Integrating circular economy and one-water approach : a sustainability assessment framework for urban water systems

Urban Water Systems (UWS) consist of water, wastewater and stormwater infrastructure, which are lifeline of any modern city. UWS are increasingly challenged by climate change, population variation, and water scarcity, all of which intensify uncertainty in long-term planning. Moreover, existing sustainability assessment approaches often lack clear goals and remain vulnerable to biases in the selection of reliable indicators and model development. These limitations highlight the urgent need for integrative, adaptive decision-support frameworks that can incorporate uncertainty while guiding future planning. This dissertation addresses this gap by developing a novel integrated framework designed to assess and enhance the sustainability of UWS planning under future stressors by combining the principles of the Circular Economy and One-Water Approach (Circular-One-Water-Approach – COWA). The proposed paradigm, COWA, bridges conceptual and methodological gaps in existing paradigms such as Integrated Water Management - the One-Water approach, and the Circular Economy. Based on a comprehensive literature review, six key principles are proposed for sustainable UWS planning: (1) waste reduction, (2) multi-scale planning, (3) flexible and adaptive infrastructure, (4) system integration, (5) community integration, and (6) nature integration. These principles are operationalized through 168 indicators organized across 17 subcriteria and four main criteria. Indicator importance was determined using a fuzzy linguistic weighting approach, followed by the development of a Fuzzy Cognitive Map (FCM) to capture causal relationships. A comparative evaluation across four paradigms namely, Business-as-Usual (BAU), Circular Economy (CE), One-Water Approach (OWA), and COWA, shows that COWA achieves sustainability performance improvements of around 30% relative to BAU. Scenario analyses under climate change conditions further emphasize the importance of coordinated multi-dimensional interventions, including nutrient and resource recovery, energy efficiency, low-impact development, and participatory governance. Overall, this research contributes (i) a comprehensive system-oriented sustainability indicator-based framework for UWS, (ii) methodological innovation through the integration of fuzzy linguistic weighting and Fuzzy Cognitive Maps for uncertainty-aware planning, and (iii) a pragmatic tool for decision-support by identifying strategies and quantifying trade-offs in sustainability interventions. The proposed COWA framework provides actionable guidance for policymakers, engineers, and urban planners seeking to design sustainable, resilient, and circular urban water systems.