Predicting biodegradable volatile solids degradation profiles in the composting process

This paper presents a new method for the prediction of the pattern of biodegradable volatile solids (BVS) degradation in the composting process. The procedure is based on a re-arrangement of the heat balance around a composting system to numerically solve for the rate of BVS carbon (BVS–C) disappearance. Input data for the model was obtained from composting experiments conducted in a laboratory-scale, constant temperature difference (CTD) reactor simulating a section of an aerated static pile, and using a simulated feedstock comprising ostrich feed, shredded paper, finished compost and woodchips. These experiments also provided validation data in the form of exit gas CO2 carbon (CO2–C) profiles. The model successfully predicted the generic shape of experimental substrate degradation profiles obtained from CO2 measurements, but under the conditions and assumptions of the experiment, the profiles were quantitatively different, giving an over-estimate of BVS–C. Both measured CO2–C and predicted BVS–C profiles were moderately to well fitted by a single exponential function, with replicated rate coefficient values of 0.08 and 0.09 d−1, and 0.06 and 0.07 d−1, respectively. In order to explore the underlying shape of the profiles, measured and predicted data at varying temperature were corrected to a constant temperature of 40 °C, using the temperature correction function of Rosso et al. [Rosso, L., Lobry, J.R., and Flandrois, J.P., 1993. An unexpected correlation between cardinal temperatures of microbial growth highlighted by a new model. Journal of Theoretical Biology, 162, 447–463], with cardinal temperatures of 5, 59 and 85 °C. Multi-phase profiles were generated for both the measured CO2–C and the predicted BVS–C data in this case. However, when alternative cardinal temperatures of 5, 55 and 80 °C, or 5, 50 and 80 °C, were used, the predicted profiles assumed an exponential shape, and excellent fits were obtained using a double exponential function. These findings support the argument that a substrate degradation curve generated under laboratory conditions at 40 °C, would, given correct cardinal temperatures, generate a correct substrate degradation profile under varying temperature conditions and that this in turn would enable an accurate and precise prediction of the temperature profile, using a heat and mass balance approach. In order to realise this prospect, it is proposed that further work to obtain experimental data under completely mixed conditions, more accurately estimate the overall heat transfer coefficient and obtain correct values for the cardinal temperatures used in the temperature correction function, is required.