Development and validation of mathematical model for aerobic composting process

By integrating the reaction kinetics with the mass and heat transfer between the three phases of the system, a new dynamic structured model for aerobic composting process was developed in this work. In order to evaluate kinetic parameters in mathematical model and to validate the model, experiments were performed with the reactor of volume 32 L, in controlled laboratory conditions. Different ratios of poultry manure to wheat straw were mixed and used as a substrate. Rosenbrock optimization method was used for parameter estimation. In order to solve the system of 12 non-linear differential (and corresponding algebraic) equations, Runge–Kutta–Fehlberg method was used, with approximation of fourth and fifth order and adjustment of step size. Both algorithms were implemented in FORTRAN programming language. In order to achieve as accurate description of the process dynamics as possible, the developed mathematical model was validated by the results of several experimentally measured dynamic state variables. Comparisons of experimental and simulation results for temperature of substrate, organic matter conversion, carbon dioxide concentration and oxygen concentration, in general showed good agreement during the whole duration of the process in a reactor. In the case of ammonia, an agreement was achieved for the first 4 days and for the last 3 days of the process. A sensitivity analysis was performed to determine the key parameters of the model. Analysis showed that two parameters had a great influence on the main characteristics of the process. With validated model for aerobic composting of mixture of poultry manure and wheat straw, optimal values were determined: initial moisture content (70%) and airflow (0.54 l min^−1 kg^−1 OM).