Kinetic Model for the Water Oxidation Method for Treating Wastewater Sludges

Abstract

AbstractA generalized kinetic model for the hydrothermal oxidation of organic matter in biological wastewater treatment sludge was developed using a simplified first‐order reaction scheme. The model was based on a series of experimental results obtained using a continuous‐flow hydrothermal reactor system used to destroy the organic component of the sludge. Forty‐eight hydrothermal treatment experiments using three sets of inflow conditions were performed. Using excess oxygen, the treatment involved sludge destruction under subcritical (<374d̀C) and supercritical (>374d̀C) water oxidation temperatures. The chemical oxygen demand (COD) was used to measure the organic content and progress of the oxidation reaction. The Arrhenius equation was used to model the reaction rate constant. In the Arrhenius equation, the pre‐exponential factor was fixed and the activation energy was found to vav with temperature. The activation energy increased up to approximately 263d̀C then stabilised at temperatures above 263d̀C. The variation of the activation energy with temperature reflected the complexity of the composition of the organic content of the sludge, which generally consists of proteins, lipids, carbohydrates and fibres. In hydrothermal oxidation, the various organic compounds oxidize at different rates, with the easily oxidized matter being removed first. As such, the activation energy reflected the changing composition of the remaining organic matter with the progress of the oxidation reaction. Above about 263d̀C, the activation energy became virtually independent of temperature. A functional relationship was established between activation energy and average temperature of the reactor. A mathematical model for the destruction of COD in the presence of excess oxygen was set using a kinetic equation having an average pre‐exponential constant and temperature dependent activation energy. With the new equation and known influent COD, efluent COD was simulated for the entire set of experiments and was compared with the actual measured effluent COD.