Seasonal patterns and ecological risks of antibiotics in municipal wastewater treatment plants and a receiving river in Northeast China.

Antibiotic contamination in wastewater treatment plants (WWTPs) and receiving rivers is of increasing concern in cold-region urban aquatic environments. This study investigated 10 antibiotics in two municipal wastewater treatment plants and a shared receiving river in Changchun, Northeast China, over four seasons from July 2024 to April 2025. The plants comprised three treatment lines: anaerobic/anoxic/oxic treatment followed by clarification, filtration, and ozonation; anaerobic/anoxic/oxic treatment followed by clarification, cloth-media filtration, and ultraviolet disinfection; and an integrated anaerobic/anoxic/oxic membrane bioreactor. Samples were collected on three weekdays per season from influents, final effluents, and upstream and downstream river sites. Antibiotics were extracted by solid-phase extraction and quantified by ultra-high-performance liquid chromatography-tandem mass spectrometry. Concentrations ranged from not detected to 2873.03 ng/L in influents, from not detected to 941.59 ng/L in effluents, and from not detected to 93.25 ng/L in river water. Fluoroquinolones and macrolides were the dominant groups, and concentrations were generally higher in winter and lower in summer. Apparent overall removal of the eight consistently detected compounds ranged from 32.99% to 100% and was generally lower in winter and spring. The membrane-bioreactor treatment line showed comparatively higher apparent overall removal of azithromycin and roxithromycin in winter than the parallel conventional line. River concentrations increased near the outfalls and generally declined downstream. Risk-quotient screening identified azithromycin as the main ecological-risk contributor, reaching moderate to high risk in winter. These findings provide seasonally resolved information on antibiotic occurrence, treatment performance, and riverine distribution in a cold-region urban system.