Wastewater treatment plant effluents as an obstacle to the full recovery of restored river sections

Freshwater ecosystems are impacted by multiple human activities, with chemical pollution emerging as a significant concern alongside well-documented stressors like climate change. While river restorations have been conducted to improve the structural condition of rivers, their ecological efficacy proved to be limited and potentially masked by chemical pollution. Conventional wastewater treatment plants (WWTPs) are unable to fully eliminate pollutants, resulting in insufficient water quality in aquatic systems, particularly in small, sensitive rivers with low dilution capacities, posing growing risks to biodiversity. This study investigated the ecotoxicological impact of WWTP effluents on downstream restored river sections. Despite the implementation of restoration measures, these river sections remained in a degraded ecological condition, likely due to the presence of chemical pollution originating from upstream WWTPs. In vitro bioassays were used to assess baseline toxicity, estrogenicity, and dioxin-like activity in water and sediment samples. Due to the complex pollution pattern, it was not possible to attribute the observed effects to the WWTPs alone. However, the results indicate that WWTP discharges act as a stressor on a larger spatial scale and may hinder ecological recovery in restored sections by prevailing toxicity levels. WWTP effluents, along with pollution from e.g. agricultural runoff, are likely obstacles to improving aquatic biodiversity. To support biodiversity recovery, integrated management approaches are necessary, focusing on reducing the impact of WWTP discharges through advanced treatment technologies alongside hydromorphological restoration efforts.