Microbial fuel cells implemented in constructed wetlands (CW-MFCs), albeit a relatively new technology still under study, have shown to improve treatment efficiency of urban wastewater. So far the vast majority of CW-MFC systems investigated were designed as lab-scale systems working under rather unrealistic hydraulic conditions using synthetic wastewater. The main objective of this work was to quantify CW-MFCs performance operated under different conditions in a more realistic setup using meso-scale systems with horizontal flow fed with real urban wastewater. Operational conditions tested were organic loading rate (4.9 ± 1.6, 6.7 ± 1.4 and 13.6 ± 3.2 g COD/m2·day) and hydraulic regime (continuous vs. intermittent feeding) as well as different electrical connections: CW control (conventional CW without electrodes), open-circuit CW-MFC (external circuit between anode and cathode not connected) and closed-circuit CW-MFC (external circuit connected). Eight horizontal subsurface flow CWs were operated for about four months. Each wetland consisted of a PVC reservoir of 0.193 m2 filled with 4/8 mm granitic riverine gravel (wetted depth 25 cm). All wetlands had intermediate sampling points for gravel and interstitial liquid sampling. The CW-MFCs were designed as three MFCs incorporated one after the other along the flow path of the CWs. Anodes consisted of gravel with an incorporated current collector (stainless steel mesh) and the cathode consisted of a graphite felt layer. Electrodes of closed-circuit CW-MFC systems were connected externally over a 220 Ω resistance. Results showed no significant differences between tested organic loading rates, hydraulic regimes or electrical connections, however, on average, systems operated in closed-circuit CW-MFC mode under continuous flow outperformed the other experimental conditions. Closed-circuit CW-MFC compared to conventional CW control systems showed around 5% and 22% higher COD and ammonium removal, respectively. Correspondingly, overall bacteria activity, as measured by the fluorescein diacetate technique, was higher (4% to 34%) in closed-circuit systems when compared to CW control systems.

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.

Global phosphorus reserves are under pressure of depletion in the near future due to increased consumption of primary phosphorus reservoirs and improper management of phosphorus. At the same time, a considerable portion of global marine water bodies has been suffering from eutrophication due to excessive nutrient loading. The marine environment can be considered as a valuable phosphorus source due to nutrient rich eutrophic seawater and sediment which could potentially serve as phosphorus mines in the near future. Hence, sustainable phosphorus recovery strategies should be adapted for marine systems to provide phosphorus for the growing market demand and simultaneously control eutrophication. In this review, possible sustainable strategies for phosphorus removal and recovery from marine environments are discussed in detail. Bio-based strategies relying on natural phosphorus uptake/release metabolism of living organisms are suggested as promising options that can provide both phosphorus removal and recovery from marine waters for achieving a sustainable marine ecosystem. Among them, the utilization of microorganisms seems promising to develop novel strategies. However, the research gap for the technical applicability of these strategies is still considerably big. Therefore, future research should focus on the technical development of the strategies through laboratory and/or field studies. Coupling phosphorus mining with other valorisation pathways (i.e., metal recovery, energy production) is also suggested to improve overall sustainability and economic viability. Environmental, economic and societal challenges should altogether be well addressed prior to real scale applications.

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.

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.