Evaluation of Wastewater Sludge as a Valuable Resource for Various Energetic Applications and Fuel Production

Thermal conversion offers opportunities for energy recovery from wastewater sludge, but its feasibility depends on feedstock properties. This study evaluates industrial sludges from food processing, refining and coking, alongside municipal sewage sludge, in comparison with coal, biomass and refuse-derived fuel. Proximate and ultimate analysis, ash composition and mercury determination were combined with slagging/fouling indices, preliminary ISO 21640 classification and chemometric analysis. Correlation and cluster analyses revealed marked differences among the sludge streams and showed that their comparison with reference fuels depends on data preprocessing. Elevated chlorine and sulfur contents in selected industrial sludges indicated potential corrosion and operational constraints. None of the samples met the low-tendency criteria of all eight slagging/fouling indices, although these indices require experimental validation for sludge. On a dry basis, eight out of 30 samples (27%; six refinery and two food-processing samples) fell within ISO 21640 classes 1–3 for net calorific value, chlorine and mercury; at measured moisture contents, none reached class 3 or better. The findings indicated that selected industrial sludge streams could serve as alternative fuels or fuel-blend components after appropriate drying, provided that energy requirements, contaminant contents, and process-specific operating risks are considered.