Ozone dose dependent formation and removal of ozonation products of pharmaceuticals in pilot and full-scale municipal wastewater treatment plants

The removal of micropollutants from municipal wastewater is challenged by the number of compounds with diverse physico-chemical properties. Ozonation is increasingly used to remove micropollutants from wastewater. However, ozonation does not necessarily result in complete mineralization of the organic micropollutants but rather transforms them into new compounds which could be persistent or have adverse environmental effects. To explore ozone dose dependency of the formation and successive removal of ozonation products, two pilot-scale and one full-scale ozonation plants were operated subsequent to a conventional activated sludge treatment. The results from these trials indicated that the concentrations of several N-oxides, such as Erythromycin N-oxide, Venlafaxine N-oxide and Tramadol N-oxide, increased up to an ozone dose of 0.56–0.61 mg O₃/mg DOC while they decreased at elevated doses of 0.7–1.0 mg O₃/mg DOC. Similar results were also obtained for two transformation products of Diclofenac (Diclofenac 2,5-quinone imine and 1-(2,6-dichlorophenyl)indolin-2,3-dione) and one transformation product of Carbamazepine (1-(2-benzoic acid)-(1H,3H)-quinazoline-2,4-dione), where the highest concentrations appeared around 0.27–0.31 mg O₃/mg DOC. The formation maximum of a given compound occurred at a specific ozone dose that is characteristic for each compound, but seemed to be independent of the wastewater used for the experiments at the two pilots and the full-scale plant.