This study evaluated the performance of a side-stream ceramic nanofiltration membrane bioreactor (NF-MBR) system with respect to basic water quality parameters as well as trace organic contaminant (TrOC) removal efficiency. The results show a stable biological performance of the continuous NF-MBR system with high effluent quality (total organic carbon < 4 mg L-1 and NH4 +-N below the detection limit). Significantly higher performance by this NF-MBR in comparison to the conventional microfiltration/ultrafiltration MBR regarding the removal of a large number of TrOCs was observed. TrOC removal efficiency depended on their hydrophobicity and molecular features. All hydrophobic compounds (LogD pH=6 > 3) were well removed (>85%), except diazinon (59 ± 7%). Hydrophilic compounds containing electron donating groups were also well removed (>90%). By contrast, hydrophilic compounds containing electron withdrawing groups were poorly removed (8-54%). Most of the 40 TrOCs investigated in this study did not accumulate in the sludge. Only three hydrophobic compounds, namely amitriptyline, triclosan and triclocarban showed considerable accumulation in sludge (>500 ng g-1). Mass balance indicated biodegradation/transformation as the most significant TrOC removal mechanism by this NF-MBR.