Metformin removal and biodegradation mechanisms in partial nitrification aerobic granular sludge

The widespread consumption of pharmaceuticals has led to their increasing occurrence in aquatic environments, posing potential risks to ecosystems and public health. Conventional wastewater treatment processes exhibit limited removal efficiencies for most pharmaceuticals, resulting in their persistence in wastewater effluents and sludge. In particular, pharmaceuticals retained in sludge can be reintroduced into receiving waters through sidestream wastewater, representing an important yet insufficiently studied migration pathway. In this study, metformin (MET), a widely used antidiabetic pharmaceutical, was selected as a model compound to investigate its removal and degradation mechanisms in sidestream partial nitrification aerobic granular sludge (PN-AGS). Granular sludge enriched for ammonia-oxidizing bacteria (AOB) was employed as the inoculum, and the degradation performance of MET was systematically evaluated under controlled temperature and pH conditions. The results elucidated the degradation pathways of MET and revealed a strong coupling between ammonia oxidation and pharmaceutical removal. This study provides mechanistic insights and theoretical support for the simultaneous removal of MET and ammonium nitrogen in sidestream wastewater treatment processes.