Empirical characterisation and mathematical modelling of settlement in composting batch reactors

The settlement of organic matter during composting was measured at different levels during the active biodegradation phase in forced-aerated static reactors loaded with different mixtures of organic wastes. The temperature evolution and the concentration of oxygen and carbon dioxide were also recorded in the exhaust gases. Two two-parameter equations and their generalised three-parameter form were fitted to the experimental data and their capability to predict settlement as a function of time was discussed. The settlement field inside the reactors was successfully described with a linear profile. At any given time and vertical position, mass settlement was proportional to the initial height after reactor loading. Furthermore, a relationship linking settlement to biological activity was also proposed. Under all tested experimental conditions, settlement showed a clear correlation with the cumulative oxygen consumption that was modelled with a three parameter equation.