Assessing drinking water treatment efficiency using passive sampling and cell-based bioassays

Introduction: Drinking water treatment plants (DWTPs) are designed to protect public health; however, residual contaminants may persist after treatment and elicit biological effects that are not fully covered by routine chemical monitoring. Methods: In this study, we combined Polar Organic Chemical Integrative Samplers (POCIS) with in vitro bioassays to evaluate residual biological activity in raw and treated water from six full-scale DWTPs in the Czech Republic. POCIS were deployed at raw- and treated-water points to collect extracts representing mixtures of polar and semi-polar contaminants under realistic exposure conditions. These extracts were evaluated using transcriptional responses in the RTL-W1 rainbow trout cell line and receptor-mediated yeast bioassays specific for estrogenic and progestogenic activity. Results: Among the evaluated biomarkers, gene expression of CYP1A was the most strongly and reliably induced, indicating the presence of AhR-active substances in raw water and, in several cases, incomplete removal of these substances during treatment. In contrast, genes related to phase II detoxification, cell stress, and oxidative stress (GST, HSP, and Nrf2) responded weakly, suggesting a predominantly receptor-mediated mechanism of action rather than generalized cytotoxicity. Estrogenic activity was detected in all raw waters but was below detection limits in all treated waters, indicating efficient removal of estrogenic substances during treatment. Progestogenic activity was not detectable. Discussion: This study highlights the importance of effect-directed analysis for assessing the efficiency of drinking water treatment processes and confirms the suitability of passive sampling combined with bioassays for identifying treatment-resistant bioactivities.