An enhanced anaerobic digestion BioModel calibrated by parameters optimization based on measured biogas plant data

Abstract

In this paper an enhanced BioModel was developed to simulate biogas production in the anaerobic digestion process of organic waste. The proposed model considers biochemical and three–phase (gas–liquid–solid) physicochemical processes, where the degradation of carbohydrates, proteins, and lipids leads to the formation of CH4 and many other by-products; the pH value is treated as a state variable. The model contains a large number of feedstock and model parameters, which were calibrated by engaging a special active set optimization procedure. This procedure exploits data from a previously made sensitivity analysis to enable gradual optimization which is started with a rather small set of the most important parameters. This set of active parameters is then gradually increased until all parameters are calibrated. The objective function was defined to minimize the daily differences between simulated and measured rates of biogas, CH4, H2, and H2S, as well as the pH value; the measured data were obtained from an actual operating full-scale biogas plant during an observed time of 365 days. The imposed constraints were related to the values of biogas, CH4, H2, H2S, and NH3 flow rates, initial bacteria concentrations in the complex substrate, and to the pH values during the whole simulation period. The needed limiting values were defined on the basis of the measured output data of the biogas plant. The results obtained confirm the accuracy of the enhanced BioModel and the usefulness of the proposed procedure for model calibration, both, in terms of numerical stability and computational efficiency.