Winter Is the Worst Season for Antibiotics Pouring Out of Cold-City Sewage Plants

In the frozen cities of Northeast China, where winter temperatures plunge far below zero and rivers run beneath lids of ice, a quiet chemical problem intensifies with the cold. A new year-long study from Changchun, the capital of Jilin Province, has tracked ten antibiotics through two municipal wastewater treatment plants and the river that receives their discharges, and the results reveal a striking seasonal rhythm: antibiotic concentrations climb as temperatures fall, treatment efficiency sags in winter and spring, and one widely used macrolide drug, azithromycin, emerges as the dominant ecological threat in the coldest months of the year.

The research, conducted by a team at Jilin Jianzhu University’s Key Laboratory of Songliao Aquatic Environment and published in Environmental Monitoring and Assessment, is one of the most seasonally resolved portraits yet of antibiotic pollution in a cold-region urban water system. Between July 2024 and April 2025, the investigators sampled three weekdays in each of the four seasons, collecting water from the influents and final effluents of two wastewater treatment plants as well as from sites upstream and downstream of their shared receiving river. The design allowed them to follow the same molecules from the sewer, through the treatment train, and into the aquatic environment across an entire annual cycle.

The analytical workflow was deliberately rigorous. Antibiotics were extracted from water samples using solid-phase extraction, a technique that concentrates trace organic compounds onto a sorbent cartridge before they are eluted for analysis. Quantification was then performed by ultra-high-performance liquid chromatography coupled with tandem mass spectrometry, an approach sensitive enough to detect individual pharmaceutical compounds at concentrations of nanograms per liter, equivalent to parts per trillion. This combination of concentration and detection methods is now the standard for pharmaceutical monitoring, because the environmental levels involved are far too low for conventional chemical assays yet high enough to exert biological effects on microorganisms.

What the measurements revealed was a steep concentration gradient across the urban water cycle. In the raw sewage entering the plants, antibiotic concentrations ranged from undetectable to a remarkable 2,873.03 nanograms per liter. After treatment, effluent concentrations fell to between not detected and 941.59 nanograms per liter, and in the receiving river itself the measured levels ranged from not detected up to 93.25 nanograms per liter. Two chemical families dominated the profile: the fluoroquinolones, a class of broad-spectrum synthetic antibiotics that includes drugs such as ciprofloxacin and norfloxacin, and the macrolides, which include azithromycin and roxithromycin. Both classes are heavily prescribed in China and are known to persist in aquatic environments because they resist both biodegradation and sunlight-driven photolysis.

The seasonal signal was unmistakable. Concentrations were generally higher in winter and lower in summer, a pattern the authors link to several interacting factors. In cold climates, winter brings higher rates of respiratory infection and correspondingly greater antibiotic consumption, which flushes more of these compounds into the sewer system. At the same time, the biological processes inside treatment plants slow down dramatically as temperatures drop, because the microbial communities that break down organic pollutants, including pharmaceuticals, become far less active in cold wastewater. Reduced dilution in frozen, low-flow rivers compounds the problem, since the same mass of discharged antibiotic ends up in a smaller volume of receiving water.

Treatment performance told the same seasonal story from the other direction. For the eight antibiotics that were consistently detected, the apparent overall removal efficiency across the plants ranged from 32.99 percent to complete elimination, but removal was generally lower in winter and spring. This matters because conventional activated-sludge treatment was never designed with pharmaceuticals in mind. Antibiotics leave wastewater plants through three main pathways: biodegradation by the microbial community, sorption onto sludge particles that are settled out of the water, and abiotic processes such as hydrolysis. When water temperatures fall, all three mechanisms weaken, and a larger fraction of the incoming drug load simply passes through the plant and into the river.

One of the most practically interesting findings concerns the comparison between treatment technologies. The two plants in the study operated three distinct treatment lines: a conventional anaerobic-anoxic-oxic configuration followed by clarification, filtration, and ozonation; a similar biological process followed by clarification, cloth-media filtration, and ultraviolet disinfection; and an integrated anaerobic-anoxic-oxic membrane bioreactor, in which fine membranes replace secondary clarifiers and allow a much higher concentration of slow-growing microorganisms to be maintained in the bioreactor. During winter, the membrane bioreactor line achieved comparatively higher apparent removal of the macrolides azithromycin and roxithromycin than the parallel conventional line. The likely explanation lies in the longer solids retention times and denser microbial populations that membrane systems sustain, giving cold-slowed bacteria more opportunity and more biomass to degrade or sorb these persistent compounds even when conditions are unfavorable.

Downstream of the outfalls, the river told a coherent spatial story as well. Antibiotic concentrations rose sharply near the discharge points and then generally declined with distance downstream, as dilution, sedimentation, photodegradation, and microbial breakdown gradually whittled away the effluent signal. In a cold-region river, however, this natural attenuation is weakened for much of the year. Ice cover reduces light penetration and suppresses photolysis, low winter flows reduce dilution capacity, and cold water slows biodegradation, so the plume of pharmaceuticals extends further and persists longer than it would in a warmer climate. This is why the study’s cold-region focus is significant: most antibiotic monitoring data come from temperate or tropical systems, and the winter dynamics of northern cities have been comparatively understudied.

The ecological risk assessment converted concentration data into a measure of biological concern using risk quotients, a screening approach that compares measured environmental concentrations with predicted no-effect concentrations derived from ecotoxicological studies. The verdict was pointed: azithromycin was the main contributor to ecological risk, reaching moderate to high risk levels in winter. This finding resonates with a growing body of international research showing that macrolides are among the most hazardous antibiotics in surface waters, partly because they are toxic to photosynthetic aquatic organisms such as algae and cyanobacteria, the foundation of aquatic food webs. Even more troubling is the resistance dimension. Laboratory studies have demonstrated that bacteria can be selected for antibiotic resistance at concentrations far below those that cause obvious toxicity, meaning that chronic exposure to even nanogram-per-liter levels in a river can act as a subtle evolutionary engine, enriching resistant strains and the genes that carry resistance.

The Changchun study arrives amid intensifying global concern about antimicrobial resistance, which the World Health Organization ranks among the top public health threats of the century. Wastewater treatment plants occupy a pivotal position in this problem: they concentrate antibiotics, resistant bacteria, and resistance genes from entire urban populations into a single discharge stream, and they are simultaneously one of the few practical points of intervention. The findings from Northeast China suggest that cold-climate cities may need seasonally adjusted strategies, from enhanced winter treatment capacity to technologies such as ozonation and membrane bioreactors that maintain performance when conventional biology falters. They also underscore the value of long-term, seasonally resolved monitoring, because a snapshot taken in summer would have dramatically underestimated both the concentrations and the risks that northern rivers actually face. As winters in many regions grow more variable and urban pharmaceutical use continues to rise, the frozen rivers of places like Changchun may offer an early warning of challenges that colder cities worldwide have only begun to confront.

Subject of Research: Seasonal occurrence, treatment removal, and ecological risk of antibiotics in municipal wastewater and a receiving river in a cold-region Chinese city

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

Article References: Sun, K., Meng, Q., Zhang, C., Wen, Z., Zhu, X., Li, R., Wang, Y., & Wang, M. (2026). Seasonal patterns and ecological risks of antibiotics in municipal wastewater treatment plants and a receiving river in Northeast China. Environmental Monitoring and Assessment, 198(11), Article 1144. https://doi.org/10.1007/s10661-026-16003-8

Image Credits: AI Generated

DOI: 10.1007/s10661-026-16003-8

Keywords: antibiotics, wastewater treatment plants, receiving river, seasonal variation, azithromycin, macrolides, fluoroquinolones, membrane bioreactor, ecological risk, antimicrobial resistance, Northeast China, cold-region water quality