Magali Dechesne1*†
Beatriz de la Loma Gonzalez2†Joana Castellar2Joaquim Bofill Añaños3Maria Dolores Hernandez Peñalver3Gary Moys4Grégoire Decamps5- 1Veolia, Montpellier, France
- 2Cetaqua Water Technology Centre, Barcelona, Spain
- 3Aquatec, Barcelona, Spain
- 4Seureca, Aubervilliers, France
- 5Veolia, Aubervilliers, France
As climate change intensifies impacts on the water cycle, adaptation of water utilities is key to maintain drinking water, wastewater and stormwater services. For Veolia, Nature-based Solutions (NBS) will play a significant role in providing more resilient water services, in combination with conventional grey infrastructures. NBS will enhance water security while contributing to biodiversity preservation, risk reduction, carbon emission reduction, and improved well-being. This green-grey integration can lead to optimized investment and operational costs, water quality compliance and cultural adoption by traditionally technological-oriented water utilities. This paper presents Veolia’s green meets grey approach including an assessment framework to assess NBS benefits, case studies, and implementation challenges for wider NBS adoption in the water management business.
1 Introduction
Given the growing challenges of climate change, water cycle disruption, biodiversity loss, and resource scarcity, Nature-based Solutions (NBS) have emerged as a key complement to traditional water management and treatment systems (Gutry-Korycka, 2020; Liu et al., 2023). By leveraging natural processes, NBS enhance water systems resilience, optimize sustainability outcomes, and contribute to biodiversity preservation. They enable lower infrastructure investment and operational costs, as well as post-climate extreme event costs in the sectors of health (heat waves, freshwater quality, etc.), food supply, and urban infrastructure protection (OECD, 2024).
Water utilities, whether public or private, have historically been more oriented towards grey infrastructures and technological solutions to provide water supply and sanitation services (Bertule et al., 2014; Browder et al., 2019; Pecharroman et al., 2021). Urban areas typically rely on large, centralized systems. Delivering quality water services to growing cities and communities is a global challenge, exacerbated by climate change (Kabisch et al., 2016; Castellari and Davis, 2021; Pryor et al., 2025). There is an imperative to adapt water systems, but critical questions persist on how, from incremental (marginal changes to existing systems) to transformative adaptation (system shifts such as NBS) (Bain et al., 2018; Dilling et al., 2023; Langendijk et al., 2025). Water utilities now have the opportunity to transform and make water systems more resilient by integrating NBS in their portfolio of solutions, potentially in combination with grey infrastructure (Depietri and McPhearson, 2017), provided land is available. Water utilities can play a central role in this adaptation to climate change and NBS upscaling: consulting and expertise, design and build, operation and maintenance, surveillance and monitoring and innovative approaches—whether technical, financial or collaborative.
Veolia provides environmental services for water, waste and energy management to public and private clients around the globe. The company manages over 3,400 water and wastewater facilities globally. In the context of increasing climate change challenges, Veolia has made ecological transformation a cornerstone of its strategy. For example, Veolia is committed to saving water resources with a target of 1.5 billion m3 of freshwater saved in 2027 with improved management solutions (Veolia, 2024). The company has begun integrating NBS into its water management portfolio as a complement to conventional infrastructures. This approach aligns with global sustainability frameworks, including the Sustainable Development Goals (SDGs) (United Nations, 2015), the European Union Green Deal and key directives such as the Restoration Law or the Urban Wastewater Treatment Directive (European Commission, 2019, 2024a,b).
NBS complement conventional water utility systems upstream, within and downstream by protecting freshwater resources, managing stormwater and controlling overflows, treating water or effluents, and recharging aquifers. This paper focuses on two NBS types that are most relevant to ongoing Veolia operations: Sustainable Urban Drainage Systems (SUDS), which may combine different NBS types, and urban wetlands, which may serve multiple functions in urban and peri-urban areas, including stormwater management, drought management, aquifer recharge, and water treatment.
Many examples of sustainable stormwater management exist at city-scale around the world, under different climates, where various NBS are implemented over urban watersheds: Copenhagen (Københavns Kommune, 2012), New York (New York City Environmental Protection, 2019), the “Sponge City” concept in Chinese cities (Chan et al., 2018), Brazil (Mendes et al., 2024), etc. The “Sponge Concept” goes beyond stormwater management, as NBS are used not only to store and retain stormwater, but also to restore the water cycle in the long term, mitigating floods and droughts, and providing lasting benefits. Current research investigates the implementation of “Sponge Measures” for entire landscapes (European Union and UK Water Industry Research, 2025; Sah et al., 2025).
Wetlands are multifunctional systems for water management. They may support SUDS, aquifer recharge and water treatment (Ferreira et al., 2023). The International Water Association (IWA) and the UK Water Industry position wetlands as a pre-treatment step for surface freshwater or stormwater, or as post-treatment for wastewater refinement (Cross et al., 2021; Pryor et al., 2025). This addresses key wastewater treatment challenges, including micropollutants, carbon footprint of wastewater treatment plants, carbon and nutrient recovery, centralised versus decentralised systems, and cost-effectiveness. While current developments for wastewater treatment explore technologies like nanomaterial-based removal (Stoller et al., 2019; Vuppala et al., 2019), photocatalysis (Leblebici et al., 2015; Ahtasham Iqbal et al., 2024), and membrane treatment technologies (Wu et al., 2016; Nasrollahi et al., 2022), wetlands and phytoremediation are effective for treating various effluents and contaminants (Riva et al., 2020; Regni et al., 2021; Lyu et al., 2024; Savvidou et al., 2024).
NBS upscaling is a challenge for many reasons. Implementation requires robust scientific evidence, diverse expertise, and long term cooperation between stakeholders (Dubovik et al., 2022). It is essential to provide clear, measurable evidence of NBS performance across economic, environmental and social dimensions (Toxopeus and Polzin, 2021). These evaluations are critical for demonstrating the value of NBS as a complementary solution to grey infrastructure, optimizing design, and ensuring alignment with evolving regulatory frameworks and sustainability goals. Additional challenges for water utilities are creating viable business models and engaging corporate decision makers and staff at each level of the organization (Giordano et al., 2020; Brill et al., 2021).
Veolia launched the Nymphae initiative to support data-driven decision-making, build trust on NBS, and bridge knowledge and market gaps. The scientific objective of the project was to create a transparent NBS assessment framework to show NBS value and outcomes, applicable to water supply and sanitation services, and validate it through real-world applications. From the market innovation standpoint, this project aims to demonstrate NBS value, engage internal stakeholders and identify pathways for successful implementation. The assessment framework supports a standardized NBS offer to systematically integrate green infrastructure in the water services portfolio for business developers globally, while addressing organizational and market barriers to adoption.
This paper introduces the comprehensive assessment framework developed, demonstrates its application through two operational case studies with their development and quantified benefits, and analyzes the business challenges, organizational drivers, and strategic considerations necessary to make NBS a viable and scalable opportunity for water utilities. The integrated approach combines technical performance evaluation with practical implementation insights to support broader NBS adoption in the water management sector.
2 Method
This section presents the development of the Nymphae Key Performance Indicator (KPI) framework designed to assess water management NBS performance across, hydrological, environmental, financial, and socio-economic services. The framework was developed through literature review of existing NBS assessment methodologies, identification of key performance categories, and creation of standardized protocols and calculation tools to enable precise performance tracking and comparison with traditional grey solutions. Two operational case studies were analyzed using a mixed-methods approach combining quantitative performance data analysis with qualitative assessment of implementation processes. Selected KPIs from the framework were applied based on data availability and relevance to each case study’s specific objectives. Performance data was collected from operational monitoring systems and compared against baseline scenarios or alternative solutions where available. Business challenges and opportunities for NBS adoption were assessed through analysis of Veolia’s internal experience, market observations, and evaluation of organizational barriers and drivers identified during the case study implementations and broader NBS portfolio development.
2.1 Background on NBS assessment
NBS may achieve a number of functions, including hydraulic control and pollution reduction, alongside ecosystem and biodiversity preservation, human health protection, and social well-being enhancement. However, a significant gap persists between these functions and the means to assess them objectively, due to knowledge limitations or inadequate monitoring methods and capabilities (Langeveld et al., 2022). Comprehensive assessment frameworks are essential not only to demonstrate NBS effectiveness but also to support decision making, business case development, and organizational adoption. These frameworks must account for the variety of NBS benefits and co-benefits, encompass multiple spatial scales involved—from local implementation to watershed management—and evaluate long term performance variation across the NBS’s lifespan (Barraud et al., 2008).
Different expert groups have worked on building frameworks and classifications, with indicators for design, performance monitoring, benefits and co-benefits assessments, to either guide potential actions, ensure efficiency and demonstrate value (Raymond et al., 2017; Huthoff et al., 2018; Brill et al., 2021; Palomo-Rios and Vandewoestijne, 2021). The European Commission suggests 12 challenge areas for comprehensive NBS assessment including Climate Resilience, Water Management, Biodiversity Enhancement, or Social Justice and Social Cohesion (Palomo-Rios and Vandewoestijne, 2021). A collaborative initiative including the CEO Water Mandate, Pacific Institute, The Nature Conservancy, Danone and LimnoTech, developed another framework with benefits classified under the five themes: Carbon, Water Quality and Quantity, Biodiversity and Environment, and Socioeconomics (Brill et al., 2021). These efforts emphasize monitoring and evaluation with indicators as an integral component from NBS project inception (Huthoff et al., 2018).
Veolia sought to identify a framework assessing NBS efficiency and benefits for water, waste and energy management services, plus environment and community co-benefits. Existing frameworks inadequately covered technical efficiency, operational aspects and client-valued benefits, and organization adoption barriers. A specific framework was therefore developed, inspired by previously cited scientific and institutional references and adapted to the scope of water, waste and energy services plus business requirements. A broad framework was firstly built, with benefits and co-benefits classified under 15 main challenges and 100 specific challenges (see Appendix 1). The framework covers a wide range of benefits and co-benefits, such as reduced flood risk, urban heat island mitigation, improved air quality, community well-being, production of biomass, thermal isolation, related to water, waste and energy systems. The water services-focused Nymphae KPI framework was derived from this comprehensive foundation.
2.2 Nymphae KPI framework for water management solutions
The Nymphae KPI framework is dedicated to water management issues such as stormwater runoff and flood mitigation, water supply quantity and quality enhancement, ecosystem restoration. It applies to specific NBS: SUDS and Wetlands. Six main challenges and 14 specific challenges were selected from the initial broad-scoped framework, and 28 KPIs were elaborated to enable systematic measurement and estimation of NBS performance in addressing each main challenge (see Figure 1). This focused approach addresses the identified gaps in existing frameworks by integrating technical efficiency metrics with operational performance indicators that are directly relevant to water utility decision-making and client value propositions.
Figure 1. Nymphae KPI framework for assessing water management NBS challenges.
The KPIs are classified using the Driver-Pressure-State-Impact-Response (DPSIR) model (Smeets and Weterings, 1999), which connects environmental drivers and pressures to ecosystem changes and societal impacts, enabling a clear evaluation of NBS effectiveness. These indicators are aligned with the Common International Classification of Ecosystem Services (CICES), ensuring consistency and relevance to key services such as water treatment and purification, hydrological regulation, and flood mitigation. This dual classification approach enables both scientific rigor and practical applicability, supporting evidence-based business cases while maintaining compatibility with international assessment standards.
2.3 KPI protocols
The Nymphae KPI protocols provide a structured methodology to monitor the 29 KPIs. Designed to support both technical precision and broad applicability, the protocols incorporate systematic classification, data-driven methodologies, and practical considerations for monitoring the selected KPIs.
Each KPI protocol is categorized by complexity levels, ranging from simple metrics to more advanced indicators that may need extensive data, resources, or computational requirements. Simple KPIs may require minimal inputs, such as manual sampling or basic statistical analysis. More complex ones integrate automated systems for real-time management to optimize NBS functioning (e.g., inflows, outflows, residence times) and to assess their performance. Leveraging its expertise in real-time monitoring of grey infrastructure and digitalization, Veolia is able to implement innovative approaches to optimize green infrastructure performance and maintenance.
The framework also incorporates economic considerations, providing a detailed analysis of the costs associated with monitoring and the evaluation of each KPI. This includes the expense of equipment, sensor installations, data collection methodologies, and the computational infrastructure required for analysis and reporting. By detailing these cost components, the Nymphae protocols allow for more informed decision-making that balances financial investment with the technical and monitoring needs of KPI implementation, ensuring both cost-efficiency and comprehensive performance assessment for NBS projects.
2.4 KPI estimation tool
The KPI Estimation Tool provides detailed estimation of NBS performance and impact against specific KPIs. It leverages quantitative data from an extensive review of scientific literature - including peer-reviewed research, technical reports, and case studies - combined with Veolia’s internal data, which includes performance outcomes from a variety of NBS projects implemented across different geographies.
Integration of over 3,000 datasets enabled development of robust ranges and averages for selected KPIs, providing a quantitative foundation for estimating the performance of various types of Wetlands and SUDS prioritized in the study. This tool enables evidence-based decision-making, supporting strategic planning, project design, and impact assessment by ensuring reliable, data-driven insights. Above all, this tool helps reassure clients (both public authorities and industries) and decision-makers, building trust in NBS adoption when KPIs demonstrate relevant performance outcomes.
3 Results
Two operation NBS case studies were selected from Veolia’s portfolio based on data availability, operational maturity, and representativeness of key NBS types. Selection criteria included: stakeholder agreement for data sharing, established monitoring systems, availability of baseline or reference scenarios, and potential for grey infrastructure comparison. This selection process highlighted common challenges in NBS performance assessment, including data standardization, monitoring protocol variations, and baseline establishment complexities. Both sites are located in Spain and demonstrate climate change adaptation through the water cycle management. The first case study, La Marjal Parque in Alicante, is a floodable park operating since 2015. The second case study examines a wetland for aquifer recharge in Barcelona, implemented in April 2024 as a drought mitigation measure.
3.1 Case study 1: La Marjal Parque in Alicante (Spain)
La Marjal Parque is a 3.6-hectare floodable park with a retention capacity of 45,000 m3. It was constructed on a former marshland in Playa de San Juan district, Alicante. The park was developed and is maintained through a public-private collaboration between the City Council of Alicante and Aguas de Alicante (AMAEM). The City Council is responsible for gardening maintenance, while AMAEM manages the hydraulic infrastructure.
La Marjal Parque operates in two distinct modes: dry and wet weather conditions. During dry periods, the park remains open to the public, while preventive maintenance of equipment and instrumentation is carried out to ensure operational readiness. During wet weather, the park functions as a stormwater storage facility. When sewer system capacity is exceeded, water is automatically diverted to the park’s retention basin. The system operates under real-time monitoring from a control center, which oversees rainfall intensity as well as the sewer network and basin capacity. Once the rain event ends, the stored water is either discharged to the sea according to water quality compliance or pumped to the wastewater treatment plant. See Figure 2 for the park under dry and wet weather conditions.
Figure 2. La Marjal Parque in Alicante (Spain): floodable park in dry and wet weather conditions.
An economic analysis comparing La Marjal floodable park with a conventional underground stormwater tank (grey infrastructure) demonstrates the substantial cost advantages of the nature-based approach. The construction cost (CAPEX) for an underground storage tank (23.2 M €) was found to be more than six times higher than that of the floodable park (3.7 M €) for an equivalent storage capacity. Additionally, the floodable park proves to be approximately 40% more cost-effective in terms of maintenance and operational expenses (OPEX) with 1.54 €/m3/year. Note this comparison excludes land availability and acquisition costs.
La Marjal Parque was designed with multiple objectives. The first two are flood risk reduction to protect the local population and to reduce sea pollution from direct discharges. Beyond flood mitigation, the park enhances the environment by creating new habitats for birds and wetland species, while also contributing to climate resilience by mitigating the urban heat island effect. Furthermore, it serves as a recreational space and an educational ornithological center. Different KPIs are evaluated for La Marjal Parque. The hydraulic management system employs a dual monitoring framework: (1) an operational monitoring system that collects data for infrastructure management (rainfall intensity, retention basin level, combined sewer overflow); and (2) a monitoring system specifically designed to quantify NBS flood protection performance metrics. Concerning the observed benefits, since its inauguration in 2015, La Marjal Parque has prevented flooding by collecting 58,350 m3 of stormwater. During a 2019 rainfall event, the infrastructure reached 52% capacity (24,000 m3), effectively protecting the surrounding area.
Regarding biodiversity monitoring, since 2018 expert ornithologists have conducted standardized observations complemented by capture-mark-recapture studies to control bird populations. A total of 105 bird species have been documented and more than 1,200 individuals ringed. Dominant species include blackcaps (Sylvia atricapilla), robins (Erithacus rubecula), and Sardinian warblers (Sylvia melanocephala), while notable records include less frequent species such as common kingfisher (Alcedo atthis) and the squacco heron (Ardeola ralloides), indicating enhanced habitat diversity within the NBS ecosystem.
Social benefit is assessed using multiple KPIs, including green space accessibility indices, and perceived well-being and environmental comfort metrics. Visitor engagement is monitored through participation rates in guided tours, educational programs, and public open days. The park functions as a significant educational and recreational asset, where bird banding activities enhance public engagement with urban biodiversity.
3.2 Case study 2: Barcelona aquifer recharge wetlands (Spain)
Barcelona has experienced severe water scarcity in recent years, including one of the most extreme droughts in decades. This crisis prompted the local water utility to release reclaimed (treated) water from the city’s tertiary wastewater treatment plant into the Llobregat River to maintain sufficient flow for both drinking water intake and ecological needs. While successful in maintaining water availability, this intervention caused significant fluctuations in surface water quality, particularly in physico-chemical parameters and total organic carbon levels, complicating drinking water production processes. The European Union-funded MARCLAIMED project (European Union, 2025a,b) addresses this challenge by recharging excess reclaimed wastewater through an infiltration wetland into the aquifer supplying the city water utility’s well fields. This natural filtration through the aquifer enhances water quality stability and provides additional treatment by reducing pathogens, nutrients, and organic contaminants.
The system, operational since April 2024, comprises a 4,000 m3 decantation wetland and a 5,600 m3 infiltration wetland, with water quality monitored through five piezometers and five abstraction wells operated by the water utility, continuous CTD sensors and groundwater sampling every 6 weeks. Preliminary results show a 5 to 20% increase in piezometric levels after recharge episodes (Figure 3). The project targets infiltration of 1 Mm3/year of reclaimed water, demonstrating Managed Aquifer Recharge (MAR) as a safe, effective additional treatment method for reclaimed water and a crucial tool for enhancing water security in water-stressed regions.
Figure 3. Graphs depicting the impact on groundwater levels downstream of the wetlands during two different episodes of aquifer recharge.
Year-round aquifer recharge, independent of rainfall variability, elevates piezometric levels and creates subsurface water storage, making this coastal aquifer less vulnerable to overexploitation and saline intrusion, particularly during climate-change intensified droughts. This subsurface buffer provides superior storage efficiency compared to grey infrastructure reservoirs, requiring reduced surface infrastructure footprint and minimizing evaporation losses.
Comparative analysis of aquifer recharge methods reveals that infiltration ponds offer multiple co-benefits beyond water storage, including urban green space creation and recreational areas for citizens, thereby enhancing urban livability. Conversely, injection wells require minimal surface area, making them suitable for space-constrained environments.
Technical and economic assessment demonstrates that infiltration ponds achieve good performance, while requiring significantly lower capital expenditure (CAPEX) and operational expenditure (OPEX) (Table 1). This combination of enhanced functionality, environmental benefits, and cost-effectiveness makes infiltration ponds a particularly attractive solution for sustainable water management in coastal regions.
Table 1. Green versus grey recharge in Barcelona (Spain): Pros and Cons—adapted from the MARCLAIMED project (European Union, 2025a).
4 Discussion
The case studies demonstrate that NBS effectively address multiple water management challenges while delivering environmental and social co-benefits that conventional grey infrastructure cannot provide. Nevertheless, widespread adoption of NBS in the water management business faces systematic challenges requiring evidence-based solutions through operations experience, standardized assessment frameworks, integration with existing practices, viable business models, stakeholder engagement and appropriate financing mechanisms (Giordano et al., 2020; Hudson et al., 2023; Hohmann et al., 2025).
4.1 Site applications and KPI framework
The case of La Marjal Parque in Alicante provides evidence for the hydraulic performance, cost efficiency and multi-functionality of NBS in urban flood management. Quantified KPIs demonstrate significant advantages over conventional grey infrastructure with biodiversity and social benefits. Long-term KPI assessment for co-benefits can be improved and more robust protocols are to be applied in the near future. Planned monitoring enhancement systems include: (1) an acoustic-based intelligent bird detection system for continuous biodiversity tracking; (2) environmental quality sensors to measure air and noise pollution parameters; (3) automated visitor flow tracking technology. The KPI framework and the monitoring protocols enable a sound evaluation of the benefits and co-benefits of the system, that shows how it functions, supports operation and maintenance, and potential changes to be made. They are also valuable from a communication perspective and to facilitate reproducibility.
In Barcelona, the implemented successive decantation and infiltration wetlands successfully enhance water security as they enable aquifer recharge with excess reclaimed wastewater. Green spaces are actively developing within and around the wetlands, but biodiversity or community recreation benefits have not yet been assessed. The long term objective to implement aquifer recharge wetlands as a structural solution to improve water supply on a regional scale represents a promising pathway for drought resilience (González et al., 2020; European Union, 2025b). Regional-scale implementation feasibility will be established through systematic demonstration of technical reliability and regulatory compliance for reclaimed water recharge. To support this objective, the MARCLAIMED project (European Union, 2025a) is strategically addressing critical knowledge requirements including long-term water quality impacts, aquifer response optimization, and public acceptance strategies for indirect potable reuse, building the scientific foundation necessary for confident regional expansion.
Both case studies benefited from adequate land availability, highlighting a critical success factor for NBS implementation. While land constraints and costs can challenge NBS deployment in dense urban areas compared to compact grey solutions (Depietri and McPhearson, 2017), innovative approaches offer viable alternatives. Distributed implementation of smaller NBS across urban watersheds can overcome space limitations while maintaining effectiveness (Chan et al., 2018). At a watershed scale, integrated NBS networks demonstrate amplified impacts on water cycle restoration and ecosystem services compared to isolated installations (Harvey and Henshaw, 2023; González-García et al., 2025; Sah et al., 2025). Both are opportunities for water utilities when they are in charge of large urban, peri-urban and rural water systems to strategically deploy NBS portfolios across multiple locations, achieving cumulative benefits that exceed the sum of individual installations. Such coordinated approaches enable utilities to optimize land use, maximize environmental co-benefits, and build system-wide resilience through diversified green infrastructure networks.
4.2 Green meets grey: a hybrid approach
Green infrastructures provide a wide range of benefits and co-benefits, but are often difficult to implement in dense urban contexts (Depietri and McPhearson, 2017). Grey infrastructures are more easily implemented, but may not be up to current climate change adaptation and society challenges (Kabisch et al., 2016; Pryor et al., 2025). An option is to mix green and grey solutions into a hybrid approach. Ecosystem functions and engineering are combined to improve water system resilience and provide co-benefits for the community (Depietri and McPhearson, 2017). In addition, such combinations may facilitate overcoming some identified barriers of water utilities to engage into NBS, such as water quality compliance issues or behavior change.
Compliance with water quality standards is usually achieved through conventional treatment processes, which have design removal efficiencies applicable under various operating conditions (Gaid, 2023). NBS may have variable removal efficiencies, depending on environmental factors that are difficult to control. They may be used to pre-treat freshwater or refine wastewater treatment, but their performance is not usually guaranteed. Additionally, NBS may require more time than grey infrastructures to become fully operational, especially for water treatment applications, in relation to the time needed for the plants to grow and become efficient (Cross et al., 2021). Combining green and grey solutions can enhance overall treatment effectiveness while ensuring compliance with regulatory standards under appropriate conditions (Lyu et al., 2024; Nazir et al., 2025). But introducing more green infrastructures in urban water systems is not solely technical, it also involves behavior change as policies and water utilities have historically relied on grey infrastructure and technological processes (Kabisch et al., 2016; Coletta et al., 2021). Practices are slowly shifting to integrate NBS, and especially hybrid approaches that combine NBS with conventional grey infrastructure (Miralles-Wilhelm et al., 2023; Nazir et al., 2025).
On a global scale, it is difficult to compare investments in hybrid green-grey versus green approaches as financial data often fail to differentiate between these categories. However, emerging evidence suggests hybrid approaches may offer superior risk-adjusted returns due to diversified performance mechanisms and reduced single-point-of-failure risks. For example, Forest Trends and The Nature Conservancy indicate that public investments in natural infrastructures for water security are growing and increasingly using hybrid approaches, but investment proportions are not available (Smith et al., 2025). A recent study conducted by the World Resources Institute and the World Bank, provides some numbers for Sub-Saharan Africa (Collins et al., 2025). Green-grey projects represented more projects and funding than solely green, with 95 projects and $8.8 billion funding versus 83 projects and $3.7 billion for green projects, between 2012 and 2021. This funding preference may reflect investors’ confidence in hybrid systems’ ability to deliver guaranteed performance while maximizing co-benefits.
4.3 Business perspectives for water services
In Veolia, the innovation team coordinates this task around four focus areas: initiation of a viable business model, corporate engagement and staff acculturation, partnerships and governance, and sources of financing.
Establishing a viable NBS business model poses significant challenges as financial returns may appear less substantial or immediate compared to those from conventional grey infrastructures. Return On Investment (ROI) may take longer to achieve (Hudson et al., 2023; Kozban et al., 2023) and this needs to be integrated in a long term business strategy. A first step for determining the ROI is to compare green or green-grey solutions versus grey-only solutions in terms of CAPEX and OPEX, as for our two case studies, and benefits such as avoided damages to assets or reduced treatment, as included in the Nymphae framework. Access to land should also be considered. Here again, combining green and grey solutions into a hybrid offer may support a viable business model. ROI analysis and payback timelines should be evaluated over the entire lifespan of the solutions, either from case studies, which are valuable proofs of concept, or from available literature or tools if case studies do not provide enough data (Brill et al., 2021; Rogéliz et al., 2022).
Monetization of ecosystem services provided by green infrastructures can underline the economical value of co-benefits for the community (watershed protection, risk mitigation, biodiversity enhancement, carbon emission reduction, social benefits…). Some may present revenue opportunities, such as carbon credits (Smith et al., 2025) or biodiversity and nutrient markets in the UK (Norfolk Environmental Credits, 2025; Wild Capital, 2025), but the lack of mature markets for most ecosystem services is to be noted (Li et al., 2020). A variety of tools and frameworks have been developed to monetize the ecosystem services provided by NBS (Milliken, 2018; Ferranti and Jaluzot, 2020), such as InVEST for a regional scale (Kareiva, 2011), B£ST for the valuation of SUDS (CIRIA, 2015), or i-Tree Eco for the value of urban trees (Moffat and Doick, 2019). Monetized benefits can include water storage and purification, flood control, temperature buffering, health and well-being, biodiversity enhancement, property value increases (Milliken, 2018). Monetization tools are especially used in urban areas where land and assets have defined monetary values (Ferranti and Jaluzot, 2020). Monetized ecosystem services can help to justify investments, support decision-making, and also facilitate local partnerships or co-financing (Smith et al., 2025).
Engaging corporate decision makers in NBS requires sound market analyses and a well-founded business model, which can be refined with time, but not only. Acculturation is also at stake within the company (Giordano et al., 2020; Brill et al., 2021). It is just as essential to inform and provide clear guidance on NBS at each level of the organization (executives, business developers, consultants, technical managers and operators), with spaces for collaboration. In Veolia, the innovation team is developing various contents for internal communication and training, targeting different audiences, such as a business strategy with business model and market perspectives, podcasts with success stories, online training, transversal core team, AI-driven business development tool based on the Nymphae framework, standardized NBS offer, or inspiring field trips. The innovation team supports many actions to promote creative thinking and foster innovation in responding to tenders or bids.
Water utilities have to secure long term partnerships with key stakeholders under a well-established project governance to implement and operate NBS, all along the NBS lifespan (Hohmann et al., 2025; Martin et al., 2025). This is particularly a challenge for urban or landscape projects involving siloed public entities and/or multiple public entities charged with different legislative responsibilities (Hudson et al., 2023). Hydrological management of the water cycle is not necessarily aligned with its administrative management and this is an obstacle in implementing NBS on the hydrologically-relevant scale. Some legislations support water cycle management with a single public authority, which can be helpful in contracting. For example, the GEMAPI legislation in France for the management of aquatic ecosystems and the prevention of flooding is the responsibility of intercommunal structures (French Ministry of Environment, 2017). Also, as NBS generate multiple co-benefits, partnering with other stakeholders, such as environmental agencies or citizen groups, will strengthen the project outcomes (Hudson et al., 2023; Kozban et al., 2023). Business-wise, it is necessary to be proactive with potential clients, raise their awareness about NBS, understand their value priorities, and anticipate market offers. Experience in Alicante and Barcelona show the importance of working closely with various municipalities to make the project successful, from the initial intention to the daily operation.
Securing financing is a central issue. In industrial projects, NBS are usually privately financed. But for public water services tenders, it may be a difficult equation to solve between ROIs, public acceptance of a potentially higher water bill, and competitive offers. The costs mobilized in the two projects presented here were invested by the municipal and regional stakeholders. Different funding mechanisms may be mobilized through stakeholder partnerships. For instance, some financial institutions or environmental agencies now dedicate resources to support the implementation of NBS, nature preservation and climate change adaptation measures (Cross et al., 2021; United Nations Environment Programme (UNEP), 2022; Smith et al., 2025). Another perspective would be to partner with insurance companies. Different schemes already exist, such as risk reduction incentives that reduce insurance premiums for public or private clients, or making previously uninsurable entities insurable (Brill et al., 2021; Castellari and Davis, 2021; Smith et al., 2025). Another scheme is co-financing resilience measures on damage pay-backs (Visser et al., 2023). Finally, as already mentioned, monetized NBS co-benefits could support stakeholder financial engagement.
5 Conclusion
NBS are very complementary to grey infrastructures, as they fulfill additional water services such as buffering flows and pollution or preserving freshwater resources. Additionally, they support many co-benefits such as watershed protection, risk mitigation, biodiversity enhancement, carbon emission reduction, and social benefits. Moreover, NBS can be more cost-effective than the grey alternative—in both CAPEX and OPEX, providing the land is available at an affordable price. However ROI and full performance, especially water treatment, may take longer to achieve for NBS than for grey infrastructures. Considering a hybrid green and grey solution is a good approach to tackle current and future water management challenges, in both rural and urban areas, and to support business development.
For Veolia, NBS implementation represents a strategic business direction that enhances water services resilience while delivering measurable biodiversity and social value. As a global water utility managing over 3,400 water and wastewater facilities, Veolia is uniquely positioned to drive the NBS agenda forward through systematic integration of nature-based approaches into conventional water management practices. The Nymphae KPI Framework and protocols were designed to allow systematic evaluation of water management NBS and show their multiple benefits, providing the evidence base necessary to build confidence among clients, decision-makers, and investors. Veolia’s expertise in real-time and digital control is particularly valuable to monitor various indicators on water flows, pollution, biodiversity counts and even visitor tracking. Along with this assessment framework, the company has developed comprehensive organizational engagement mechanisms including business model, online training programs, AI-driven business development tools, and other approaches to embed NBS thinking at all levels of the organization, from executives to field operators.
The case studies in Alicante and Barcelona demonstrate how thoughtfully designed and monitored NBS can outperform conventional infrastructures, in both performance and cost-effectiveness, when properly contextualized within local ecological and social systems. Strong stakeholder involvement and adequate governance are also key success factors.
Despite demonstrated benefits, several critical barriers limit widespread NBS adoption across the water management sector. Policies primarily designed for grey infrastructure inadequately address NBS performance variability, creating uncertainty for project approval and compliance verification. The lack of standardized design and monitoring protocols increases project risks and costs, making business case development more challenging. Limited insurance products exist to adequately cover NBS-specific risks. Addressing these systemic barriers requires coordinated action across industry, regulatory, and financial sectors.
Nevertheless, the momentum toward NBS adoption is building globally. Recent evidence shows significant growth in investments in NBS for water security, with both public and private sectors increasingly recognizing their value (Smith et al., 2025). Hybrid green-grey approaches are gaining particular traction, as demonstrated by growing project portfolios in regions from Sub-Saharan Africa to Europe (Collins et al., 2025). As climate change intensifies water-related challenges, the integration of NBS into mainstream water management is transitioning from an innovative option to an essential strategy for resilient, sustainable water services (Castellari and Davis, 2021). With continued development of robust assessment frameworks, supportive policies, and innovative financing mechanisms, NBS are poised to become a cornerstone of climate-adapted water infrastructure worldwide.
Data availability statement
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.
Author contributions
MD: Conceptualization, Investigation, Methodology, Writing – original draft, Writing – review & editing. BL: Conceptualization, Supervision, Validation, Writing – original draft, Writing – review & editing. JC: Investigation, Methodology, Project administration, Writing – review & editing. JA: Resources, Writing – original draft. MP: Resources, Writing – original draft. GM: Conceptualization, Writing – review & editing. GD: Conceptualization, Writing – review & editing.
Funding
The author(s) declare that no financial support was received for the research and/or publication of this article.
Acknowledgments
The authors would like to acknowledge the contribution of Laura Flores Rosell from Cetaqua on the Nymphae KPI protocols, and the previous work from Alice Peyrard and Clara Bercovici at Veolia on maturing the subject of NBS and their relevance for the adaptation of Veolia services to climate change.
Conflict of interest
MD was employed by Veolia. JA and MP were employed by Aquatec. GM was employed by Seureca. GD was employed by Veolia.
The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Supplementary material
The Supplementary material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/frsus.2025.1584758/full#supplementary-material
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Keywords: green infrastructure, nature-based solution, key performance indicator, aquifer recharge, floodable park, biodiversity, society, real-time monitoring
Citation: Dechesne M, de la Loma Gonzalez B, Castellar J, Añaños JB, Peñalver MDH, Moys G and Decamps G (2025) Green meets grey: Veolia’s approach for resilient and sustainable water management. Front. Sustain. 6:1584758. doi: 10.3389/frsus.2025.1584758
Edited by:
Gregg Brill, Pacific Institute, United StatesReviewed by:
Amalendu Singha Mahapatra, Techno India Group, IndiaAngela Marchetti, Sapienza University of Rome, Italy
Copyright © 2025 Dechesne, de la Loma Gonzalez, Castellar, Añaños, Peñalver, Moys and Decamps. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Magali Dechesne, bWFnYWxpLmRlY2hlc25lQHZlb2xpYS5jb20=
†These authors have contributed equally to this work and share first authorship