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The Effect of Hydrothermal Carbonization Temperature on Microplastic Content in Digested Sewage Sludge and Its Relation to the Fuel Properties of Hydrochars

Zuzanna Prus, Klaudia Szkadłubowicz, Joanna Mikusińska, Agnieszka Dróżdż, Irena Brunarska, Joanna Chwiej, Katarzyna Styszko and Małgorzata Wilk ()
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Zuzanna Prus: Department of Heat Engineering & Environment Protection, Faculty of Metals Engineering and Industrial Computer Science, AGH University of Krakow, Mickiewicza 30 Ave., 30-059 Krakow, Poland
Klaudia Szkadłubowicz: Department of Heat Engineering & Environment Protection, Faculty of Metals Engineering and Industrial Computer Science, AGH University of Krakow, Mickiewicza 30 Ave., 30-059 Krakow, Poland
Joanna Mikusińska: Department of Heat Engineering & Environment Protection, Faculty of Metals Engineering and Industrial Computer Science, AGH University of Krakow, Mickiewicza 30 Ave., 30-059 Krakow, Poland
Agnieszka Dróżdż: Department of Medical Physics and Biophysics, Faculty of Physics and Applied Computer Science, AGH University of Krakow, Mickiewicza 30 Ave., 30-059 Krakow, Poland
Irena Brunarska: Institute of Geological Sciences, Jagiellonian University, Gronostajowa 3a Ave., 30-387 Kraków, Poland
Joanna Chwiej: Department of Medical Physics and Biophysics, Faculty of Physics and Applied Computer Science, AGH University of Krakow, Mickiewicza 30 Ave., 30-059 Krakow, Poland
Katarzyna Styszko: Department of Fuel Technology, Faculty of Energy and Fuels, AGH University of Krakow, Mickiewicza 30 Ave., 30-059 Krakow, Poland
Małgorzata Wilk: Department of Heat Engineering & Environment Protection, Faculty of Metals Engineering and Industrial Computer Science, AGH University of Krakow, Mickiewicza 30 Ave., 30-059 Krakow, Poland

Energies, 2025, vol. 18, issue 19, 1-15

Abstract: Microplastics (MPs) represent a persistent class of emerging contaminants, of which significant amounts can be found in sewage sludge. In this study, the effect of hydrothermal carbonization (HTC) temperature on MPs and the properties of digested sewage sludge (DSS) was evaluated. The HTC process was carried out at temperatures of 200, 210, and 220 °C for 2 h in a batch reactor, and the solid products were subjected to (i) mass balance and fuel properties and (ii) microplastic occurrence analysis using Confocal Raman Microspectroscopy and Scanning Electron Microscopy. In digested sludge, 2700 ± 475 MP particles/100 g d.m. were detected, mostly fragments with ~350 ± 100 fibers. Hydrocharcontained only black and brown fragments in the following amounts: 4175 ± 575 (200 °C), 4450 ± 700 (210 °C), and 1450 ± 590 (220 °C), respectively, after 2 h. The microplastic removal rate was 54% for the highest temperature. Polystyrene (PS) was identified in untreated sludge, while only PE was detected after HTC at 200–210 °C, and no MPs were identifiable at 220 °C. The surfaces of post-MPs exhibited progressive degradation with increasing HTC temperature. The results confirm that HTC lowers the content and alters the physicochemical properties of microplastics, reducing their thermal stability and degrading their structure, while simultaneously improving the fuel properties of hydrochars by increasing the calorific value and carbon content.

Keywords: microplastics; sewage sludge; hydrothermal carbonization; biosolids; Raman spectroscopy; scanning electron microscopy (search for similar items in EconPapers)
JEL-codes: Q Q0 Q4 Q40 Q41 Q42 Q43 Q47 Q48 Q49 (search for similar items in EconPapers)
Date: 2025
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