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

The timing of this warning could hardly be more significant. Wastewater-based surveillance earned its reputation during the COVID-19 pandemic, when measurements of SARS-CoV-2 RNA in sewage proved to be a leading indicator of clinical case waves across cities and continents. That success spawned an entire discipline, and models such as the wastewater-based epidemic model developed for SARS-CoV-2 in three Canadian cities demonstrated that, under the right conditions, gene copy concentrations in influent sewage could be translated into estimates of infected individuals in a catchment. The European Union has since embedded wastewater surveillance into law through the recast Urban Wastewater Treatment Directive of 2024, and the European Centre for Disease Prevention and Control has published a framework for integrating wastewater data into infectious disease surveillance at the EU level. Antimicrobial resistance is now a priority target of these systems.

Yet the Cyprus-based authors argue that the epidemiological machinery that made viral surveillance interpretable does not yet exist for resistance. For viruses, researchers have invested heavily in quantifying shedding dynamics: how many genome copies an infected person excretes per gram of faeces, how those rates change over the course of infection, and how they differ between variants. Longitudinal studies of SARS-CoV-2, pepper mild mottle virus and crAssphage shedding, together with time-varying shedding models revisiting the 2013 poliovirus outbreak in Israel, have provided the conversion factors that turn a wastewater concentration into an approximate count of infected people. For antimicrobial resistance genes and resistant bacteria, no equivalent body of calibration data exists. The authors point out that faecal carriage of extended-spectrum beta-lactamase-producing Enterobacteriaceae in the general population, the duration of colonisation after international travel, and the effects of antibiotic exposure on the gut resistome are all documented in clinical studies, but these threads have never been woven into transfer functions that connect a gene abundance measured in sewage to a prevalence of carriage in the community above.

The technical reasons for this gap are formidable. A wastewater concentration is the product of a long and noisy chain of processes. It begins with the heterogeneous shedding of resistance determinants by carriers, which depends on the species of bacterium, the type of gene, whether the gene is carried on a chromosome or a plasmid, and whether it resides inside intact cells, in faecal particles, or as free extracellular DNA. It continues with in-sewer decay and partitioning, processes that have been characterised for pathogens such as Campylobacter jejuni and Campylobacter coli but that differ for every gene and microbe of interest. Hydraulic conditions, temperature, sewer biofilms and sediment resuspension all modulate what arrives at the sampling point. Sampling itself, whether flow-weighted composite or grab, introduces further uncertainty. By the time a sample reaches the laboratory, the signal is a composite whose relationship to the original biological state in the population is mediated by dozens of parameters, most of which are unknown for antimicrobial resistance.

Measurement technology compounds the problem. Quantitative metagenomics, which promises absolute quantification of resistance genes across thousands of loci simultaneously, is advancing quickly, and recent work has begun to establish detection limits for environmental monitoring of antibiotic resistance and for viruses. But the Cyprus team emphasises that analytical quantification is not the same as epidemiological calibration. A measurement can be perfectly reproducible and still be uninterpretable if nobody knows what concentration corresponds to what burden of carriage, what change in concentration corresponds to what change in incidence, or what effluent concentration corresponds to what environmental exposure risk. The authors describe this as quantifying variation without resolving biological states, a phrase that captures the epistemological core of their argument: surveillance data are being collected and compared, but the mapping from data to meaning remains undefined.

The downstream half of the problem, the effluent side, is equally under-calibrated. Wastewater treatment plants are known to substantially reduce the load of resistance genes and resistant bacteria leaving a catchment, with studies showing that extended-spectrum beta-lactamase and carbapenemase genes are substantially and sequentially reduced during conveyance and treatment of urban sewage. But removal efficiency varies enormously with treatment technology, capacity overloading, and the fate of intracellular versus extracellular DNA during processes such as ultrafiltration, ultraviolet disinfection and chlorination. Paradoxically, some disinfection steps may promote rather than reduce resistance spread: chlorine disinfection has been shown to facilitate natural transformation of bacteria through reactive oxygen species-mediated oxidative stress, and disinfection byproducts can increase transformation rates of environmental DNA in model organisms such as Acinetobacter baylyi. Without calibrated models of these processes, an effluent measurement cannot be translated into a statement about the resistance burden delivered to a receiving river, coastal water or irrigation system.

That environmental endpoint matters because the receiving environments are not passive sinks. Research along the wastewater-river continuum has documented the dissemination and persistence of antimicrobial resistance far downstream of discharge points. Treated wastewater irrigation has been shown to alter the resistance profiles of agricultural soils, subsoil pore-water and rhizospheres, while biofilms in reclaimed water distribution systems act as hot spots of horizontal gene transfer. Heavy metals such as copper accelerate the conjugative transfer of resistance genes in freshwater microcosms, and even herbicide selection can promote antibiotic resistance in soil microbiomes. Sludge and compost application to land introduces further pathways. Each of these environmental compartments hosts its own ecology of gene exchange, shaped by pH, soil type, moisture and microbial community composition. The authors argue that a calibrated surveillance framework must therefore serve two distinct interpretive goals simultaneously: translating influent signals into population-level epidemiological burden, and translating effluent signals into quantified risk of environmental introduction and subsequent exposure of humans, animals and crops.

The Perspective, illustrated with a conceptual overview of the calibration gap and a proposed calibration solution for interpreting epidemiological data and environmental introduction risk, does not merely diagnose the problem; it sketches what a solution would require. Drawing on the authors’ experience validating the national-scale SARS-CoV-2 surveillance system in Cyprus, and on emerging methods such as metagenomic absolute quantification with cellular internal standards and microbiome-based microbial risk assessment across multiple environments, they call for systematic studies that establish the missing transfer functions. These include quantifying shedding of resistance determinants by carriers in the community, characterising in-sewer and treatment-plant fate for priority genes and bacteria, and linking measured effluent concentrations to quantified exposure in receiving environments through models that account for dilution, decay, regrowth and gene transfer. Global comparative work, such as the exploration of human faecal and sewage resistomes as a function of socio-economic status, and metagenomic source attribution of resistance in wastewater, provides the kind of comparative baseline that calibration will ultimately require.

The stakes of getting this right are considerable. Antimicrobial resistance is one of the leading global threats to public health, and wastewater offers something clinical surveillance cannot: a continuous, catchment-wide, pathogen-agnostic signal that does not depend on patients seeking care or on laboratories reporting results. Wastewater surveillance has already revealed potential underreporting of disease cases in the Detroit area for Salmonella enterica, Campylobacter jejuni and norovirus, and has detected early introductions of the emerging fungal pathogen Candida auris in Utah. Small-scale wastewater-based epidemiology studies of antibiotic resistance are proliferating worldwide. But the Cyprus authors warn that without calibration, these signals risk being over-interpreted, with policy decisions made on the basis of numbers whose epidemiological and environmental meaning is unknown. Conversely, a properly calibrated system could convert routine sewage measurements into actionable estimates of community carriage prevalence and quantified environmental release, transforming wastewater plants from passive sentinels into genuine early-warning infrastructure.

The message of the Perspective is ultimately one of disciplined optimism. The raw ingredients for calibration already exist, scattered across clinical cohort studies of gut colonisation, environmental microbiology of sewers and treatment plants, and risk assessment frameworks developed for water reuse under EU Regulation 2020/741. What is missing is the deliberate, coordinated effort to assemble these ingredients into validated models, much as the viral surveillance community did in the years after 2020. The authors, writing with support from the SPORE-MED project under the PRIMA Programme and the EU-WISH joint action on integrated wastewater surveillance for public health, frame calibration not as an academic luxury but as the precondition for the next phase of the field. As regulators across Europe and beyond write antimicrobial resistance monitoring into wastewater law, the question they pose is blunt: a signal is only as valuable as the model that gives it meaning, and for antimicrobial resistance, that model has yet to be built.

Subject of Research: Calibration of wastewater-based antimicrobial resistance surveillance signals for epidemiological and environmental risk interpretation

Article Title: Calibrating antimicrobial resistance influent and effluent signals in wastewater systems

Article References: Naziri, Α., Chatzimichail, S., Karaolia, P., & Fatta-Kassinos, D. (2026). Calibrating antimicrobial resistance influent and effluent signals in wastewater systems. Nature Water. https://doi.org/10.1038/s44221-026-00701-1

Image Credits: AI Generated

DOI: 10.1038/s44221-026-00701-1

Keywords: antimicrobial resistance, wastewater surveillance, wastewater-based epidemiology, resistance genes, sewage, wastewater treatment plants, environmental risk, public health, metagenomics, horizontal gene transfer, effluent, Nature Water