The goal of this study is to investigate the occurrence and removal of emerging organic contaminants (EOCs) during wastewater treatment processes and understand the role of enhanced nitrification treatment in removing EOCs. The influent and effluent of each treatment step at two local wastewater treatment plants (WWTPs) were analyzed by high-performance liquid chromatography coupled with high-resolution mass spectrometry. A suspect screening method was applied with a comprehensive self-compiled suspect list. The list is composed of 1225 EOCs in 5 categories: pharmaceuticals, personal care products, pesticides, industrial chemicals, and metabolites. A total of 292–341 suspect hits were retrieved. The structures of 56 out of 86 selected suspect hits were further validated. The estimated influent concentrations of different EOCs varied from several ng L−1 to less than a hundred μg L−1. Paracetamol, caffeine, benzotriazole, methylbenzotriazole, DEET, gabapentin, metformin, N4-acetylsulfamethoxazole, atenolol acid, triethanolamine, and triethylphosphate were detected in the influent with high abundance. The results also revealed that many EOCs, such as fexofenadine, citalopram, flecainide, had low removal efficiencies. Seasonal effects were larger than the location influence on EOC removal. The secondary biological treatment played the most crucial role in EOC removal, whereas other steps except the enhanced nitrification step had a minor contribution. The enhanced nitrification step substantially removed a small number of EOC suspects. Notably, a reverse transformation from product to parent was also observed. This study provides systematic information on the occurrence and removal of EOCs in WWTPs and first important insights into the roles of the enhanced nitrification step in wastewater treatment.
