Nature-based units as building blocks for resource recovery systems in cities

Cities are producers of high quantities of secondary liquid and solid streams still poorly utilized within urban systems. In order to tackle this issue, there has been an ever-growing push for more efficient resource management and waste prevention in urban areas, following the concept of a circular economy. This work provides a characterization of urban solid and liquid resource flows (including water, nutrients, metals, potential energy, and organics), which pass through selected Nature-Based Solutions (NBS) and Supporting Units (SU), expanding on that characteri-zation through the study of real-life cases. In particular, this paper presents the currently im-plemented NBS units for resource recovery, the applicable solid and liquid urban waste streams and the SU dedicated to increase the quality and minimize hazards of specific streams at the source level (e.g., concentrated fertilizers, disinfected recovered products). Recovery efficiency of systems, where NBS and SU are combined, operated at micro or meso-scale and applied at Technology Readiness Levels higher than 5 is reviewed. The importance of collection and transport infrastructure, treatment and recovery technology and (urban) agricultural or urban green reuse on the quantity and quality of input and output materials is debated, also regarding the current main circularity and application challenges.