Advanced wastewater treatment is widely regarded as a final safeguard before treated water is released into rivers or reused. Yet new research suggests that some of the most powerful purification technologies may carry a hidden trade-off: they can reduce certain hormone-disrupting effects while intensifying another. In a study of wastewater from Tianjin, China, researchers found that ultraviolet irradiation and ozonation increased anti-androgenic activity after conventional treatment, in some cases pushing it above a biological risk threshold. The finding challenges the assumption that cleaner water, measured by the disappearance of familiar pollutants, is always biologically safer.
The study, published in Energy & Environment Nexus, examined wastewater as it moved through a conventional anaerobic-anoxic-oxic, or A2O, treatment system and then through pilot-scale advanced treatment processes. The researchers focused on biological activity linked to three human nuclear hormone receptors: the estrogen receptor, androgen receptor and thyroid receptor. These receptors regulate processes including reproduction, development, metabolism and tissue maintenance. Chemicals that activate or block them can interfere with hormonal signaling in humans and wildlife, sometimes at concentrations too low to be obvious through conventional chemical monitoring.
Instead of testing only for a predefined list of contaminants, the team used yeast-based bioassays engineered to contain human nuclear receptor genes. These tests are designed to measure the combined activity of complex chemical mixtures. That distinction is important because treated wastewater can contain hundreds or thousands of compounds, including pharmaceuticals, industrial chemicals, personal-care product ingredients and the transformation products created when those substances react during treatment. A chemical-by-chemical analysis may miss the combined effect of compounds that individually occur at very low concentrations but collectively influence a hormone receptor.
The A2O process removed much of the endocrine-disrupting activity detected in the incoming wastewater. Estrogen receptor activity declined substantially and fell below the assay’s detection limit after secondary sedimentation. Advanced treatment also generally reduced estrogen-related and thyroid-related effects. These results are consistent with the intended role of technologies such as ozonation and ultraviolet treatment. Ozone is a powerful oxidant that attacks the molecular structures of micropollutants, while ultraviolet light damages the genetic material of microorganisms and can also initiate photochemical reactions involving dissolved organic compounds.
The androgen receptor results revealed a more complicated pattern. The researchers did not detect androgenic activity, which would mimic the action of male sex hormones. They did, however, observe anti-androgenic activity throughout the treatment system. This activity decreased from 156.6 micrograms per liter, expressed as flutamide equivalents, in the influent to 12.4 micrograms per liter after A2O treatment. Following ultraviolet irradiation and ozonation, however, the anti-androgenic signal increased again. In other words, the advanced processes removed some forms of endocrine activity but appeared to generate or release compounds capable of interfering with androgen signaling.
To place the results in a risk-assessment context, the researchers derived an effect-based trigger value of 7.97 micrograms of flutamide equivalents per liter for anti-androgenic activity. Flutamide is a drug known to block androgen receptors, so expressing the measured activity in flutamide equivalents provides a common reference for the potency of a mixture. Activity after ultraviolet treatment exceeded this benchmark. The value is not a direct prediction of harm to people drinking reclaimed water or to organisms exposed to the discharge, but it signals that the biological activity is high enough to warrant further investigation and possible treatment optimization.
The most likely explanation is the formation of transformation products during oxidation and photochemical treatment. When ozone or ultraviolet light reacts with a contaminant, it may split the original molecule into smaller fragments. Some fragments are less persistent or less biologically active, but others can retain activity or interact with hormone receptors in new ways. The original compounds may also be converted into a larger number of products that collectively produce a stronger receptor response. Because the chemical identities of many wastewater transformation products remain unknown, linking a bioassay signal to specific molecules is one of the most difficult challenges in environmental toxicology.
The study also showed that treatment effects depended on the biological endpoint being measured. Ozonation and biological activated carbon were effective at reducing antagonistic activity involving the thyroid receptor, whereas coagulation produced a slight increase in that activity. Biological activated carbon can provide an additional polishing step because contaminants and transformation products may adsorb to the carbon surface or undergo further biodegradation by microorganisms living within the filter. These contrasting results demonstrate why no single indicator can fully describe treatment performance. A process that appears highly successful when judged by estrogenic activity or the concentration of a selected pharmaceutical may produce a different result when evaluated for androgen or thyroid receptor effects.
The findings have implications for water reuse, wastewater discharge permits and the design of next-generation treatment plants. Advanced treatment is essential for removing pathogens and persistent micropollutants, but performance should not be evaluated solely by measuring how much of a known chemical disappears. Researchers argue that effect-based monitoring, which measures the biological activity of the entire mixture, should be used alongside chemical analysis. Such monitoring could reveal unexpected hazards early enough for operators to adjust ozone doses, ultraviolet conditions, residence times or downstream polishing steps. It may also help identify which transformation products deserve targeted chemical investigation.
The authors emphasize that the study does not show that treated wastewater is universally unsafe, nor does it establish that ultraviolet irradiation or ozonation should be abandoned. Rather, it exposes a blind spot in the way treatment success is often defined. Water can contain fewer known pollutants and still exhibit a different, potentially important biological profile after processing. The research team, led by Yingnan Han, Kongrui Zhu, Na Li, Wei Shang and Yu Zhang with corresponding author Mei Ma, is calling for broader testing across multiple hormone receptors and additional work to identify the compounds responsible for the anti-androgenic response. As cities expand water reuse and rely more heavily on advanced purification, understanding what treatment removes—and what treatment may unintentionally create—will be central to making reclaimed water genuinely safe.
Subject of Research: Endocrine-disrupting activity and transformation products in advanced wastewater treatment
Article Title: Integrated effect-based analysis of endocrine disruption risks in advanced wastewater treatment: elevated hazards from ozonation and ultraviolet processes
News Publication Date: 27-Jul-2026
Web References: https://doi.org/10.48130/een-0026-0014
References: Han Y, Zhu K, Li N, Shang W, Zhang Y, et al. 2026. “Integrated effect-based analysis of endocrine disruption risks in advanced wastewater treatment: elevated hazards from ozonation and ultraviolet processes.” Energy & Environment Nexus 2: e021. DOI: 10.48130/een-0026-0014
Image Credits: Yingnan Han, Kongrui Zhu, Na Li, Wei Shang, Yu Zhang & Mei Ma
Keywords
Wastewater treatment, endocrine disruption, anti-androgenic activity, ozonation, ultraviolet irradiation, effect-based monitoring, transformation products, hormone receptors, water reuse, environmental toxicology
Tags: advanced wastewater purification technologiesbioassay-based detection of endocrine disruptorsbiological risk thresholds in water treatmentconventional vs. advanced wastewater treatmenteffects of water treatment on hormone signaling pathwaysEndocrine disruption in wastewater treatmentenvironmental endocrine disruptorshidden chemical risks in water reusehormone receptor bioassays in water quality assessmenthormone-disrupting chemicalsimpact of wastewater on human and wildlife hormonal healthultraviolet irradiation and ozonation effects
