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Water supply and reuse through non-conventional water resources can significantly decrease the stress on natural water resources. Decentralized systems can help not only to alleviate issues of water security in arid areas, but also to create a sustainable framework within a circular economy. Although these small-scale innovative technologies are able to achieve ready-to-use, high quality of recovered/treated water on-site, the loop cannot be closed in most cases due to legislative barriers. Similarly, the end-use of sewage sludge after treatment in decentralized systems still lacks specific regulations that limit its valorization. This work analyzes the current policy and legislations related to water supply, wastewater treatment, water reuse and resource valorization within the context of decentralized state-of-the-art technologies applied in rural areas. The drawbacks in the current EU legislation that set barriers to close water-related loops in European countries are highlighted. A regulatory fitness check was applied to each type of loop to identify the key factors to accomplish the legislative compliance, and financing pathways were further evaluated at the EU level. As a possible solution, further development of an innovation deal approach is recommended to address the environmental, regulatory and financial gaps in water management through an integrated framework, providing ad-hoc policies and prescriptions for sustainable reuse of all water resources. • Regulatory fitness check was applied to closed water loops to identify legislative and financing barriers. • Current EU legislatives do not provide ad-hoc guidelines to decentralized systems. • No general barriers were detected for the reuse of reclaimed water in the EU legislation. • Serious limitations exist on drinking water production from non-conventional water resources. • Major constraints were determined for EC-marked compost for organic farming. [ABSTRACT FROM AUTHOR]

An increasing world population is projected to increase water, energy and food requirements, three vital resources for humankind. Projected climate change impacts will aggravate water availability, as well as flood risks, especially in urban areas. Nature-based solutions (NBS) have been identified as key concepts to defuse the expected tensions within the Water-Energy-Food (W-E-F) nexus due to their multiple benefits. In this paper, the authors outlined the theories and concepts, analyzed real-life case studies, and discussed the potential of NBS to address the future W-E-F nexus. For this purpose, we performed a systematic literature review on the theories of NBS that address the W-E-F nexus, and we summarized 19 representative real-life case studies to identify the current knowledge gaps and challenges. The quantitative and qualitative data was used to differentiate and discuss the direct and indirect potential benefits of NBS to the W-E-F nexus. The study further expanded on the challenges for the implementation of NBS and highlighted the growing possibilities in the context of circularity and the implementation of NBS in urban planning. It was concluded that the potential impacts of NBS on the W-E-F nexus have been identified, but the quantitative effects have not been analyzed in-depth. Moreover, indicators are mostly single-purpose and not multipurpose, as required to fully characterize the W-E-F nexus and circularity holistically. Overall, there is a need to adopt systemic thinking and promote the multipurpose design of NBS. [Display omitted] • This work combines a systematic literature review and the analysis of case studies. • The aim was to understand the link between urban NBS and the W-E-F nexus. • The literature review provides some examples of specific NBS addressing specific nexus. • Potential benefits are demonstrated for several case studies, but quantification is scarce. • Challenges were identified covering technical, sustainability, and governance issues. [ABSTRACT FROM AUTHOR]