Guenter Langergraber

Nature-based solutions for water treatment (NbSwt), such as bioswales, raingardens, green roofs, and treatment wetlands, are multifunctional and cost-efficient wastewater and stormwater solutions, increasingly supported by climate adaptation policy. Yet they remain marginal in urban water infrastructure investment. While barriers to NbS adoption have been widely documented, the decision criteria and underlying assumptions of actors choosing between green and grey remain poorly understood. This study maps the subjective decision logics of three groups of decision-makers in European cities: water and wastewater infrastructure managers, city public authorities, and real-estate developers. Through semi-structured interviews with 19 decision-makers across 10 European countries, analysed via qualitative coding and cognitive models, we find that flood prevention, rather than water treatment, is the primary service driving NbSwt adoption. Each group operates within distinct system boundaries, mandates, and planning horizons, producing fundamentally different decision architectures: City authorities frame NbSwt as instruments of strategic urban greening and climate goals, with multifunctionality as the core value proposition. Water and wastewater managers evaluate NbSwt primarily against cost-competitiveness with grey systems, constrained by a feedback loop in which lack of deployment prevents cost learning. Real-estate developers treat NbSwt as compliance measures filtered through budgets. Across all groups, hesitation reflects both genuine gaps in market infrastructure and subjective assessments of reliability and familiarity independent of available evidence. By surfacing these differentiated decision architectures, the study provides an empirical basis for tailoring NbSwt value propositions to each investor group and identifies where targeted interventions can unlock adoption by addressing each group’s decision logic.

This study explores the contribution of Resource-Oriented Sanitation (ROS) to sustainable food systems, using the United Nations Sustainable Development Goals (SDGs) as the guiding framework for a systemic assessment. Combining expert-based SDG scoring and interdisciplinary analytical contextualisation, the study identifies and analyses 42 positive interactions between ROS and SDG targets. By providing a target-level assessment of SDG interactions and contextualising these through a food-system lens, this study supports an integrated understanding of how ROS relates to SDG targets and indicators beyond SDG 6, and of their relevance for the food-system and its dimensions. The findings highlight the environmental, economic, and social co-benefits of ROS, including improved nutrient and water cycles, enhanced food security and increased resource efficiency. Strong reinforcing links emerge with goals related to sustainable production, water quality, innovation, and climate resilience.

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.

Natural wetlands are critical water quality regulators, especially in developing tropical countries. The Lubigi wetland is a large urban wetland in Kampala, the largest city in Uganda in Africa. We studied whether stormwater discharge and wastewater effluent from a nearby stormwater channel and a sewage treatment plant in the western part of the city were cleaned as they flowed through the wetland. Despite the significant pollution, the wetland removed ammonium-nitrogen, orthophosphate, and particulate nutrients during both seasons, achieving removal rates ranging from 50 to 60% for orthophosphate but only 20–40% for ammonium-nitrogen. Overall, seasonal differences in loads and retention rates of nutrient and organic matter inputs were minimal. Interestingly, the wetland mostly released nitrate and nitrite during water passage through the wetland, most likely due to the mineralization of organic nitrogen and agricultural run-off during rainy events in the wet season. However, the limited capacity of the sewage treatment plant and untreated stormwater discharge from the Nsooba main channel reduced the wetland’s ability to clean water. The insufficient carrying capacity of the treatment plant and the release of untreated sewage into the wetland significantly impact the self-purification capacity of the Lubigi wetland. Thus, the concept of Nature-Based Solutions is ineffective if the wetland systems are overloaded.

Impacts of natural and anthropogenic factors on microbiological water quality indicators along an urban riverine tropical wetland Flavia Byekwaso1,3,6, Guenter Langergraber2, Gabriele Weigelhofer1,3, Rose Kaggwa4, Frank Kansiime5, Thomas Hein1,31 University of Natural Resources and Life Sciences, Vienna (BOKU), Department of Ecosystem Management, Climate and Biodiversity, Institute of Hydrobiology and Aquatic Ecosystem Management, Gregor-Mendel-Strasse 33, 1180 Vienna, Austria2 University of Natural Resources and Life Sciences, Vienna (BOKU), Department of Landscape, Water and Infrastructure, Institute of Sanitary Engineering and Water Pollution Control, Muthgasse 18, 1190 Vienna, Austria3 WasserCluster Lunz, Dr. Kupelwieser-Promenade 5, 3293 Lunz am See, Austria4National Water and Sewerage Corporation, P.O. BOX 7053, Kampala, Uganda5 Makerere University, Department of Environmental Management, P.O. BOX 7062, Kampala, Uganda6 Ministry of Water and Environment, Climate Change Department, P.O BOX 20026, Kampala, UgandaABSTRACT Water quality monitoring is essential for understanding seasonal variations in microbiological indicators and their implications for public health. Waterborne bacteria and pathogens are a significant cause of human diseases, especially in developing countries. The study aimed to understand the factors that cause seasonal changes in the concentrations of microbiological water quality indicators along a riverine tropical wetland. In total, 144 water samples were collected for 12 months at six sites along Lubigi wetland in Kampala, Uganda, receiving varying stormwater and wastewater inputs from urban water infrastructure during the dry and wet seasons. Water samples were analysed using specific microbiological assay tests for Escherichia coli, faecal coliforms, heterotrophic plate counts, Enterococcus and Salmonella species. Generally, the highest concentrations of microbial contamination were detected during the dry season at all sites. There was a decreasing trend in microbial contamination for all the selected five microbiological indicators with increasing distances from the sources of stormwater and wastewater inflows in the upstream reaches towards the downstream areas of Lubigi wetland. Nitrogen compounds, Escherichia coli, faecal coliforms, Enterococcus and Salmonella species originated from stormwater, whereas wastewater discharges delivered primarily phosphorus compounds, organic matter and heterotroph plate counts. E. coli and heterotrophic plate counts were positively correlated with water temperature and salinity. E. coli, faecal coliforms and heterotroph plate counts were positively associated with Biological Oxygen Demand (BOD5). Escherichia coli, faecal coliforms and Enterococcus species were positively correlated with NH4-N. Escherichia coli, faecal coliforms, and heterotrophs dominated with high concentrations during the dry seasons, while Enterococcus and Salmonella species were more prevalent in the wet season. Escherichia coli, faecal coliforms and Salmonella showed insignificant logarithmic reductions during both seasons, showing when the carrying capacities of Nsooba main channel and Lubigi sewage treatment plant systems were exceeded. Enterococcus species showed no reduction in both seasons, which implied continuous high in-stream contamination. Heterotrophs showed significantly higher logarithmic reduction during the wet season than in the dry season. This suggested a concentration reduction during the wet season and loading/increase in the dry seasons. Our research findings may be used by the public health sector to understand relationships between the occurrence of surface water quality microbiological indicators and the prevalence of diseases through strategic seasonal monitoring and evaluation in Kampala and the region.

Nature-based solutions (NBS) are frequently implemented without taking the system’s perspective into account and with the main focus on technical and economic issues of implementation. This study was conducted to test the hypothesis on the potential synergistic effects between circularity and NBS to holistically tackle urban challenges. The main objective is to establish preliminary insights on the obstacles and perspectives of NBS integration and implementation, through a questionnaire set up by the network of experts gathered within the COST Action CA17133 Circular City. The following research questions arise: (i) what differences exist in the level of NBS application according to the variance of engaged countries; and (ii) what are the main obstacles and perspectives for the NBS implementation in order to holistically tackle urban challenges, enhancing the sustainable connection among urban environment, nature, and human well-being. To go beyond the current state-of-the-art and reflect on the research conducted within the Circular City Action, this study aims to open a multi-geographical academic dialogue across Europe and beyond and to move towards a holistic approach to circular cities. Accordingly, this study is: (1) multi-geographical and context-based, providing input for thirty-three EU countries and four non-EU countries to give an overview of the main obstacles and perspectives of NBS implementation, and (2) approach-directed, aiming to formulate a holistic approach to deal with societal challenges. This document intends to provide qualitative and quantitative insight into the potentials and obstacles of NBS implementation in Europe, as well as to motivate further discussion and research to achieve holistic and sustainable cities.

This short review outlines the implementation of nature-based solutions in the urban built environment which can contribute to a circular economy as well as the multiple benefits related to the ecosystem services they can provide. The novel Circular City framework on the mainstreaming of nature-based solutions for the enhancement of urban resource management, which was developed within the COST Action CA17133, is presented. Urban circularity challenges addressed by nature-based solutions are assessed in the built environment following three different levels of implementation—i.e., green building materials, systems for the greening of buildings envelope, and green building sites as vegetated open spaces and water-sensitively designed. Considering the possibilities of implementing nature-based solutions in the built environment, we also highlight the circularity processes that can take place through the integration of nature-based solutions at some or all of the proposed scales towards the achievement of at least one of the seven urban circularity challenges. A collection of representative actual case studies exemplifying the development and implementation of nature-based solutions towards circular cities is also included.