Occurrence and Emission Evaluation of Oxypurinol and Metformin from Effluent in WWTPs across Australia

Oxypurinol is a metabolite of allopurinol, a drug used to treat gout, and metformin is a first-line treatment for type 2 diabetes. Both oxypurinol and metformin enter the wastewater treatment plants (WWTPs) in high loads, but information about their potential environmental risks remains scarce. This study systematically investigated the occurrence, emission, removal rate, and environmental risks of oxypurinol and metformin in effluents from 88 and 65 WWTPs across Australia in 2016 and 2021, respectively. Oxypurinol and metformin were frequently detected in effluents (>90%), with concentrations reaching up to 233 μg/L. The estimated per capita emissions of oxypurinol were 12 and 15 mg·day^-1·capita^-1, and those of metformin were 2.7 and 2.0 mg·day^-1·capita^-1 in 2016 and 2021, respectively. Both compounds were detected in effluents at concentrations up to 166 times above their respective PNECs. On average, dilution factors of 3.5 and 10 for metformin and 34 and 405 for oxypurinol are required for being released into river and ocean environments, respectively. During wastewater treatment, oxypurinol exhibited high persistence, with 54% of the WWTP sites showing no observed removal, whereas metformin showed an average removal rate of 88%. These findings provide valuable evidence of oxypurinol and metformin emissions and their environmental risk in Australia.