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 (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. • Aerobic granule sludge (AGS) sizes affect micropollutant (OMP) biotransformation • Aerobic OMP biotransformation was AGS-size dependent, faster in smaller fractions • K bio values positively correlate with nitrifiers and aerobic heterotroph activity • OMP biotransformation is lower under anoxic denitrification than aerobic conditions • Eight OMPs were biotransformed in at least one of the bioconversion conditions