Greywater
Girona, Spain
Key facts
Environmental &
Socio-economic impacts
Girona is currently involved in a transformation process towards a more resilient, livable, and sustainable city. The MULTISOURCE Environmental and New Technology Solutions pilot in Spain will provide new ways for cities and communities to achieve improved resource recovery and reuse. Such “green wall” technologies can be implemented in different types of buildings, on empty vertical spaces, in touristic, private residences, and commercial/public/industrial facilities. Therefore, Nature-Based Solutions for Water Treatment have great potential to diversify current business models of private companies and to facilitate the transition of public water companies towards more circular and decentralized water urban management systems.
Main Activities
The project focuses on two green walls for greywater treatment. One, nearing completion on the ICRA building, will undergo five months of performance monitoring. The second, at Agora School in Girona, will treat greywater for garden reuse and serve as an environmental education tool.
A third green wall in Sant Quirze del Vallès has been monitored for greywater and rainwater treatment to be reused for toilet flushing.  The evaluation covered treatment efficiency, co-benefits (e.g., stakeholder engagement, green areas, biodiversity, carbon sequestration), and operating costs, providing valuable insights into its economic feasibility.
This work underscores the technical, environmental, and economic aspects of scaling up decentralized greywater treatment using green walls.
Results
The green walls showed high efficiency in greywater treatment, reducing contaminants like suspended solids, COD, and BOD5 by up to 90%, making the water suitable for non-potable uses in line with Spanish regulations. They also removed microbiological contaminants, such as E. coli, and reduced organic micropollutants by 60% to 99%, outperforming traditional methods.
The systems offered co-benefits like increased stakeholder engagement, expanded green spaces (+222.6%), biodiversity support, and carbon sequestration (505 gCO₂/m² annually). Economic viability depends on system scale, with larger setups becoming more cost-effective, especially with rising water prices.
The Sant Quirze del Vallès green wall confirmed strong long-term performance, meeting legal limits with low maintenance, making it a viable option for decentralized urban water management.
References
Da Cunha, J.A.C., Arias, C.A., Carvalho, P., Rysulova, M., Canals, J.M., Pérez, G., Gonzale Z, M.B., Morató, J.F., 2018. “WETWALL” — an innovative design concept for the treatment of wastewater at an urban scale. Desalination Water Treat. 109, 205–220. https://doi.org/10.5004/dwt.2018.22143
Impact generated
The development and evaluation of green walls for greywater treatment have shown significant environmental, educational, and social benefits. The first green wall, installed in January 2025 on the ICRA building, treats greywater and has been monitored for five months to optimize treatment and inform future projects.
The second green wall at Agora School in Girona will support sustainable greywater reuse for irrigation and serves as an educational tool, raising student awareness about sustainable water management.
The third green wall in Sant Quirze del Vallès has demonstrated effective greywater and rainwater treatment for toilet flushing, with valuable insights from monitoring. Co-benefits such as increased stakeholder engagement, green space expansion, biodiversity enhancement, and carbon sequestration highlight its broader advantages.
The comprehensive assessment of treatment performance, environmental benefits, and cost analysis has highlighted the potential for large-scale implementation of decentralized green infrastructure. These results offer a solid foundation for promoting the widespread adoption of green walls as a sustainable and cost-effective solution, when considering social and environmental benefits for urban water management.
Partners involved
