Sewage Sludge Treatment May Transform, Not Destroy, Microplastics in Wastewater

Hydrothermal carbonization has been hailed as one of the most promising ways to deal with the mountains of sewage sludge that modern cities produce every day, a process that uses hot, pressurized water to convert wet organic waste into a carbon-rich solid called hydrochar without the need for energy-hungry drying. But a nagging question has lingered beneath the enthusiasm: what actually happens to the microplastics that concentrate in sludge during wastewater treatment? A new study from researchers at AGH University of Krakow in Poland, published in Energy Reports, offers one of the most detailed answers yet, and its conclusions are more sobering than the optimistic headlines suggest. Under the very conditions used to process sewage sludge, the two dominant microplastic pollutants in biosolids are not eliminated. They are transformed, and much of their material migrates into the process water, where it could pose new treatment challenges.

The research team, led by Zuzanna Prus-Frączek and including senior authors Katarzyna Styszko and Małgorzata Wilk, focused on polyethylene terephthalate, or PET, the polymer of water bottles and food packaging, and polystyrene, or PS, the aromatic plastic found in insulation, packaging foam, and countless disposable items. Both are among the most common polymers recovered from treated sludge, which can harbor hundreds to thousands of microplastic particles per kilogram of dry matter. Because European Union policy increasingly promotes the reuse of sludge as fertilizer and soil amendment on farmland, understanding whether hydrothermal treatment truly breaks these particles down is not an academic detail. It is central to whether plastic pollution quietly rides the circular economy back into the food chain.

Earlier studies had reported that hydrothermal carbonization can slash the number of microplastic particles detectable in sludge by up to 90 percent, with one study showing removal of particles larger than 50 micrometers jumping from about 11 percent to 85 percent as temperatures rose from 170 to 220 degrees Celsius. But a reduction in detectable particles is not the same as destruction. Fragments can shatter below the size limits of analytical instruments, and polymer-derived compounds can dissolve into the surrounding water without ever being mineralized. The Polish team set out to determine which of these fates actually befalls PET and PS, treating the two polymers separately in pure water at 220 degrees Celsius for one, two, and three hours, conditions carefully chosen to mirror real sewage sludge processing while eliminating interference from sludge-derived organic matter.

The results revealed a striking asymmetry between the two plastics. PET, whose backbone contains ester bonds that water can chemically attack, proved far more vulnerable. Thermogravimetric analysis showed that its degradation onset temperature, which stands at roughly 402 degrees Celsius in the raw polymer, plummeted to between 246 and 284 degrees Celsius after treatment. The pyrolytic residue left at 800 degrees Celsius collapsed from 12.7 percent to as little as 1.3 percent, indicating a severely diminished tendency to form stable carbon structures. Infrared and Raman spectroscopy painted a consistent picture: the sharp ester carbonyl band at 1714 inverse centimeters gave way to new dominant components near 1671 to 1677 inverse centimeters, signaling a heterogeneous mix of partially hydrolyzed ester bonds and newly formed oxygen-containing structures. Scanning electron microscopy showed surfaces transformed from smooth and compact into rough, cracked, and cavity-riddled landscapes.

Polystyrene, by contrast, emerged from the reactor remarkably intact. Its hydrophobic aromatic skeleton, held together by carbon-carbon bonds that water cannot easily cleave, resisted the conditions that visibly weakened PET. Treated PS samples retained a single-step thermal degradation profile with maximum decomposition temperatures of around 416 to 417 degrees Celsius, essentially unchanged from the raw material. The principal FTIR and Raman bands, including the characteristic aromatic ring breathing mode near 1001 inverse centimeters, remained within measurement uncertainty of their original positions. Only slight band broadening after three hours hinted at limited local disorder at the surface. The finding aligns with earlier work showing that complete PS decomposition demands 360 degrees Celsius and four hours, conditions far more severe than any sewage sludge reactor would employ.

Perhaps the most consequential discovery came from analyzing the process water itself. When the researchers measured chemical oxygen demand and total organic carbon in the liquid phase, PET proved to be a prodigious polluter. Normalized to the initial polymer mass, PET released approximately 737 to 876 milligrams of COD and 320 to 343 milligrams of TOC per gram of plastic into the water, with absolute concentrations reaching 6145 to 7300 milligrams per liter of COD and around 2700 to 2855 milligrams per liter of TOC. Polystyrene, in comparison, contributed only 35 to 83 milligrams of COD and 99 to 140 milligrams of TOC per gram of polymer. The pH of the PET process water remained stubbornly acidic, between 3.8 and 3.9, consistent with the accumulation of acidic products from ester bond cleavage, while phenolic compounds slowly increased with longer treatment, suggesting the formation of refractory aromatic derivatives that resist further degradation.

These numbers carry a direct message for engineers designing full-scale sludge hydrothermal carbonization plants. Process water from sludge HTC is already heavily loaded with dissolved organics and can contain inhibitory or toxic compounds, and adding a substantial polymer-derived burden, particularly from PET-rich sludge, would amplify the challenge of treating or safely recirculating that water. Previous pilot-scale studies have shown that biological treatment of HTC process water can achieve COD removals of roughly 71 to 75 percent in membrane bioreactors and sequencing batch reactors, but the researchers caution that the added plastic-derived load could strain these systems further. Hydrochar yields added another layer of nuance: the most intensive apparent conversion for both polymers occurred at two hours, with yields for PS dropping to 75.6 percent and PET to 53.6 percent, and the non-monotonic trends hint at competing processes of depolymerization, dissolution, and secondary condensation of soluble intermediates back onto the solid phase.

The environmental implications extend beyond the treatment plant. Hydrochar from PET-containing sludge, with its weakened structure and earlier degradation onset, would likely be more susceptible to further fragmentation, oxidation, and weathering if applied to soils, potentially generating new generations of smaller plastic particles. The dramatic reduction in pyrolytic residue also suggests a diminished capacity for stable carbon sequestration in PET-derived char, undermining one of the climate arguments for hydrochar use in agriculture. Polystyrene-derived material, meanwhile, may remain embedded within or incorporated into the carbonized matrix despite surface alteration, retaining its aromatic identity. In neither case does the plastic vanish. As the authors put it, HTC should be understood primarily as a polymer-transformation and redistribution process, shifting material between the solid, liquid, and gas phases rather than eliminating it from the system.

The study does have limits that the team acknowledges candidly. The experiments used pristine, manually fragmented waste plastics in distilled water at a single temperature, rather than environmentally aged particles in a real sludge matrix, and the analyses could not identify specific molecular transformation products, distinguish hydrolysis from oxidation, or assess nanoplastic formation because samples were ground before imaging. Future work, the researchers argue, should incorporate real sewage sludge, replicate experiments across broader operating windows, and deploy gas and liquid chromatography coupled with mass spectrometry to identify individual compounds, alongside leaching and ecotoxicity tests to evaluate the fate of released organics in soils. For now, the message for the water sector is clear and somewhat uncomfortable: hydrothermal carbonization remains a valuable technology for valorizing wet sludge, but claims of microplastic removal must be read as claims of microplastic transformation, and any credible plant design must treat the process water as a first-class waste stream, not an afterthought.

Subject of Research: Hydrothermal carbonization of PET and polystyrene microplastics under sewage sludge treatment conditions

Article Title: Hydrothermal carbonization of PET and PS microplastics under conditions relevant to sewage sludge treatment

Article References: Prus-Frączek, Z., Kalemba-Rec, I., Magdziarz, A., Dróżdż, A., Chwiej, J., Styszko, K., & Wilk, M. (2026). Hydrothermal carbonization of PET and PS microplastics under conditions relevant to sewage sludge treatment. Energy Reports, 16, Article 109697. https://doi.org/10.1016/j.egyr.2026.109697

Image Credits: AI Generated

DOI: 10.1016/j.egyr.2026.109697

Keywords: microplastics, hydrothermal carbonization, sewage sludge, PET, polystyrene, hydrochar, process water, wastewater treatment, COD, TOC, polymer degradation, circular economy