Removal of acetaminophen and total nitrogen from secondary effluent via a UV/PAA-biofilter process: Efficiency, biofilm characteristics, and reaction mechanism.

Simultaneous removal of pharmaceutical contaminants and excess nitrogen from secondary effluent remains a critical challenge for wastewater reuse. In this study, the UV/peracetic acid (UV/PAA) process achieved 99% degradation of acetaminophen (ACE) within 30 min under ultrapure water conditions, primarily due to the synergistic effects of hydroxyl radicals (•OH) and other reactive species. Twelve intermediates and three degradation pathways were identified by liquid chromatograph mass spectrometer (LC-MS) and density functional theory (DFT). During the degradation of ACE, the formation of several mildly toxic intermediates led to a slight increase in overall ecotoxicity. The UV/PAA-biofilter process achieved >95% ACE removal from secondary effluent, with contributions of 87.44% from UV/PAA and 9.85% from the biofilter. TN decreased from 35.75 to 7.03 mg/L (80.34% removal). Biological treatment effectively reduced acute toxicity, while EPS exhibited spatial variations along the biofilter. Genus-level microbial communities also varied significantly with filter height. Overall, the UV/PAA-biofilter process demonstrates high feasibility for simultaneous ACE and Total nitrogen (TN) removal from secondary effluent, offering valuable insights for optimizing advanced wastewater treatment systems.