Toxic risk assessment of multi-endpoint toxicity and toxicity transformation mechanisms during UV-combined advanced treatments in wastewater treatment plants: Development of effect-based trigger values, in vitro bioassays, and FT-ICR MS analysis.

The ultraviolet (UV) process is recognized as an environmentally friendly treatment, typically producing fewer byproducts compared to conventional chemical oxidation methods. However, research on the mechanisms underlying the removal of toxic effects by UV and UV-based combined processes during wastewater treatment remains insufficient. In this study, effect-based trigger values (EBTs) for acute toxicity, genotoxicity, and estrogen receptor (ER) agonist activity were derived and subsequently applied to assess three categories of toxicity variations in both full-scale wastewater treatment plants (WWTPs) and pilot-scale systems. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was further employed to elucidate the mechanisms underlying these toxicity changes. EBTs for acute toxicity, genotoxicity, and estrogen receptor agonist activity were determined to be 1.695 mg Phenol EQ/L, 57.15 ng 4-NQO EQ/L, and 0.5 ng EEQ/L, respectively. The results reveal a significant reduction in toxic effects downstream of the receiving water, with levels remaining below the corresponding EBTs. However, the sulfur autotrophic and UV process increase acute toxicity and genotoxicity, respectively. Pilot-scale experiments demonstrated that standalone UV treatment exhibited limited efficacy in toxicity removal (0-29.0 %), whereas ozonation achieved significantly higher removal rates (59.3-76.8 %). Additionally, the ultraviolet/sodium sulfite followed by ozonation (UV/NaSO-O) process exhibited the highest removal efficiency. Analysis of FT-ICR MS results revealed that the observed toxicity changes were likely attributed to the UV/NaSO process significantly removing chlorinated compounds while generating more small molecular species, thereby simplifying subsequent treatment processes. This study demonstrates the feasibility of applying EBTs for water quality assessment. Notably, while UV processes pose genotoxicity risks that cannot be ignored, the UV/NaSO-O emerges as an effective technology for mitigating toxicity risks.