Integrating multicriteria analysis and economic valuation to assess nature-based water reuse systems (LIFE Renaturwat).

The new Urban Wastewater Treatment Directive (EU) 2024/3019 introduces a substantially more stringent regulatory framework by extending its scope to all urban agglomerations of at least 1000 population equivalents, in response to evidence showing that small agglomerations contribute significantly to pressure on approximately 11% of the European Union's surface water bodies-a source of pollution not fully addressed by previous regulations. Against this background, this study assesses a nature-based treatment upgrade implemented at two wastewater treatment plants (WWTPs) in the province of Valencia (Spain): Carrícola and Los Monasterios. In both plants, the secondary effluent is further treated in constructed wetlands filled with aluminium-rich drinking water treatment sludge, enhancing phosphorus removal while integrating circular economy principles into the urban water cycle. To capture and monetise the resulting environmental and social benefits, the AMUVAM-ANP framework was applied. The framework combines a panel of 12 multidisciplinary experts, who assess the interdependencies among ecosystem services, with an economic valuation that monetises non-market benefits using a pivot value of €1.09/m3, corresponding to the market price of drinking water supply in Spain. Water availability emerged as the dominant service, accounting for 33.27% of total relevance. By category, overall relevance was distributed across provisioning (42.23%), fauna and flora (25.84%), cultural (22.54%), and regulating services (9.40%). Over a 25-year horizon, the systems generate substantial economic benefits, estimated at approximately €242,200 for Carrícola and €880,726 for Los Monasterios, with benefit-cost ratios of 6.6-7.5 that remain above unity even under conservative scenarios. By making benefits such as biodiversity enhancement and climate mitigation explicit, the approach provides a transparent and replicable economic basis for decision-making and supports the scalability of sustainable and resilient wastewater treatment models for small populations facing water stress.