Multi-criteria risk assessment for prioritising contaminants of emerging concern in treated wastewater and surface waters

Contaminants of emerging concern (CECs) pose complex and cumulative risks to aquatic ecosystems due to their persistence, toxicity, bioaccumulation potential and co-occurrence as chemical mixtures. This study applies a multi-criteria risk assessment framework to prioritise CECs in treated wastewater and receiving surface waters by integrating three complementary methodologies: Occurrence-Persistence-Bioaccumulation-Toxicity, deterministic Risk Quotients and Toxic Units. The framework harmonises hazard- and effect-based indicators through a normalised ranking scheme and explicitly incorporates mixture toxicity using concentration addition metrics. The approach was demonstrated as a proof of concept on a real-world dataset derived from wide-spectrum chemical screening of treated effluent from a decentralised wastewater treatment plant, where 66 CECs were detected. Mixture risk assessment revealed that cumulative risks exceeded accepted thresholds by more than one order of magnitude, despite most individual compounds exhibiting low or moderate risk when assessed separately. A small number of substances dominated overall mixture toxicity, while a substantial fraction of cumulative risk would have been overlooked under single-compound evaluations. Compounds such as galaxolidone, meclofenamic acid, fipronil, clomipramine, niflumic acid and diclofenac consistently ranked high across hazard-, risk and mixture-based metrics. The study further highlights the strong dependence of prioritisation outcomes on analytical coverage and site-specific conditions.