Forever Chemicals Slip Through a Modern Water Recycling Plant, Year After Year

Per- and polyfluoroalkyl substances, the notoriously persistent industrial chemicals better known as PFAS, have become one of the most stubborn pollution stories of our time. Dubbed forever chemicals because their carbon-fluorine bonds resist virtually every natural breakdown process, they now appear in rainwater, blood, polar bear tissue, and the sludge from municipal sewage plants. A new year-long study of a full-scale municipal water resource recycling center in Taiwan adds a sobering data point to that global picture: even a modern treatment train combining conventional biological stages with a membrane bioreactor leaves measurable quantities of these compounds flowing out in the final effluent, day after day, month after month. The research, published in the journal Environmental Geochemistry and Health, offers one of the most detailed stage-by-stage portraits yet of how PFAS behave inside a working reclamation facility.

The team, led by Ferlian Vida Satriaji and Jheng-Jie Jiang of Chung Yuan Christian University together with collaborators in Taiwan, the Czech Republic, and Japan, tracked nine target PFAS compounds through five points in the treatment process: the raw influent, primary sedimentation, final sedimentation, the membrane bioreactor, and the final effluent. Their facility employed an anaerobic-anoxic-oxic biological process followed by membrane filtration, a configuration widely regarded as state of the art for municipal wastewater recycling. Sampling was unusually thorough. Twelve monthly grab-sampling events ran from June 2018 to May 2019, and a separate intensive campaign collected daily samples over seven consecutive days from 21 to 27 May 2019, allowing the researchers to separate seasonal patterns from short-term variability.

Seven of the nine target compounds were detected above their compound-specific method detection limits in at least one sample. Only the two longest-chain compounds examined, perfluoroundecanoic acid (PFUnA) and perfluorododecanoic acid (PFDoA), were never detected, a finding consistent with the phase-out and declining use of long-chain PFAS chemistry in consumer and industrial products over the past two decades. The workhorse compounds of the legacy PFAS era, perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS), dominated the profile. Together they accounted for 70.4 percent of the estimated annual mass of PFAS leaving the plant, underscoring how these two legacy chemicals continue to define the PFAS burden of municipal wastewater even years after their production was curtailed in much of the world.

The headline numbers from the monthly dataset tell a subtle story. Mean concentrations of the seven quantified compounds, summed as sigma-PFAS, were 20.84 nanograms per liter in the influent, rose to 29.62 nanograms per liter after the membrane bioreactor, and stood at 26.21 nanograms per liter in the final effluent. In other words, the plant did not remove PFAS on balance; concentrations in the treated water were actually higher than in the incoming wastewater. This counterintuitive pattern, observed repeatedly at treatment plants around the world, is usually attributed to the transformation of precursor chemicals. Fluorinated precursors that were not directly measured in this study can be converted by microbial and chemical activity into the stable perfluoroalkyl acids that the analytical methods capture, so the mass of measurable PFAS can grow even as the total fluorinated burden stays constant or slowly declines.

Statistical analysis sharpened that picture. Friedman tests, a nonparametric method suited to repeated measurements across matched treatment stages, revealed significant differences in summed PFAS concentrations among the five sampling points in both the monthly dataset and the seven-day campaign, with p-values of 0.002 and 0.013 respectively. Yet the direct comparison between influent and final effluent, the pair that matters most for assessing what a plant sends to receiving waters, was not statistically significant in either campaign. The stage-wise differences were real, but the net change across the whole plant was small enough to be lost within the variability of the data. That distinction matters for regulators and engineers alike: a treatment train can reshuffle PFAS concentrations internally without meaningfully reducing the load discharged to rivers or reuse systems.

Translating concentrations into mass gives the study its practical punch. The researchers estimated that the facility discharges approximately 0.314 kilograms of the seven quantified PFAS compounds into the environment each year through its final effluent. That figure may sound modest, but it represents a continuous, diffuse input of essentially nondegradable chemicals into a receiving watershed, accumulating alongside contributions from every other upstream discharge. Because PFAS do not break down, each kilogram released persists and can migrate through surface water, groundwater, soil, and eventually drinking water supplies. Mass discharge estimates of this kind are increasingly recognized as the critical metric for prioritizing which sources to control first, since a plant with moderate concentrations but enormous flow volumes can contribute more pollution than a smaller source with higher concentrations.

Risk assessment in the study offered reassurance, with caveats. The team calculated compound-specific risk quotients by comparing measured effluent concentrations against ecotoxicological benchmarks drawn from resources such as the United States Environmental Protection Agency’s ECOTOX knowledgebase. The resulting quotients ranged from 1.30 times ten to the minus nine to 1.65 times ten to the minus five, all classified as negligible under the criteria applied. Such calculations, however, only cover the compounds measured and the endpoints selected. PFAS mixtures can act additively or synergistically, regulatory toxicity thresholds continue to tighten as new health data emerge, and the risk landscape can shift dramatically if precursor transformation downstream generates additional perfluoroalkyl acids after discharge. A negligible quotient today is not a guarantee of negligible risk under tomorrow’s benchmarks.

The authors are candid about the limits of their mass balance. Precursor compounds, sludge, suspended solids, and total organic fluorine were not measured, which means the observed differences between treatment stages cannot be used to definitively establish precursor transformation, phase partitioning between water and solids, or a complete accounting of fluorinated mass through the plant. Sludge is a particularly important blind spot. PFAS sorb to solids and biosolids, and when sludge is land-applied or landfilled, those chemicals can re-enter the environment through entirely separate pathways. The oxidizable precursor assay, a technique increasingly used to convert hidden precursors into measurable perfluoroalkyl acids, would be a natural next step for facilities seeking a fuller picture, as recent screening work on mixed liquor and effluents has demonstrated.

Why does any of this matter beyond one plant in Taoyuan? Water resource recycling centers are the linchpin of circular water economies, especially in water-stressed regions like Taiwan, where reclaimed water is being promoted for industrial and environmental uses. If PFAS pass through reclamation essentially untouched, then every liter of recycled water carries a legacy chemical load, and every year of operation adds to the environmental stock. Conventional biological treatment and membrane bioreactors were simply never designed to break carbon-fluorine bonds. Emerging technologies such as granular activated carbon, ion exchange, reverse osmosis, and destructive techniques including supercritical water oxidation and electrochemical treatment show promise, but they carry substantial costs and energy demands, and they concentrate rather than eliminate the problem unless destruction is included.

The study’s greatest value may lie in its template. A full year of monthly sampling plus an intensive daily campaign, applied across five treatment stages with compound-specific detection limits and nonparametric statistics, provides exactly the kind of facility-specific evidence that regulators need to design monitoring programs and that engineers need to justify targeted PFAS treatment investments. As governments worldwide move to restrict PFAS manufacture, designate them as hazardous substances, and set enforceable limits in effluents and drinking water, studies like this one quantify the scale of the challenge: the forever chemicals we have already released keep cycling back through our pipes, and the plants built to clean our water are, for now, largely transparent to them. Closing that gap is one of the defining environmental engineering tasks of the coming decade.

Subject of Research: Fate and mass discharge of per- and polyfluoroalkyl substances across treatment stages at a municipal water resource recycling center

Article Title: Treatment-stage variation and final-effluent mass discharge of per- and polyfluoroalkyl substances at a municipal water resource recycling center

Article References: Satriaji, F. V., Li, M.-C., Jachimowicz, P., Solayman, H. M., Mori, A. M. J. P., Le Tong, C. T., Widyastuti, A., Nguyen, T.-B., Horie, Y., Chao, H.-R., & Jiang, J.-J. (2026). Treatment-stage variation and final-effluent mass discharge of per- and polyfluoroalkyl substances at a municipal water resource recycling center. Environmental Geochemistry and Health, 48(14), Article 576. https://doi.org/10.1007/s10653-026-03471-7

Image Credits: AI Generated

DOI: 10.1007/s10653-026-03471-7

Keywords: PFAS, forever chemicals, wastewater treatment, membrane bioreactor, water recycling, PFOA, PFOS, mass discharge, effluent, ecological risk, Taiwan, water reclamation