Model calibration and validation for OFMSW and sewage sludge co-digestion reactors

A mathematical model was recently proposed by the Authors to simulate the biochemical processes prevailing in a co-digestion reactor fed with sewage sludge and the organic fraction of municipal solid waste. This model is based on the Anaerobic Digestion Model n°1 of the International Water Association, which has been extended to include the co-digestion processes, using a surface-based kinetics to model the organic waste disintegration and conversion to carbohydrates, proteins and lipids. When organic waste solids are present in the reactor influent, the disintegration process is the rate limiting step of the overall co-digestion process and the main advantage of the proposed modelling approach is that the kinetic constant of such a process does not depend on the waste particles size dimension (PSD), but only on its nature and composition. Therefore model calibration aimed at assessing the kinetic constant of the disintegration process can be carried out using organic waste samples of any PSD, and the resulting value will be valid for all organic wastes with the same nature of the investigated samples, independently on their PSD. This assumption was proven in this study by means of biomethane potential experiments carried out on organic waste samples with different particle sizes. The results of these experiments were used to calibrate and validate the mathematical model, resulting in a good agreement between simulated and observed data for any investigated particle size of the solid waste. Three calibration methods were applied, obtaining identical results. This confirms the validity of the proposed model calibration procedure, which thus can be used to apply the model to assess the treatment efficiency and predict the methane production of full-scale digesters.