Marco Hartl

Urban areas increasingly face compound hazards linked to climate change, including intensified pluvial flooding, heat stress and water scarcity. As a result, cities are turning towards green infrastructure (GI) and nature-based solutions (NbS) that can simultaneously reduce risk, enhance urban livability and enable circular resource management. In this contribution, we present alchemia-nova’s experiences from implementing and monitoring two building-integrated vertical NbS for decentralized water treatment and reuse: the vertECO® vertical constructed wetland system at the eco-community Cambium (Fehring, southeastern Austria) and the GRETA™ modular green wall at the St. Quirze social housing pilot (Barcelona metropolitan area, Spain).At Cambium, vertECO® was installed in a wintergarden and represents, to our knowledge, the first full-scale vertical green wall receiving all fractions of mechanically pre-treated domestic wastewater (including blackwater, and not only greywater), with the aim of water and nutrient reuse in local agriculture . The system consists of four parallel (each 2-m long) modules with four stepwise aligned, aerated subsurface horizontal-flow basins, followed by treated water storage and ozonation recirculation . Monitoring results demonstrate that vertECO® alone already achieved average effluent quality compliant with the EU water reuse regulation thresholds for reclaimed water quality Class C (drip irrigation), while vertECO® combined with ozonation achieved Class B (broader irrigation methods), also meeting local Austrian permit requirements . The wintergarden setting maintained operational temperatures above freezing conditions during the monitoring period, supporting year-round performance in a temperate climate with cold winters.In parallel, the GRETA™ pilot at St. Quirze demonstrates a vertical green wall for residential water management, combining bathroom greywater (three showers and two sinks) with rainwater harvested from a 120 m² roof area. The system was integrated into a renovated social housing building with dedicated greywater separation, highlighting the value of implementing source separation during new construction or refurbishment. GRETA™ treats ~125 L/day (peaks up to 180 L/day) using four parallel treatment lines across four stages of horizontal subsurface flow through modular planted units. Treated water is collected, disinfected via ozonation, and reused for toilet flushing in four apartments, with emergency tap water feeding options to improve reliability.Monitoring from May 2023 to October 2024 (15-day intervals) indicates consistent performance, including strong reductions of turbidity, suspended solids, organic load, and ammonium. Hygiene indicators were already low in the influent and reached non-detectable levels after treatment and ozonation, supporting compliance with Spanish reuse requirements for urban non-potable applications. The pilot also yielded operational lessons: elevated installation reduced vandalism risk, and a heat period combined with automation failure caused major plant die-off. However, the system recovered quickly and maintained stable treatment efficiency, highlighting vertical GI resilience under disturbances.Across sites, we show how vertical GI can contribute to integrated urban hazard management by reducing freshwater demand, strengthening resilience to drought and shortages, supporting rainwater buffering strategies, and acting as visible, community-facing infrastructure. We conclude with key research needs on scaling, cost–benefit assessment including co-benefits (e.g., greening and cooling), long-term robustness, and governance models for operation and maintenance.

Urban sprawl poses a persistent challenge for stormwater management, as low-density development patterns increase impervious surfaces while limiting the effectiveness and affordability of conventional, centralised drainage infrastructure. Nature-based Solutions (NbS) for urban areas, such as rain gardens, bioswales, and bioretention areas, have demonstrated strong potential to address stormwater quantity and quality challenges while delivering co-benefits such as urban cooling, biodiversity enhancement, and recreational value. However, despite extensive piloting, NbS for stormwater services are insufficiently mainstreamed in many urban regions and grey infrastructure often remain the default. A key barrier lies in the difficulty of developing scalable business models and governance arrangements that enable their long-term provision as part of regular stormwater services, particularly in dispersed urban environments.This paper examines how co-creation processes can inform the development of business models for mainstreaming decentralised NbS for stormwater management in urban sprawl. Empirical insights are drawn from structured co-creation processes conducted in the metropolitan cities of Lyon (France) and Milan (Italy), involving the metropolitan authorities responsible for stormwater management, water utilities, planners, and researchers. The co-creation activities aimed to identify priority planning units or contexts, relevant stakeholder groups, and feasible implementation arrangements for NbS by aligning technical performance requirements with regional policies and governance structures, financing mechanisms, and stakeholder roles.Across both case studies, three distinct urban environments emerged as particularly relevant for NbS-based stormwater service delivery in urban sprawl: (i) single household units, (ii) parking lots, and (iii) public parks. These environments differ substantially in terms of land ownership, regulatory context, investment logic, and operation and maintenance responsibilities, resulting in divergent requirements for viable business models. Rather than proposing a one-size-fits-all solution, the paper demonstrates how each urban environment is associated with a specific set of business model logics and governance pathways.For single household units, mainstreaming NbS depends on incentive-based and technical assistance models that minimise transaction costs for private property owners and enable aggregation at neighbourhood scale. Parking lots, typically characterised by mixed ownership, offer opportunities for public–private partnership models that integrate NbS into asset management and redevelopment cycles. Public parks provide a setting for utility- or municipality-led models in which NbS are embedded into existing public service provision and justified through multi-functional value creation.The findings highlight the importance of distinguishing between urban environments as planning and business model units when seeking to mainstream NbS in urban contexts. Co-creation proved instrumental in revealing institutional opportunities and constraints, aligning actor expectations, and identifying realistic pathways from pilot projects to standard practice. The paper concludes that successful mainstreaming of NbS for decentralised stormwater management requires environment-specific business models supported by coherent governance arrangements. Consistently, focusing on specific urban environments significantly reduces the complexity of navigating urban governance systems and can accelerate the development of scalable business models for NbS.

Resource-oriented sanitation (ROS) is increasingly discussed as an innovative approach for circular resource use, yet its cross-sectoral sustainability implications are rarely assessed at the level of specific Sustainable Development Goal (SDG) targets. An expert-based scoring approach adapted from Nilsson et al. (2016) was applied within the Austrian UniNEtZ project to assess interactions between ROS and 123 SDG targets. The analysis identified 42 non-neutral interactions, particularly related to water management, nutrient cycling, food production, resource efficiency, health, innovation, and governance. ROS has the potential to improve water quality and reduce pollution loads through direct sanitation pathways as well as indirect effects linked to the reuse of reclaimed water and nutrients, with decentralised and nature-based solutions representing important implementation pathways. The identified interactions were contextualised through a food-system perspective to examine cross-sectoral pathways relevant for integrated governance and policy-relevant sustainability assessment in infrastructure-mature contexts.

Nature-Based Solutions (NBS) have been proven to effectively mitigate and solve resource depletion and climate-related challenges in urban areas. The COST (Cooperation in Science and Technology) Action CA17133 entitled “Implementing nature-based solutions (NBS) for building a resourceful circular city” has established seven urban circularity challenges (UCC) that can be addressed effectively with NBS. This paper presents the outcomes of five elucidation workshops with more than 20 European experts from different backgrounds. These international workshops were used to examine the effectiveness of NBS to address UCC and foster NBS implementation towards circular urban water management. A major outcome was the identification of the two most relevant challenges for water resources in urban areas: ‘Restoring and maintaining the water cycle’ (UCC1) and ‘Water and waste treatment, recovery, and reuse’ (UCC2). s Moreover, significant synergies with ‘Nutrient recovery and reuse’, ‘Material recovery and reuse’, ‘Food and biomass production’, ‘Energy efficiency and recovery’, and ‘Building system recovery’ were identified. Additionally, the paper presents real-life case studies to demonstrate how different NBS and supporting units can contribute to the UCC. Finally, a case-based semi-quantitative assessment of the presented NBS was performed. Most notably, this paper identifies the most typically employed NBS that enable processes for UCC1 and UCC2. While current consensus is well established by experts in individual NBS, we presently highlight the potential to address UCC by combining different NBS and synergize enabling processes. This study presents a new paradigm and aims to enhance awareness on the ability of NBS to solve multiple urban circularity issues.

Cities are producers of high quantities of secondary liquid and solid streams that are 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 review paper 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 characterization through the study of existing cases. In particular, this paper presents the currently implemented NBS units for resource recovery, the applicable solid and liquid urban waste streams and the SU dedicated to increasing the quality and minimizing hazards of specific streams at the source level (e.g., concentrated fertilizers, disinfected recovered products). The recovery efficiency of systems, where NBS and SU are combined, operated at a 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 are discussed, also regarding the current main circularity and application challenges.