Ozonation of Pharmaceuticals and Their Human Metabolites in Wastewater─Insights from Laboratory Experiments and Field Data

Pharmaceuticals and their human metabolites are contaminants of emerging concern in aquatic environments. While monitoring usually targets parent compounds, metabolites, often excreted at higher loads, are largely overlooked. This study investigates the behavior of both during wastewater treatment, focusing on ozonation as an advanced treatment step. Second-order rate constants for reactions of 87 parent compounds and 130 metabolites with ozone were determined using a multicompound competition kinetics method, enabling exploration of functional group-specific reactivity trends. Aromatic hydroxylation generally increased ozone reactivity (up to 5 orders of magnitude), whereas <i>N</i>-oxides (up to 5 orders of magnitude) and <i>N</i>-dealkylated metabolites (up to 2 orders of magnitude) showed reduced reactivity compared to their parents. These constants, combined with predicted second-order rate constants for the reaction with hydroxyl radicals, as well as estimated ozone exposures and experimentally determined hydroxyl radical exposures, were used to model the abatement in three Swiss wastewater treatment plants. Modeled and observed abatement agreed well. A derivative-based sensitivity analysis highlighted ozone exposure as most crucial for moderately to highly ozone-reactive compounds, whereas hydroxyl radical exposure dominated for ozone-resistant compounds. This study emphasizes the importance of considering both parent compounds and metabolites in wastewater treatment processes to address contaminants holistically.