What does it take to renature cities? An expert-based analysis of barriers and strategies for the implementation of nature-based solutions.

This paper uses an expert-based methodology to survey the barriers and strategies related to the implementation of nature-based solutions (NBS). The ambition of the paper is to offer a bird's eye overview of the difficulties encountered by NBS deployment and ways to overcome them. With a wide participation of 80 experts from COST Action Circular City, we identify barriers specific to 35 pre-defined NBS of the following four categories: Vertical Greening Systems and Green Roofs; Food and Biomass Production; Rainwater Management; and Remediation, Treatment, and Recovery. The research sheds light on how a major interdisciplinary - yet predominantly technically-oriented - community of scientists and practitioners views this important topic. Overall, the most relevant barriers are related to technological complexity, lack of skilled staff and training programs and the lack of awareness that NBS is an option. Our results highlight concerns related to post implementation issues, especially operation and maintenance, which subsequently affect social acceptance. The paper identifies a "chain" effect across barriers, meaning that one barrier can affect the existence or the relevance of other barriers. In terms of strategies, most of them target governance, information, and education aspects, despite the predominantly technical expertise of the participants. The study innovates with respect to state-of-the-art research by showing a fine-grained connection between barriers, strategies and individual NBS and categories, a level of detail which is not encountered in any other study to date.

Influence of design parameters on the treatment performance of VF wetlands - a simulation study.

The main approach for designing vertical flow (VF) treatment wetlands is based on areal requirements ranging from 2 to 4 m2 per person equivalent (PE). Other design parameters are the granularity of the filter material, filter depth, hydraulic and organic loading rates, loading intervals, amount of single doses as well as the number of openings in the distribution pipes. The influence of these parameters is investigated by running simulations using the HYDRUS Wetland Module for three VF wetlands with different granularity of the filter material (0.06-4 mm, 1-4 mm, and 4-8 mm, respectively). For each VF wetland, simulations are carried out at different temperatures for different organic loading rates, loading intervals and number of distribution points. Using coarser filter material results in reduced removal of pollutants and higher effluent concentrations if VF wetlands are operated under the same conditions. However, the treatment efficiency can be increased by applying more loadings and/or a higher density of the distribution network. For finer filter material, longer loading intervals are suggested to guarantee sufficient aeration of the VF filter between successive loadings.

Numerical simulation of vertical flow wetlands with special emphasis on treatment performance during winter.

In Austria, single-stage vertical flow (VF) wetlands with intermittent loading are a state-of-the-art technology for treating domestic wastewater. They are designed according to the Austrian design standard with a specific surface area of 4 m2 per person (i.e. 20 g COD/(m2·d)) and thus demand a bigger amount of land to treat the same amount of wastewater compared to intensified technical treatment systems. In order to reduce the amount of land needed, a modified design for VF wetlands has been proposed. The modified design has a specific surface area of 2.5 m2 per person (i.e. 32 g COD/(m2·d)) and it has been shown to be able to meet the Austrian effluent requirements. To allow higher organic loading, more loadings per day but lower volume of a single loading, a constant loading interval, and increased number of openings per m2 are applied. A simulation study using the HYDRUS Wetland Module was carried out to compare the treatment efficiencies of single-stage VF wetlands with classical and modified design. Data from a classical Austrian single-stage VF wetland was used for calibration of the model using the standard parameter set for the CW2D biokinetic model. The influent COD fractionation was calibrated to adapt to the wastewater. The simulations showed a good performance of the modified design compared to a classical VF wetland for COD removal with COD effluent concentrations in winter (effluent water temperature of 4.5 °C) of 35 and 29 mg/L, respectively. The simulation study showed that during high-loading events the VF wetland with modified design has lower maximum NH4-N effluent concentrations. Single-stage VF wetlands with modified design seem to be very effective and allow application of higher organic loads compared to single-stage VF wetlands with classical design.

Using numerical simulation of a one stage vertical flow wetland to optimize the depth of a zeolite layer.

This simulation study investigates the treatment performance of a compact French vertical flow wetland using a zeolite layer in order to increase ammonium nitrogen removal. For the modelling exercise, the biokinetic model CW2D of the HYDRUS Wetland Module is used. The calibrated model is able to predict the effect of different depths of the zeolite layer on ammonium nitrogen removal in order to optimize the design of the system. For the model calibration, the hydraulic effluent flow rates as well as influent and effluent concentrations of chemical oxygen demand (COD) and NH4-N have been measured. To model the adsorption capacity of zeolite, Freundlich isotherms have been used. The results present the simulated treatment performance with three different depths of the zeolite layer, 10 cm (default), 15 cm and 20 cm, respectively. The increase of the zeolite layer leads to a significant decrease of the simulated NH4-N effluent concentration.

Evaluation of the multifunctionality of a vertical greening system using different irrigation strategies on cooling, plant development and greywater use.

Vertical greening systems (VGS) are implemented in the building envelope to address challenges such as the urban heat island effect, energy reduction, air purification, support of biodiversity and recently greywater treatment (wastewater without urine and faeces) for reuse purposes. In this context, providing and using treated wastewater is a crucial point, as generally VGS are irrigated with tap water and thereby increase urban water depletion and pollution. In this study, we evaluate the multifunctionality of a pot-based VGS irrigated with untreated greywater and capable, as well, of acting as a greywater treatment system. The full-scale experimental system uses a low-tech irrigation technique and was investigated for different irrigation water volumes to identify the needed water demand to maximize local cooling by evapotranspiration and suitable plants for the different water conditions and water types. Plant development and greywater treatment capabilities were monitored from April 2020 until September 2021. Based on the highest irrigation volume, a local air temperature reduction of up to 3.4 °C was measured. The removal efficiencies for treating greywater were COD 80 %, TOC 74 %, TNb 70 %, NH4-N 81 % and Turbidity 79 %, respectively, and showed a decrease in the second year of operation. Therefore, the results support the need to develop more robust systems, since up to now mainly short-term experiments have been reported in literature.

Application of leaf analysis in addition to growth assessment to evaluate the suitability of greywater for irrigation of Tilia cordata and Acer pseudoplatanus.

Water is the key resource in fulfilling the cooling function of plants in urban environments and needs to be supplied reliably and adequately, especially during dry periods. To avoid an unsustainable use of high-quality drinking water for irrigation, the reuse of greywater should be implemented for Green Infrastructure irrigation in the sense of the circular economy. In this study, the influence of greywater irrigation on vitality of two trees species, Tilia cordata and Acer pseudoplatanus, was determined by investigating the effect of irrigation with raw or treated greywater in comparison to municipal tap water. Plant growth parameters were measured, including leaf area, number of leaves, average leaf area and annual growth. In addition, the relative chlorophyll content was determined and image analysis was used to identify vital and necrotic leaf parts. While treatment did not affect growth after one growing season A. pseudoplatanus had significantly higher leaf necrosis (34.8%) when irrigated with raw greywater compared to treated greywater (15.5%) and tap water (5.8%). Relative chlorophyll content of T. cordata irrigated with tap water decreased over time until it was significantly lower (28.5) then the greywater treatments (34.5 and 35). Image analysis of leaves to quantify necrosis proved to be a sensitive method to quantify plant health and showed negative effects earlier than an analysis of growth. Anionic surfactants and electrical conductivity had a significant influence on plant vitality. Therefore, plant selection should take these parameters into account, when planning green infrastructure irrigated with greywater.

The coupled socio-ecohydrological evolution of river systems: Towards an integrative perspective of river systems in the 21st century.

River systems have undergone a massive transformation since the Anthropocene. The natural properties of river systems have been drastically altered and reshaped, limiting the use of management frameworks, their scientific knowledge base and their ability to provide adequate solutions for current problems and those of the future, such as climate change, biodiversity crisis and increased demands for water resources. To address these challenges, a socioecologically driven research agenda for river systems that complements current approaches is needed and proposed. The implementation of the concepts of social metabolism and the colonisation of natural systems into existing concepts can provide a new basis to analyse the coevolutionary coupling of social systems with ecological and hydrological (i.e., 'socio-ecohydrological') systems within rivers. To operationalize this research agenda, we highlight four initial core topics defined as research clusters (RCs) to address specific system properties in an integrative manner. The colonisation of natural systems by social systems is seen as a significant driver of the transformation processes in river systems. These transformation processes are influenced by connectivity (RC 1), which primarily addresses biophysical aspects and governance (RC 2), which focuses on the changes in social systems. The metabolism (RC 3) and vulnerability (RC 4) of the social and natural systems are significant aspects of the coupling of social systems and ecohydrological systems with investments, energy, resources, services and associated risks and impacts. This socio-ecohydrological research agenda complements other recent approaches, such as 'socio-ecological', 'socio-hydrological' or 'socio-geomorphological' systems, by focusing on the coupling of social systems with natural systems in rivers and thus, by viewing the socioeconomic features of river systems as being just as important as their natural characteristics. The proposed research agenda builds on interdisciplinarity and transdisciplinarity and requires the implementation of such programmes into the education of a new generation of river system scientists, managers and engineers who are aware of the transformation processes and the coupling between systems.