Abstract Summary
The concept of a circular city envisions urban areas as natural systems where resources are continuously reused, contrasting with traditional cities that follow a "use and dispose" model. Circular cities aim to close material and energy loops, minimizing waste and maximizing efficiency through practices like local production, recycling, sustainable mobility, and renewable energy use. The Spanish hub of the UNITED CIRCLES project, based in Salamanca, embodies this vision by promoting the transition to a circular and sustainable economy. This initiative fosters collaboration between industries and communities to achieve zero net emissions by regenerating wastewater, valorizing sludge, and recovering high value bioproducts, thus closing material and energy cycles in urban environments. The hub comprises leading biowaste management companies and key industrial end-users, focusing on four pillars: urban and industrial wastewater reuse, sludge valorization into energy, safe drinking water production, and resource recovery. Innovative approaches for wastewater technologies, such as capacitive deionization coupled with forward osmosis and various membrane technologies (RO, NF, UF, MF), will be validated at a relevant scale (TRL 8). Similar methodologies will be applied to sludge valorization: co-digestion, hydrothermal liquefaction, tubular anaerobic digestion, and biological methanation will be also validated at TRL8. Removal of pathogens and contaminants of emerging concern (CECs) in drinking water will be addressed by upcycling activated carbon and implementing sensors for continuous microbiological control. Additionally, resources like cellulose and struvite will be recovered from urban wastewater for use in the pulp & paper and fertilizer industries, respectively. The outcomes of this project will not only benefit the environment but also drive innovation, create new employment opportunities, and strengthen community resilience. This work is part of the UNITED CIRCLES project, co-funded by the European Union under the Horizon Europe research and innovation programme, aiming to set a precedent for sustainable urban development and industrial biowaste management.
