Coupling of UV/H₂O₂ and biological treatment for the removal of the pharmaceuticals metoprolol and metoprolol acid from hospital wastewater

Advanced oxidation processes (AOPs) are considered efficient tools for the elimination effluents containing recalcitrant compounds. Therefore, AOPs can be used as a tertiary treatment to remove those compounds not eliminated during conventional wastewater treatment plants (WWTPs). AOPs can also be used as a pre-treatment step to degrade the initial persistent contaminants and further coupled to a secondary biological treatment for a more efficient biodegradation. During these combined processes, the transformation products generated may actually be more toxic or persistent that the initial ones. Therefore, monitoring the presence of these substances along the treatments is necessary. Among the pharmaceuticals from the group of β-blockers, metoprolol has been detected in wastewater in the range 160-2000 ng/l. Moreover, its main human metabolite, metoprolol acid, has been found at much higher concentration and shown to be a recalcitrant in certain biological treatment processes, which highlights its potential environmental relevance. In this study, a comprehensive overview of the elimination of metoprolol and metoprolol acid was performed coupling fungal (FG) and activated sludge (CAS) treatments with UV/H2O2 processes. Two different combinations were tested including UV/H2O2 as pre-treatment and post-treatment step of hospital wastewater. The extent on compound transformation was evaluated via a suspect screening methodology using a liquid chromatography system coupled to a high resolution mass spectrometer (LC-LTQ-Orbitrap-MS/MS). While the different combinations attained high elimination rates of target pollutants, the extent on compound transformation differed in large proportion depending on the set-up and the matrix: higher abundance of persistent intermediates was found at the end of the experiments when CAS+AOP, FG+AOP and AOP+FG combinations were performed compared to AOP + CAS. Results reveal how combined treatments may represent a feasible solution to improve total pollutant removal when treating complex wastewater matrices. Comprehensive studies of the transformation products generated are highly recommended when evaluation of combined treatments, since the target analysis of parent compound did not provide complete information to draw conclusions of the best treatment performance to use.