Investigating chemical changes, material properties, and off-gas emissions in sewage sludge through thermal plasma treatment

Abstract

This study aimed to investigate the thermal plasma treatment of sewage sludge as an alternative waste management solution. Samples were collected from a sewage treatment facility in São Paulo, Brazil, and subjected to thermal treatments in a furnace at temperatures of 400, 600, 800, and 900 °C to assess moisture content, mass loss, and ash composition. Subsequently, the samples were processed in a thermal plasma reactor operating at an average power of 30 kW. Comprehensive characterization using X-ray fluorescence, X-ray diffraction, and Fourier-transform infrared spectrometry revealed the presence of oxides, particularly those of K, Ca, and Fe, which influence material melting points. Plasma treatment significantly reduced the mass and volume of the sewage sludge, losing up to 72 and 90%, respectively. Gas analysis indicated syngas (CO + H2) production with a high heating value (HHV) of 9049 kJ/m³, underscoring the energy recovery potential. Leaching and solubility tests confirmed the treated material as inert and suitable for safe disposal or reuse in construction, with chemical and structural analyses showing that the vitrified material was primarily composed of SiO₂. These findings highlight the effectiveness of thermal plasma treatment in achieving sustainable sewage sludge management by combining waste reduction, energy recovery, and the production of environmentally safe by-products.