Stage-Resolved Characterization of Wastewater Sludge Streams for Thermochemical Valorization: Moisture, Fuel Value, and Ash Mineralogy Constraints

Wastewater sludges are often treated as a single feedstock in thermochemical studies, despite substantial variations across treatment stages. Five sludge streams from the Ankara Central Wastewater Treatment Plant, primary sludge, waste activated sludge (WAS), raw sludge, anaerobically digested sludge, and dewatered cake were characterized by gravimetric moisture analysis, TGA-based proximate analysis, oxygen bomb calorimetry for higher heating value, XRD of dried solids and 950 °C residues, and SEM/EDS of selected residues. The as-received moisture contents were very high for four sludge streams, exceeding 97%, indicating that drying would be the first practical barrier before thermal conversion. Even after mechanical dewatering, the cake still contained 77.2 ± 1.4% moisture. On a dry basis, primary sludge, WAS, raw sludge showed similar volatile matter (67%) and lower ash (25-28%), whereas anaerobic sludge and sludge cake had lower volatile matter (~54%) and higher ash (40-42%), with HHV decreasing from 19.9 ± 0.6 MJ/kg (primary) to 13.5-13.6 MJ/kg (anaerobic/cake). Dried sludges showed quartz/calcite/Fe-oxide signatures, with an amorphous contribution; 950 °C residues were mineral-dominated, with aluminosilicate and Ca-P phases and microstructural heterogeneity. Treatment stage shifted the dominant constraint from moisture limited energy recovery in upstream streams to ash and residue limited management in downstream streams. These results demonstrate that sludge should not be treated as a single feedstock class and that thermochemical pathway screening should evaluate moisture burden, dry-basis fuel value, ash loading, and mineral residue behavior together.