Risk-based prioritisation of WWTPs for quaternary treatment: a practical workflow informed by Swedish experience

The recently revised European Urban Wastewater Treatment Directive (Directive (EU) 2024/3019) requires Member States to identify where organic micropollutants (OMPs) pose risks and to prioritize wastewater treatment plants (WWTPs) for advanced (quaternary) treatment. Micropollutant-related obligations are thereafter phased in over time. In this perspective, we propose a cost-effective three-phase workflow to prioritize WWTPs for quaternary treatment, grounded in two decades of Swedish experience from monitoring, environmental risk assessments, pilot trials, and full-scale implementation. We argue that harmonized minimum data requirements and a structured phased approach are essential to implement risk-based prioritization efficiently. Phase 1 screens WWTP-recipients using harmonized chemical and system data to model predicted environmental concentrations (PEC) and compare them with predicted no-effect concentrations (PNEC) and environmental quality standards (EQS). Phase 2 validates identified risks through targeted chemical analysis and, where relevant, effect-based monitoring to capture mixture and seasonal variability. Phase 3 supports implementation and follow-up of quaternary treatment focusing on high-risk OMPs and performance verification. Our intention is that the proposed workflow will help authorities and utilities allocate investments where environmental benefits are largest relative to costs and to support harmonized implementation across Sweden and the EU.