8/12/2025
Sustainable water management calls for innovative strategies, and the circular economy (CE) has emerged as a promising approach, gaining traction as a response to global resource and environmental challenges—including depletion, pollution, and excess greenhouse gas emissions. Learn more>>
A white paper from the Center for Secure Water (C4SW)
Increasingly erratic weather patterns, marked by more frequent droughts and floods, threaten the availability and reliability of freshwater supplies. Rising demand from agriculture, industry, urban populations and technology expansion (e.g., demand for data centers water cooling) further strains already limited resources. Pollution and declining water quality drive up treatment costs, while aging infrastructure and chronic underfunding hinder effective delivery and storage. Governance remains fragmented, with competing stakeholder interests obstructing basin-wide coordination. Meanwhile, sparse monitoring data and high uncertainty complicate forecasting and adaptive management. Adding to the challenge, safeguarding environmental flows and aquatic ecosystems remains essential for long-term sustainability.
Tackling these complex challenges calls for innovative strategies, and the circular economy (CE) has emerged as a promising approach. By minimizing waste and maximizing resource efficiency at scale, CE principles offer potential solutions to two of the most pressing water- related engineering challenges of the 21st century: water scarcity and water pollution. While more efficient use of existing freshwater sources can help address rising demand, these sources are already under significant stress and cannot meet society’s growing needs alone. As a result, producing clean water from non-traditional sources—such as treated wastewater—has become essential.
In line with CE principles, the water sector has introduced the concept of a “circular water economy” [1]. Circular water systems integrate a range of water sources, including freshwater, greywater (i.e., wastewater without fecal contamination), marginal water (e.g., saline or brackish water), treated wastewater, and seawater (via desalination), all managed through interconnected infrastructure and treatment facilities.
We advocate that the analysis, design, and operation of circular water systems can significantly enhance sustainable water resource management at local, regional, and national levels. To support this transition, stronger collaboration between water professionals and experts from other disciplines is essential—both to advance sustainable water solutions and to contribute meaningfully to the broader implementation of CE principles.
The CE proposes an economic model aimed at minimizing waste and making most effective use of resources. Unlike a traditional linear economy using a “take-make-dispose” approach, a circular economy emphasizes the creation of a closed-loop system, where resources such as water, nutrient, and energy are continually cycled back into the economy (Figure 1), reducing the need for new raw materials and minimizing environmental impact [2]. CE aims to move beyond sector-specific applications—such as industrial [3] or ecosystem-level approaches [4]—toward integrated implementation at regional and national scales. This broader perspective considers the interconnections among industrial, municipal, agricultural, energy, and other sectors through a “system of systems” approach.
Drawing on the research and outcomes of recent national and international initiatives—such as the Ellen MacArthur Foundation [5] and the Circular Bioeconomy Systems Institute [6]—five core principles of the CE have been proposed: (I) enhance resource use efficiency, (II) design out waste and pollution, (III) keep products and materials in use, (IV) regenerate natural systems, and (V) generate economic value.
The CE approach is gaining traction as a response to global resource and environmental challenges—including depletion, pollution, and excess greenhouse gas emissions. To enable this shift, CE calls for supportive legislation, financial systems, market structures, institutional capacity, and infrastructure. It also offers a concrete pathway toward achieving Sustainable Development Goal 12: Responsible Consumption and Production, with broad relevance to all other SDGs.
Phosphorous (P) recovery and reuse can help address the "P" paradox
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CE principles are already being applied in localized water systems, though typically at a small scale. Examples include the use of green roofs and rain gardens for water harvesting, reuse of wastewater for non-potable purposes, and desalination of brackish water in coastal cities. Realizing a Circular Water Economy at the regional and national scale will, however, require overcoming a range of technological, environmental, and social challenges. Many of these intersect with longstanding issues in water resources management, including urban water system design and operation, water reuse in agriculture, and integrated watershed management. To address these challenges, we propose several key areas of focus outlined below.
Scaling Existing Circular Water Systems
This section explores three areas of CE implementation—water reuse, treated wastewater irrigation, and nutrient recovery—while examining their benefits, challenges, and opportunities for integrated, cross-sectoral solutions.
Circular urban water systems—defined at the city or sub-city level—integrate multiple water sources such as freshwater, stormwater, graywater, and wastewater to support water supply, flood control, water quality, and ecosystem health, while aligning with social, economic, and environmental goals. These systems present major opportunities, including reduced freshwater dependence, enhanced climate resilience, improved urban livability, and resource recovery (e.g., water, energy, and nutrients). However, the implementation of circular water systems faces several challenges: fragmented governance, lack of integrated infrastructure, uncertain regulatory frameworks, limited public acceptance, and the high upfront costs of retrofitting legacy systems. In addition, inconsistent water quality across sources and the complexity of managing interlinked systems present operational hurdles. To scale up urban circular water systems, cities need coordinated planning, cross-sector collaboration (e.g., with energy, waste, and agriculture sectors), supportive policies, investment in decentralized infrastructure, and public education to build trust and acceptance. Data-driven tools, such as digital twins and integrated water models, can also help optimize system design and adaptive management.
Reusing treated wastewater for irrigation
Reusing treated wastewater for irrigation—often termed “recycled” or “reclaimed” water—supports circular water systems by conserving freshwater, enhancing drought resilience, and recovering nutrients. It offers significant benefits for agriculture, particularly in water-scarce regions. However, challenges include health risks from potential pathogen contamination, chemical pollutants, bioaccumulation, and antimicrobial resistance—especially when reused water is applied to edible crops— and public concerns about safety and acceptance. Scaling up requires careful consideration of discharge sources, treatment processes, crop types, and environmental impacts on soil and nearby water bodies. Sustainable reuse depends on balancing crop profitability, system costs, and risks to public and environmental health.
Nutrient recovery from watersheds
Nutrient recovery from watersheds—the process of capturing and reusing nutrients, particularly nitrogen and phosphorus, that would otherwise be lost through agricultural runoff or industrial discharged— is another key process for the circular economy by transforming waste streams into valuable resources. Integrating nutrient recovery into watershed-scale management offers several advantages by improving water quality, reducing nutrient pollution, and supporting more sustainable agriculture. It enables the reuse of valuable nutrients like nitrogen and phosphorus, decreasing reliance on non-renewable resources and lowering fertilizer production costs. Despite its potential, nutrient recovery remains underutilized due to several challenges. Current efforts are often siloed, with limited interdisciplinary collaboration between hydrologic, agricultural, and environmental engineers, lacking an integrated understanding of how nutrient recovery influences other processes at the watershed scale. Nutrient recovery is frequently modeled within discipline-specific systems—such as biorefineries or wastewater treatment plants—without accounting for cross-sectoral effects at the watershed level. Scaling up nutrient circularity introduces further complexity, and requires coordinated, interdisciplinary collaboration beyond current siloed approaches, bringing together hydrologic, agricultural, and environmental engineers within innovative institutional frameworks [7].
New classes of renewable biocatalysts help remove contaminants from wastewater
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Develop Cost-Effective Recovery & Reuse Technology
The development of technologies that remove contaminants that are harmful to the environment and human health is critical to advancing water reclamation and reuse and activate reclaimed water, nutrient and carbon markets. However, most state-of-the-art treatment technologies are energy- and chemically intensive, and/or generate unwanted by-products. Enzymes, which are highly efficient biological agents that can degrade toxic compounds to benign products, are a promising platform for the development of innovative water reclamation and reuse technologies [12]. Therefore, we can apply biocatalysis-- the use of natural catalysts, such as enzymes, to speed up or catalyze chemical reactions-- as a green chemistry approach to develop advanced treatment technologies in water reclamation and reuse.
Develop Multi-Centric Infrastructure Systems
Traditional infrastructure systems have been developed and operated in silos, with water, energy, waste, and other sectors planning independently. This single-sector approach limits efficiency and fails to address the interconnected challenges of resource use, environmental impact, and infrastructure resilience.
To address this, the implementation of a CE requires a holistic, multi-sector framework that enables integrated infrastructure planning, investment, and policy innovation. Specifically, coordinated planning, design, and operation across water supply, treatment, storage, distribution, and wastewater reuse systems must recognize their interdependence with other sectors like energy, agriculture, and waste. Such an integrated approach can lead to greater resource efficiency, improved environmental outcomes, and long-term sustainability by breaking down silos and enabling shared infrastructure, data, and funding strategies.
Uncoordinated water management decisions can have severe environmental consequences
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Upgrade Water Resource Systems Models & Tools
Current water resources modeling tools are not equipped to capture the complexity and interconnectedness of CE systems, particularly when dealing with integrated water, energy, and food systems. These systems—often referred to as systems of systems (SOS)—involve multi-resource flows and governance structures, yet traditional models remain too narrow in scope to support informed decision-making for planning, design, and operations [14].
To address this gap, there is an urgent need to develop next-generation modeling tools that can simulate, analyze, and predict the performance of circular systems. These tools are to incorporate water quantity, quality, and reuse across multiple sources (e.g., freshwater, greywater, wastewater), while tracking interactions with nutrient and energy flows. The models will account for interdependencies, constraints, and trade-offs across sectors, using approaches such as techno-economic analysis (TEA) and decision-support systems. They must also integrate environmental, social, and economic criteria, enabling more holistic and adaptive planning.
These advanced models will play a critical role in supporting the transition to a CE by enabling a more comprehensive and data-driven approach to sustainability. They will help quantify circularity across key dimensions—such as water, carbon, nitrogen, and energy—and identify missing or weak-link technologies to guide targeted infrastructure development. By providing tools to benchmark circularity across different scales and assess trade-offs and synergies, these models will enhance multi-stakeholder decision-making and foster knowledge exchange across sectors. Additionally, they will support the education and training of future engineers and scientists, equipping them with systems-thinking approaches. Ultimately, this integrated modeling framework will enable smarter investments, more resilient and adaptive infrastructure, and better environmental and social outcomes by recognizing co-benefits, anticipating unintended consequences, and informing balanced policy and design choices. Artificial Intelligence offers opportunities to enhance this field through multi-variable optimization approaches and considerations.
Build User-Inspired Solutions through Engagement
Developing effective solutions for a CE, especially in water resources, often lacks meaningful stakeholder engagement and fails to incorporate the diverse perspectives of end users. Despite water’s fundamental role in life and production, science communication and trust-building between researchers and communities remain limited. This disconnect hampers public understanding of priorities, potential synergies, and trade-offs, ultimately weakening the implementation and impact of CE initiatives.
A “user-inspired solutions” approach emphasizes early and continuous stakeholder involvement—from defining problems and designing projects to testing and validation. This inclusive “incubation” process ensures that innovations reflect real-world needs and priorities. Applying this model to water systems can help identify feasible, acceptable, and sustainable strategies.
Furthermore, aligning these efforts with principles of equity ensures that benefits and risks are shared fairly, especially in communities most affected by environmental and economic transitions.
Despite growing recognition of its importance, stakeholder engagement is often under-resourced and poorly structured. Communication gaps, lack of trust, and limited outreach to marginalized communities can hinder progress. Additionally, as CE initiatives scale, they may unintentionally introduce new equity concerns, such as uneven distribution of benefits, increased costs for vulnerable groups, or gentrification effects tied to improved infrastructure or environmental quality.
Engaging stakeholders from the start fosters transparency, collaboration, and trust. It leads to more resilient and inclusive solutions, strengthens public support, and promotes shared ownership of outcomes. In the context of water resources and CE initiatives, this approach can help address long-standing issues of access, affordability, and environmental justice while improving decision-making quality and accountability.
Integrating circular water economy principles into data centers can help minimize its impacts
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Neither the concept nor the practices of circularity in water resource systems are new, but extending circularity to economies of scale is both novel and challenging in the context of circular economy. The realization of a CE, especially a circular water economy for water resources communities, can help us achieve the Sustainable Development Goals (SDGs) as a “framework for planet and prosperity,” highlighting the inter-connected of many goals and the need of a lens to de-silo and optimize resources uses to create richer, multi-faceted outcomes for people and the environment. Along with professionals in other areas, the water resources community faces challenges and opportunities in realizing a circular economy. These include advanced water treatment technologies, new regulatory standards for environmental and health risk management, additional monitoring and risk assessment, and expanded public education, awareness, trust, and acceptance.
Within the water sector, hydrologic and hydraulic scientists and engineers, as well as those in traditional environmental engineering, can enhance their collaboration for watershed management, urban water systems operation, irrigation, etc. Innovations are needed to overcome interwoven issues and concurrently address multidimensional solutions considering tradeoffs and synergies across multiple areas, and predicting any unintended consequences, with a common goal of environmental sustainability. Scaling up circular water systems to achieve an economy of scale will require more advanced technological development, more rigorous environmental and public health impact assessments, more careful consideration of social equity, and broader acceptance of resource reuse by the public.
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