Mathematical Modeling of the Mesophilic and Thermophilic Stages of a Composting Tubular Reactor for Sewage Sludge Sanitization

Abstract

A tubular composting reactor was constructed to study the physical phenomenology of the two first stages of a typical composting process; the mesophilic and thermophilic phases. The insulated reactor, with a height of 2.7 m and a diameter of 0.254 m, was filled with a mixture of aerobically digested sewage sludge from a local municipal wastewater treatment plant, wood chips, and grass. The present study consisted in the analyses of the heat and mass transfer processes, considering dispersion and convection processes coupled with a modified Contois kinetics model. The mathematical model was solved using the method of lines employing second-order finite differences to discretize the radial and axial coordinates and a fourth-order Runge Kutta method to integrate in time. Several physical parameters were estimated using a multivariable optimization routine after measuring temperature and oxygen concentration profiles. The reactor reached temperatures above 60 °C in the first 40 h and kept its temperature above or close to 50 °C for the next 80 h, thus maintaining conditions necessary to sanitize the sludge; fecal coliforms were reduced from 43,000 to 75 MPN/g 25 days after the end of sanitization. The mathematical model was able to predict temperature and oxygen concentration profiles, after running an experiment and fitting the model to obtain the value of selected parameters, for a change in length and gas velocity.