The water–energy nexus is a key area of focus in efforts to achieve the United Nation's Sustainable Development Goals. As population growth and climate change continue to strain both water and energy systems, innovative and integrated solutions are required that take into account the inextricable interdependencies of these systems. Optimizing resource use, reducing carbon emissions, and increasing system resilience are no longer optional; they are critical imperatives for a sustainable future. This special issue of Engineering presents groundbreaking research exploring the synergies and tradeoffs within the water–energy nexus, offering invaluable insights for policymakers, engineers, and researchers.

This issue features diverse, multidisciplinary research addressing water–energy challenges from technical, policy, and modeling perspectives. One of the key innovations presented herein is the advancement of decentralized water systems (DWSs), particularly those incorporating source-separation technologies. Pan et al. demonstrate that these systems can reduce costs by 40% and greenhouse gas (GHG) emissions by 56% compared with conventional centralized models, while enabling efficient resource recovery. Their strategy integrates rainwater harvesting with solar-powered treatment systems to promote circular water management, significantly reducing energy consumption by 38%.

Xu et al. identify combined sewer overflows (CSOs) as significant yet often overlooked sources of GHGs. Their research highlights methane emissions from sewers and nitrous oxide emissions from rivers, emphasizing the importance of green infrastructure and real-time control for CSO mitigation. They advocate for standardized CSO emissions monitoring to better inform urban carbon accounting. In a similar vein, Qu et al. conduct a nationwide assessment of carbon emissions from municipal wastewater plants in China, identifying electricity and chemical use as the primary drivers of emissions. They suggest that operational improvements could reduce emissions by 12.6%, thereby supporting the sector’s decarbonization efforts.
On the renewable energy front, Guo et al. present an innovative biohybrid system leveraging solar energy to convert nitrate into ammonium with 95% selectivity and quantum efficiency, providing a sustainable alternative to the carbon-intensive Haber–Bosch process. This biohybrid technology exemplifies how renewable energy can play a transformative role in wastewater treatment. Xia et al. review emerging technologies that convert hydraulic energy into electricity, focusing on applications such as antifouling piezoelectric membranes, reactive oxygen species (ROS)-generating catalysts, and efficient sludge dewatering systems. These technologies hold significant potential for energy recovery in decentralized treatment systems, increasing operational efficiency and sustainability.

In the field of water treatment, Ao et al. develop a cetyltrimethylammonium bromide (CTAB)-modified montmorillonite material that achieves 90% perchlorate removal, showcasing the role of hydrophobic cavities and unconventional CH···O hydrogen bonding in ensuring selectivity. Their work offers an efficient solution to drinking-water safety concerns. Niu et al. introduce an electroactive biofiltration dynamic membrane (EBDM) that not only mitigates fouling but also increases methane production by 7.2%, while achieving 93% chemical oxygen demand (COD) removal with low energy input. Their work demonstrates how synergies between water treatment performance and energy recovery can lead to more efficient and sustainable systems. Liu et al. present a rapid sludge-drying technique that reduces moisture content from 80% to 15% in seconds using high-speed particle rotation and quartz sand. This method achieves 89% non-phase-change drying efficiency, offering a low-energy alternative to traditional thermal drying techniques.
Ahmad et al. use coupled long-range energy alternatives planning (LEAP) and water evaluation and planning (WEAP) models to evaluate resource tradeoffs; they find that integrated policies could result in a 15% reduction in energy consumption and a 30% reduction in water use by 2050. Their work underscores the importance of holistic planning in managing the water–energy nexus.

From these contributions, several key insights emerge: Integrated cross-sectoral models such as LEAP–WEAP enable more accurate resource planning and policy alignment; decentralized systems and resource recovery align with the principles of circular economy, offering both economic and environmental benefits; and understanding and navigating the tradeoffs between water-conservation technologies and energy use is essential for sustainable policy design.

As we look to the future, the scaling of emerging technologies such as biohybrids, EBDMs, and cyclone dryers should be prioritized in order to move them from pilot-scale to commercial deployment. Moreover, policy coordination across the water and energy sectors is essential to unlock synergies and maximize efficiency. Expanding the monitoring of non-traditional emission sources, such as CSOs and sludge drying, will also be critical in informing more comprehensive mitigation strategies.

The water–energy nexus is more than a technical intersection: It is a convergence point for innovation, policy reform, and sustainable development. The research featured in this special issue demonstrates how reimagining traditional systems, deploying cutting-edge technologies, and embracing integrated planning can lead to significant progress toward a more resilient, low-carbon future. We hope this collection will inspire further research, investment, and action to achieve a sustainable water–energy nexus. We express our sincere gratitude to the authors, reviewers, and editors whose expertise and dedication have made this special issue possible. Their contributions provide a rich resource for advancing the global dialogue on sustainable water and energy systems.

Cite this article: Jiuhui Qu, Gang Liu. A Focus on the Water–Energy Nexus for Sustainable Development, Engineering, Volume 50, 2025, Pages 1-2. https://doi.org/10.1016/j.eng.2025.06.013

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