Energies, Vol. 18, Pages 6071: Hydrothermal Treatment of Sewage Sludge Under Different Process Conditions with a Focus on Energy Properties and Resource Recovery
Energies doi: 10.3390/en18226071
Authors:
Klaudia Szkadłubowicz
Joanna Mikusińska
Artur Pozarlik
Małgorzata Wilk
Hydrothermal carbonization (HTC) is an effective method for processing wet sewage sludge without prior drying. This study investigates the influence of temperature (200 °C and 210 °C), residence time (15 and 30 min), and pH (neutral and acidic, pH = 2) on the properties of hydrochar and the liquid fraction. Increasing process severity enhanced carbonization, increasing carbon content from 36% in raw sludge to 43% in acidified samples. Under neutral HTC conditions, ash content exceeded 40%, while acidic conditions reduced it to 28%, indicating mineral dissolution and transfer into the liquid phase. Hydrogen and nitrogen contents remained within 3–6%, contributing to the fuel characteristics. The solid yield decreased from 1.04% in raw sludge to 0.22–0.37% after HTC, confirming intensified organic matter conversion. Acidic conditions significantly improved nutrient release to the liquid phase. PO43− concentration increased from 337 to 375 mg/L under neutral conditions to over 675 mg/L, while P2O5 exceeded 509 mg/L. Conductivity rose from approximately 2.0 to 4.25 mS/cm, reflecting high ionic content. These results highlight the potential of the liquid fraction as a nutrient-rich stream that can be used for fertilizer recovery, particularly via struvite precipitation, and confirm that precise HTC parameter control supports resource recovery in line with circular economy principles.