Hybrid green wall - ozonation system for greywater and rainwater reclamation in a residential building: technical performance and social insights

Growing water scarcity calls for sustainable and decentralized strategies to reduce demand for freshwater resources. Greywater reuse is a promising option, though concerns about pathogens and organic micropollutants (OMPs) necessitate effective treatment. This study evaluates a full-scale system integrating a green wall with ozonation, installed on a residential building in Sant Quirze del Vallès (Barcelona, Spain), for greywater and rainwater treatment and reuse in toilet flushing. Over one year, the green wall achieved high removal of COD (84%), BOD 5 (96%), TOC (87%), TSS (94%), and ammonium (>99%), with ozonation providing additional polishing and ensuring microbial safety ( E. coli below the detection limit). Several OMPs, including caffeine, benzotriazole, and tramadol, were effectively removed by the green wall, while ozonation addressed more persistent compounds such as DEET or valsartan. The treated effluent consistently met Spanish quality standards for non-potable reuse. Social acceptance was assessed through resident surveys and stakeholder interviews. While participants acknowledged the environmental value of water reuse, acceptance was influenced by practical concerns (odour, noise or water colour) and trust in the managing entity. Findings highlight the need to complement strong technical performance with effective communication and reliable maintenance to foster long-term adoption of such nature-based and hybrid solutions.