Advanced Wastewater Treatment Can Backfire, Raising Hidden Hormone Risks

Water scarcity is pushing cities around the world to recycle their wastewater on a scale never seen before, and the technologies that make this possible—ozonation, ultraviolet irradiation, and biological activated carbon filtration—are widely celebrated as the gold standard for purifying reclaimed water. But a new study from China delivers a sobering warning: the very processes designed to scrub contaminants from effluent can create new biological hazards that conventional monitoring never sees. By tracking hormone-disrupting activity across every stage of a full-scale treatment plant and two pilot systems, researchers found that while advanced treatments tamed estrogen- and thyroid-related effects, they simultaneously pushed anti-androgenic activity above a newly derived safety threshold, raising the possibility that polished, crystal-clear reclaimed water may still carry hidden endocrine risks.

The study, published on 27 July 2026 in the journal Energy & Environment Nexus by Mei Ma’s team at the Chinese Academy of Sciences, is notable for its unusually comprehensive scope. Rather than measuring a handful of known chemicals, the team assessed the total biological activity of wastewater samples using effect-based tools that capture the combined action of entire chemical mixtures. This matters because oxidation and photochemical reactions during advanced treatment can generate transformation products whose biological properties differ sharply from those of their parent compounds. Some of these byproducts may be cytotoxic, genotoxic, or endocrine-disrupting, yet routine chemical analysis—built to detect known target compounds—can miss their combined effects entirely.

The researchers worked at a typical wastewater treatment plant in Tianjin, China, supplemented by two pilot-scale treatment systems. Samples were collected from 11 locations spanning the full treatment train: the plant inlet, preliminary sedimentation, anaerobic-anoxic-oxic (A2O) biological treatment, secondary sedimentation, ultraviolet disinfection, ozonation, coagulation, and biological activated carbon filtration. Organic compounds were isolated from each sample using solid-phase extraction, concentrating the dissolved chemistry so that even trace activity could be measured.

The heart of the method lay in a panel of recombinant yeast assays. Each assay carried a human hormone receptor—estrogen receptor alpha, androgen receptor, or thyroid receptor beta—wired to a reporter system, allowing the team to quantify both receptor-agonistic effects, in which chemicals mimic a hormone, and receptor-antagonistic effects, in which chemicals block a hormone from acting. Bioactivity was expressed as equivalent concentrations of reference chemicals, giving the results a tangible toxicological meaning. Crucially, the team also derived an effect-based trigger value of 7.97 micrograms of flutamide equivalents per liter for androgen receptor-antagonistic activity, providing a benchmark against which treated effluents could be judged.

For estrogenic activity, the treatment train performed largely as intended. Estrogen receptor-agonistic activity was prominent in untreated and preliminarily treated wastewater but declined progressively through the plant, falling below the detection limit after secondary sedimentation. Estrogen receptor-antagonistic activity followed a similar trajectory, dropping from 58.4 micrograms per liter in the influent to roughly 7.8 to 9.2 micrograms per liter after treatment. These results confirm that conventional biological treatment, particularly the A2O process, removes most detectable estrogen-linked endocrine activity before water ever reaches the advanced polishing stages.

Thyroid-related activity told a similar, though slightly more complicated, story. Thyroid receptor-antagonistic activity fell dramatically from 0.56 milligrams per liter to 0.0085 milligrams per liter through the main plant. In the pilot systems, ozonation—and ozonation followed by biological activated carbon filtration—further suppressed this activity, demonstrating that oxidative processes can genuinely help control certain endocrine endpoints. Yet coagulation, a standard physicochemical step, produced a slight rebound in thyroid receptor antagonism. The researchers suggest this may occur because residual endocrine-disrupting chemicals compete with other organic matter during removal, escaping the coagulation process while bulk organic material is stripped away.

The most alarming findings concerned the androgen receptor. No androgen receptor-agonistic activity was detected anywhere in the treatment train—good news in itself—but antagonistic activity, which can interfere with male hormone signaling, was present throughout. The A2O biological process reduced it markedly, from 156.6 to 12.4 micrograms of flutamide equivalents per liter, a roughly twelvefold decrease that brought the effluent comfortably below the trigger value. What happened next undermined that progress. Both ultraviolet irradiation and ozonation increased anti-androgenic activity, pushing the treated effluents above the newly established threshold of 7.97 micrograms per liter. Even biological activated carbon filtration applied after ozonation did not fully reverse the increase.

The researchers propose a chemical explanation for this counterintuitive result. Advanced oxidation and UV-driven photochemistry rarely mineralize organic contaminants completely; instead, they transform parent compounds into partially degraded products through reactions such as hydroxylation, dealkylation, decarboxylation, and deamination. These transformation products may retain or acquire the ability to bind and block the androgen receptor, even as the original pollutants disappear from chemical inventories. In other words, a treatment plant can appear to be succeeding by every conventional metric—removing parent pollutants, reducing chemical oxygen demand, eliminating pathogens—while its effluent grows biologically more hazardous along a specific endocrine pathway.

The broader implication is that treatment performance cannot be judged solely by pollutant removal or by measuring a single biological endpoint. UV irradiation and ozonation remain valuable technologies for pathogen control and contaminant degradation, and the study does not argue for abandoning them. Instead, it calls for optimizing their operating conditions and downstream treatment configurations to prevent the formation or persistence of hazardous transformation products. It also argues for embedding multi-endpoint, effect-based monitoring into routine wastewater management, so that risks invisible to chemical analysis—such as mixture effects that simultaneously activate or inhibit different hormone receptors—can be detected before reclaimed water is discharged or reused.

As water reuse expands from arid regions into mainstream urban planning, the study offers a timely reminder that purification is not the same as safety. Effect-based trigger values like the one derived for anti-androgenic activity could become a template for regulators seeking to balance purification efficiency against ecological and human health protection. For now, the message for engineers and water utilities is clear: the cleanest-looking effluent is not necessarily the safest, and the next generation of wastewater reclamation systems must be designed with the full spectrum of hormone receptor activity in view.

Subject of Research: Endocrine-disrupting effects of transformation products formed during advanced wastewater treatment

Article Title: Advanced wastewater treatments may intensify hidden endocrine risks

Article References: Advanced wastewater treatments may intensify hidden endocrine risks. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: wastewater treatment, endocrine disruption, ozonation, ultraviolet irradiation, androgen receptor, estrogen receptor, thyroid receptor, water reclamation, transformation products, effect-based monitoring, A2O treatment, flutamide equivalents