Potential integration of nature-based solutions as building blocks for resource recovery systems in cities

Cities are centres of human and economic activities, and in its flurry, are also producers of great quantities of discarded materials and products, effectively functioning as concentrators of natural resources, perpetuating the current linear system of "take-make-dispose" (EMF - Ellen MacArthur Foundation, 2017). In order to tackle this issue, there has been an evergrowing push for the adoption of better resource management and waste prevention in urban areas, going in line with the concept of Circular Economy (Zeller et al., 2019). The definition of Circular Economy, given in the first publication of the COST Action Circular City (Langergraber et al., 2020), is an economic system that aims at minimising waste and input of energy and return them as many resources as possible. In order to do so, the different economic systems must bear in mind to minimise resources input and output, emissions to air, water and soil and energy leakage. This could be achieved by slowing, closing and narrowing energy and material loops. In urban environments, specifically, this can be ensured by managing the energy and material flows specific to this environment (water, nutrients, commodities, organics, energy) in such a way that the amount of waste generated is minimal or none at all (Langergraber et al., 2020). However, the implementation of circularity in urban areas comes with added challenges which needs must be addressed. Paiho et al. (2020) attempted to develop a comprehensive list of these challenges, which belong to the following categories: i) "Business" category, ii) "Policy" category, iii) "Technical" category and iv) "Knowledge" category. To deal with these circularity challenges, Langergraber et al. (2020) proposed to apply Nature-Based Solutions (NBS) as the basis for achieving the Circular City concept. By establishing solutions which are simultaneously cost-effective, resource efficient and locally adapted, NBS can serve as tools of application of circular economy within cities (Langergraber et al., 2020; Nika et al., 2020). NBS methodology is extremely interesting, as the inherent focus on resource recovery ensures an improved management of not just water but also the carbon, nutrients, energy and potentially other elements that can be used in interconnected systems (Nika et al., 2020). Several NBS units can together form resource recovery system to cycle above mentioned elements. In that way, the ethodology introduced with the concept of NBS can be proved to fulfil the goals previously set out. While biological processes are the foundation for NBS, other units based on chemical and physical principles may be required to effectively "close the loop". These additional units are designated as "Supporting Units", and their mass and energy balances need also to be studied in detail. As a follow-up of the paper of Kisser et al. (2020), this work will provide a characterisation of urban solid and liquid resource flows, (including water, nutrients, potential energy and organics) which pass through selected NBS and Supporting Units, expanding on that characterization through the study of real-life cases. In particular, we will present the currently implemented NBS units for resource recovery and the applicable solid and liquid urban waste streams and the supporting units dedicated for producing separate input streams at the source level (like urine or concentrated black water) or concentrated fertilizers and disinfected recovered products. Recovery efficiency of systems (where NBS and supporting units are combined) operated at micro or meso scale, applied at TRL 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 inputs and outputs material will be debated. Finally, the current main circularity and application challenges to be considered (e.g. from the infrastructure, legislation, social and environmental services, multi-stakeholders view-points) will be discussed.