Energy-integrated water cycles: Critical review of optimisation strategies and renewable energy synergies

The urban water cycle, encompassing every stage from extraction and treatment to distribution and reuse, is inherently energy-intensive. The deep and growing interdependency between water and energy systems creates a critical imperative to enhance efficiency and reduce consumption. This paper reviews optimisation strategies and the integration of renewable energy in both water supply and wastewater systems. Existing literature on optimisation and renewable energy integration often promises significant energy savings but overlooks practical implementation challenges. Consequently, this paper places a strong emphasis on industrial feasibility, identifying key research gaps between theoretical potential and real-world application. A key finding is the emergence of two distinct optimisation paradigms: water supply systems are suited for external, grid-interactive strategies by leveraging their inherent storage capacity, whereas wastewater systems, constrained by operational limitations, require internal process optimisation. The core argument of this paper is that the most impactful and capital-efficient frontier for innovation lies not in new, high-cost infrastructure, but in leveraging the inherent flexibility of existing water systems. Through advanced optimisation and control, this untapped potential can be unlocked safely and in some cases without significant capital investment. A case study of a pump and reservoir system confirms this conclusion, demonstrating how using the system's inherent flexibility can achieve significant energy and cost savings while satisfying water demand.