Disentangling micropollutant toxicity from major ion effects in treated water effluents: Advancing invertebrate biotests for enhanced ecotoxicological assessment.

Water treatment systems generate effluents with varying compositions of major geochemical ions and diverse micropollutants. Well-established procedures exist to assess the toxicity of effluents as a complete and unique matrix. Although informative, these approaches may fail to identify the toxicity specifically attributable to micropollutants. This review provides a bibliographic overview of the physicochemical and ecotoxicological characterization of 181 effluents reported across 88 publications. It highlights a general lack of information on effluent ionic composition while assessments are predominantly conducted using Daphnidae (e.g. CerioDaphnia dubia, Daphnia magna) as model organisms. By comparing toxicity thresholds of major geochemical ions for these species with the ionic composition of the consolidated effluents, Daphnidae appear to be adversely impacted by the presence of these ions alone. To disentangle micropollutant toxicity from major geochemical ion effects, a bioassay strategy is proposed that consists in 1) selecting species tolerant to high concentrations of major geochemical ions, and 2) employing control water with major-ion concentrations similar to those of the assessed effluents.