Wet Oxidation of sewage sludge: a mathematical model for estimating the performance based on the VSS/TSS ratio

Wet Oxidation (WO) models are generally employed for reactor design, thus WO modeling is a topic of great interest. Indeed, WO is one of the different technologies studied for sludge minimization and treatment, which represent crucial aspects in the management of wastewater treatment plants (WWTP). Different Generalized Lumped Kinetic Models (GLKM) for sewage sludge WO were proposed in the literature for describing the complex transformations occurring in the process. Usually, works are related to a single type of sludge and, consequently, kinetic constants cannot be used for different case studies. In the present paper an alternative approach has been followed. A mathematical model was proposed and identified on a set of six different types of sludge (primary, secondary and mixed sludge from both urban and industrial WWTP). Kinetic parameters were determined for the different types of sludge with the non-linear least-squares method. In general, the predicted COD values obtained with the estimated kinetic constants closely fit experimental data: the mean absolute error associated with total (dissolved) COD is 3.2% (3.7%) and 10% (17%), respectively for the best and worst case. Kinetic constants values were finally correlated to the sludge VSS/TSS ratio (VSS = Volatile Suspended Solids; TSS = Total Suspended Solids) in order to provide a tool for a gross estimation of WO kinetic parameters, for a given sludge, relying on simple analytical measurement (VSS and TSS), commonly available at every WWTP.