Granule size-dependent biotransformation of organic micropollutants in aerobic granular sludge under different bioconversion conditions

Organic micropollutants (OMPs) are commonly detected in municipal wastewater. Conventional activated sludge processes partially remove these compounds, allowing them to enter receiving waters and pose ecological risks. Biotransformation, governed by microbial community composition and activity, is the main pathway for OMP removal. Aerobic granular sludge (AGS), with its distinct structure and microbial communities compared to conventional activated sludge, has emerged as a promising alternative. Full-scale AGS reactors contain predominately large granules (>1 mm), alongside medium (0.2-1 mm), and small (<0.2 mm) fractions, which differ in morphology and microbial composition and may influence OMP biotransformation. To date, the potential of different AGS size fractions for OMP biotransformation at environmentally relevant concentrations (1 µg L^-1) remains poorly understood. This study evaluated the biotransformation of 23 OMPs (pharmaceuticals and industrial compounds) under nitrifying, aerobic heterotrophic, and denitrifying conditions, using batch microcosm with six AGS size fractions collected from a full-scale AGS plant. Eight OMPs (sulfamethoxazole, atenolol, furosemide, benzotriazole, trimethoprim, diclofenac, metoprolol, and gabapentin) showed biotransformation efficiencies above 10 % under at least one condition. Under aerobic conditions, smaller fractions showed higher biotransformation rate (K_bio), reflecting increased nitrifier and aerobic heterotroph activity. Under denitrifying conditions, three OMPs were biotransformed > 10 %, but K_bio did not correlate clearly with denitrifying activity, likely due to heterogeneous denitrifier distribution across size fractions. At the system level, AGS showed slightly lower K_bio than activated sludge, as smaller, more active AGS fractions comprised less than 40 % of total biomass in full-scale reactors. This study is the first to assess OMP biotransformation across AGS size fractions, highlighting the combined effects of granule size and bioconversion conditions. The findings provide insights for optimizing AGS systems, including potential granule size adjustments, to enhance OMP biotransformation and reduce environmental impacts.