The impact of Urban Waste Water Directive on European water quality: a scenario analysis

The Urban Waste Water Treatment Directive (91/271/EEC; UWWTD) set the framework for the development of the European wastewater treatment system. More than 25 years after its inception, the European Commission undertook a retrospective evaluation of the Directive as part of the Better Regulation initiative and the Regulatory Fitness and Performance Programme. Within the evaluation, this study assessed the reduction of nutrients and organic matter in European waters brought by the UWWTD. Politically relevant scenarios were defined with stakeholders, including present situation, the past (1990), future full implementation of UWWTD, and a ’what-if no Directive’ scenario. Further, we investigated the importance of individual and other appropriate systems (IAS) to collect and treat parts of agglomeration waste where connections to sewer networks are not economically justified. Loads and concentrations of 5-days Biochemical Oxygen Demand, total nitrogen, and total phosphorus under the scenarios were quantified with European scale conceptual models. Coherence of approaches with regard to policy questions was evaluated through stakeholder workshops and regular conversations with policy-makers, allowing adapting models and data structure to the scope of the evaluation. Model transparency and reliability were ensured via scientific peer-review publication process. Results quantify the significant reduction of wastewater emissions of pollutants to surface waters achieved with implementation of the UWWTD. IAS treat only a small part of the EU population equivalents, their impact at the EU scale is comparatively small. Implementing wastewater treatment to full compliance with UWWTD would further bring important reductions of pollutant loads and these are comparable with other sources of pollution, namely urban runoff and combined sewer overflows. Both of these sources are not specifically addressed by the Directive, which contains only general principles regarding diffuse urban pollution and stormwater. Finally, while reducing urban loads is important, reducing loads of organic matter and nutrients to receiving water bodies requires addressing also agricultural sources. Defining a common glossary and setting clear objectives at the onset of the work and continuous exchange among all involved helped establishing a constructive dialogue between policy and science that drove the analysis to a successful conclusion and posed the basis for supporting further policy development.