Advanced wastewater treatments may create unexpected environmental risks

Advanced wastewater treatment technologies are widely used to remove pollutants and pathogens before treated water is discharged or reused. However, a new study suggests that some of these processes may also create an overlooked environmental risk.

Researchers found that ultraviolet irradiation and ozonation, although effective at reducing several types of endocrine-disrupting activity, increased anti-androgenic activity in treated wastewater to levels above an effect-based risk threshold.

The study, published in Energy & Environment Nexus, evaluated wastewater from a treatment plant in Tianjin, China, together with two pilot-scale advanced treatment systems. The researchers examined how different treatment stages affected biological activity associated with three important hormone receptors: the estrogen receptor, androgen receptor, and thyroid receptor.

Advanced treatment should not be evaluated only by how efficiently it removes known pollutants. We also need to understand whether the treatment process produces new compounds with biological effects that were not present, or were less pronounced, before treatment."

Mei Ma, corresponding author, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences

Many advanced treatment technologies work by transforming organic contaminants. Ozone, for example, is a powerful oxidant that can break down a wide range of micropollutants, while ultraviolet treatment is commonly used to inactivate microorganisms. Yet these reactions do not always completely mineralize contaminants. Instead, they may generate transformation products with biological properties different from those of the original chemicals.

To capture these effects, the researchers used yeast-based bioassays containing human nuclear receptor genes. Rather than measuring individual chemicals alone, this approach detects the combined biological activity of complex mixtures in wastewater.

The conventional anaerobic-anoxic-oxic, or A2O, treatment process removed much of the endocrine-disrupting activity. Estrogen receptor activity declined substantially, and estrogenic activity fell below the detection limit after secondary sedimentation. Subsequent UV and ozonation treatments were also effective at controlling estrogen receptor and thyroid receptor effects.

The androgen receptor results, however, showed a different pattern. No androgenic activity was detected, but anti-androgenic activity was observed throughout the treatment system. It declined from 156.6 μg/L in the wastewater influent to 12.4 μg/L after A2O treatment, before increasing again following UV treatment and ozonation.

The researchers derived an effect-based trigger value of 7.97 μg flutamide equivalents per liter for anti-androgenic activity. The activity measured after UV treatment exceeded this benchmark, indicating a potentially important ecological risk.

The authors suggest that transformation products generated during oxidation may contribute to the increased anti-androgenic effects, although the specific chemicals responsible have not yet been identified. Limited reference data for many transformation products make it difficult to connect individual compounds with their toxicological effects.

The study also found that ozonation and biological activated carbon treatment effectively reduced thyroid receptor antagonistic activity, while coagulation caused a slight increase in this activity.

The findings show that advanced wastewater treatment can simultaneously remove some biological hazards while increasing others. The researchers therefore recommend evaluating treatment performance across multiple toxicological endpoints rather than relying solely on concentrations of known pollutants.

"Effect-based monitoring can reveal risks that conventional chemical analysis may miss," Ma said. "This information can help wastewater treatment facilities optimize advanced processes while minimizing unintended biological effects."

The study emphasizes that safer water reuse will require balancing pollutant removal efficiency with careful assessment of transformation products and their combined biological effects.