opportunities within a circular bioeconomy. In order to realize this potential, it is necessary to treat not only organic and green waste, but all urban wastewater streams in urban biorefineries. Finally, an urban biorefinery concept is proposed that optimally matches waste streams and specific biorefinery technologies, enabling nutrient recovery from organic waste and wastewater and supporting the development of regionally adapted circular strategies.