Thermal oxidation kinetics and mechanism of sludge from a wastewater treatment plant

The organic fraction of a sludge from a wastewater biological treatment plant is characterized by the total organic carbon, TOC, content, cyclohexane and toluene extractions, and thermal desorptions in nitrogen and air flow at different temperatures. The inorganic fraction is characterized by water extraction, FT-IR spectroscopy, thermogravimetric analysis, and scanning electron microscopy/energy dispersion X-ray analysis. The thermal degradation rate of organic carbon is studied in batch experiments in air, in the 250-500 degrees C temperature range. The sample TOC is used to measure the decrease of reagent concentration with time. The TOC vs time data are well fitted by a generalized kinetic model, previously proposed for the MSWIs fly ash thermal degradation. The rate constants of the immediate carbon gasification, k2, and of the dissociative oxygen chemisorption, k1, followed by C(O) intermediate gasification, k3, together with activation and thermodynamic parameters are calculated. The rate determining step is the C(O) oxidation. The influence of desorbed or extracted organic compounds on kinetics and the role of the C(O) formation in explaining the reaction mechanism as well as the comparison with fly ash kinetics are discussed.