Thermal valorisation of sewage sludge into artificial aggregates: A critical review of processes, environmental performance, and circular bioeconomy implications

The continuous generation of Sewage Sludge (SS) from wastewater treatment plants poses major environmental and management challenges while offering significant opportunities for bioresource valorisation. This review critically analyses the transformation of SS into Artificial Aggregates (AAs) through thermal processes such as sintering, highlighting its potential as a sustainable route for resource recovery and circular bioeconomy integration. The unique physicochemical composition of SS rich in SiO₂, Al₂O₃, CaO, Fe₂O₃, and organic matter enables granulation, bloating, and vitrification during sintering, producing lightweight aggregates with densities below 1200 kg m−3 and compressive strengths above 6 MPa. Incorporating supplementary binders such as fly ash, rice husk ash, and Na₂SiO₃ optimises sintering behaviour, enhances densification, and reduces water absorption. High-temperature sintering (>1050 °C) effectively immobilises toxic metals including Cd, Cr, and Pb, ensuring leachate concentrations remain within European Waste Acceptance Criteria limits. Beyond technical performance, SS-derived AAs contribute to climate change mitigation by offsetting the extraction of natural aggregates, lowering carbon emissions, and enabling waste to resource pathways consistent with Sustainable Development Goals (SDGs 6, 12, and 13). This review consolidates more than 180 studies, providing a state-of-the-art synthesis of process optimisation, binder synergy, environmental safety, and techno-economic perspectives. Key research gaps related to energy efficiency, scalability, and long-term durability are identified to guide future innovations in integrating SS derived aggregates into sustainable water resource recovery and bioengineering systems.