Ecotoxicological risk and seasonal removal of licit and illicit drugs in wastewater treatment plants: A comparison of secondary and tertiary processes

This study evaluates the effectiveness of different wastewater treatment methods in removing emerging contaminants, including licit and illicit drugs. 168 samples were collected over four seasons from three wastewater treatment plants (WWTPs), comparing conventional secondary biological treatments with advanced oxidation processes (ozone and ozone-UV). Hydrophilic compounds like caffeine and ethyl sulfate showed high removal efficiencies (>85 %), while compounds such as MDMA and methamphetamine had lower and more variable removal rates. No consistent seasonal trend was found for most substances, although some exhibited improved removal in warmer months, likely due to enhanced microbial and photodegradation. Ecotoxicological risk was assessed using toxic units (TU_chem), calculated from effluent concentrations and EC₅₀/LC₅₀ values for algae, daphnia, and fish. WWTPs with tertiary treatments generally showed lower TU_chem values, indicating a potential reduction in environmental risk. Algae were the most sensitive organisms. The combined ozone and UV treatment achieved the lowest TU_chem values, reflecting lower residual concentrations of the measured compounds. These results reveal limitations of conventional secondary treatments and emphasize the need to optimize tertiary processes. The observed compound-specific variability highlights the importance of regular monitoring and prioritization to improve the ecological safety of treated wastewater discharged into aquatic environments.