Can Wastewater Sludge Become a Raw Material for Sanitaryware?

Introduction

The transition from a linear to a circular economy is essential to reduce waste and promote sustainable resource use. In 2024, only 12.2 wt.% of waste was recycled in the European Union. In Portugal, the rate was 3 wt.%, highlighting the urgency of circular strategies that turn waste into valuable resources. One approach is the substitution of raw materials by wastewater treatment sludge (WWTS) in ceramic formulations.

Methods

Substitution from 0 to 10 wt.% of feldspar, a critical raw material, by WWTS in sanitaryware paste. WWTS was characterized - chemical and mineralogical composition, particle size distribution, and loss on ignition (at 1000 °C). Ceramic suspensions were homogenized in a turbo diluter, adjusted with water and sodium silicate to achieve density of ≈1890 g/cm³ and viscosity of ≈55 s (Ford cup, 4 mm). Specimens (12×2×1 cm³) were slip cast, dried, and fired at 1210 °C (5 °C/min heating, 1 h dwell).

Results

WWTS consisted mainly of SiO₂ and Al₂O₃ (≈79 wt.%) and showed a mean particle size of 10.4 µm. Rheological analysis showed thixotropic behaviour in suspensions with sludge. However, substitution levels up to 5 wt.% did not compromise the fired ceramics’ properties.

Conclusion

Substitution of feldspar, a critical raw material, by WWTS in sanitaryware paste is technically feasible. Future studies should study alternative wastewater treatment to avoid the interaction of additives in the paste formulation.

Acknowledgements

This work was developed within the scope of the project CICECO Aveiro Institute of Materials, UID/50011/2025 (DOI 10.54499/UID/50011/2025) & LA/P/0006/2020 (DOI 10.54499/LA/P/0006/2020), financed by national funds through the FCT/MCTES (PIDDAC). This research was funded by the PRR - Recovery and Resilience Plan and the NextGenerationEU European Funds, at the University of Aveiro, through the Agenda for Business Innovation ‘Ecoceramica e Cristalaria de Portugal’ (Project no. 76, application C644916391-00000029).