Optimization of the degradation of sewage sludge by wet air oxidation. Study of the reaction mechanism on a cellulose model compound

Abstract
The aim of this work is to optimize the degradation yield of an activated sludge and to study the reaction mechanism on a cellulose model compound. This optimization is performed with a Simplex design with four parameters: reaction temperature (280-350 °C), injected air pressure (40-60 MPa), reaction time (10-45 minutes) and COD concentration (0.5-5 g L^-1). The better yield obtained on TOC concentration is 82,5% at 330 °C. Over this temperature, the yield substantially decreases. The use of cellulose as a model compound for activated sludge confirms optimization results and leads to the identification of by-products and free radical species participating in the reaction. Two major organic compounds have been detected: 4-oxo pentanoïc acid and 3-oxo butanoïc acid, and three types of radicals: HO°, CO and °CH(OH)CH₃.