Circular economy in the context of water smart industrial symbioses (project ULTIMATE)

The Green Deal aims to make Europe climate-neutral by 2050.The circular economy is an important step towards achieving this goal.The European Horizon 2020 ULTIMATE project promoted circular economy concepts within the framework of water-smart industrial symbiosis.The project focused on industrial wastewater streams as a source of water, energy and materials.Through nine case studies, ULTIMATE developed and demonstrated 24 technologies related to the circular economy.Within this framework, ULTIMATE promoted, established and extended water-smart industrial symbioses (WSIS) between the industrial sector and water service providers.Industrial symbiosis involves the collaboration of industrial facilities or companies, whereby one company's waste or by-products become another company's raw materialscreating a winwin situation for both partners.In WSIS, water and wastewater play a key role as reusable resources and as a means of extracting, treating, storing and reusing energy and materials.This special issue of Circular Economy in the Context of Water Smart Industrial Symbioses (Project ULTIMATE) comprises 10 papers.The basis for implementing circular economy concepts in WSIS is stakeholder engagement.Perkis et al.(2025) presented the crucial role of stakeholder engagement in the successful adoption and acceptance of symbiotic technology and strategy solutions at the nine ULTIMATE WSIS.They focused exemplarily on three approaches in particular: extended reality technologies for immersive media experiences, communities of practices and water-oriented living laboratories.These approaches created a framework for fostering collaboration, innovation, and sustainable practices in industrial symbiosis.Stakeholder engagement is closely related to the decision-making process, which is also crucial for implementing symbiotic concepts.Khoury et al. (2025) developed a web-based decision support tool for industrial symbioses that applies circular economy principles.The online platform helps decision-makers explore different industrial symbiosis configurations to find the optimal metabolic pathway solution, and can also be used as a teaching tool.This work is also in line with the quest for digital transition, where digital technologies are used to achieve the EU's Green Deal goals of climate neutrality and environmental sustainability.WSIS requires collaboration among multiple stakeholders working towards a shared goal, which can pose challenges in achieving mutually beneficial outcomes.Chen et al. (2025) proposed a hybrid modelling framework to support decisionmaking, incorporating stakeholder behaviour, operational processes and dynamic interactions within the WSIS, as well as stakeholder perspectives.The implementation of this framework was demonstrated using an Israeli case study involving the treatment of olive mill wastewater.This is another work presenting digital solutions linked to stakeholder engagement.In terms of technological development and demonstration, ULTIMATE focused on technologies for recovering water, materials and energy from the wastewater of various industrial sectors.In the Dutch case study, for example, the greenhouse sector played a significant part.To facilitate the reuse of greenhouse wastewater, Guleria et al. (2024) presented advancements in cation exchange membranes for the selective separation of Na /K using electrodialysis systems.The study provides recommendations for improving Na /K selective cation exchange membranes and machine learning-driven optimisation strategies.In the Danish case study, Jhrig et al.