Variable kinetic approach to modelling an industrial waste anaerobic digester

• Variable kinetic coefficient is proposed in order to follow the acclimation of microorganisms.
• Batch test were used to obtain biodegradation potential.
• Monte Carlo methods were applied to obtain confidence interval of parameters.
• Continuous reactor was used to calibrate and validate the model.

Anaerobic co-digestion of agro-industrial wastes is a promising option for the stabilisation of residues with biogas production. A mixture of bovine ruminal content, tannery carving fat and activated sludge purge was considered for this study. Biodegradability tests for individual wastes and a mixture of wastes were performed in batch conditions. Additionally, a completely mixed reactor with an average residence time of 30 days and a loading rate of 3.0 gVS/L.d. was employed for the mixture of wastes. A Volatile Solids (VS) removal efficiency of 66% with a methane production of 0.38 LCH4/kgVSadded or 0.58 LCH4/kgVSremoved was attained. A first-order kinetic model with lag time was employed to describe the behaviour of the batch tests. Parameter determination was performed using direct search methods. Monte Carlo methods were utilised to determine the range of parameters. An ultimate methanation of 90 ± 6% was obtained from the batch tests. For the continuous system, a simple model with a variable kinetic constant that reflects the microbiological activity was proposed. The model was calibrated by adjusting the stoichiometric coefficients using solids outlet data, and the kinetic constant was deduced using experimental methane flow data. The kinetic constant doubles for approximately four residence times, which demonstrates the acclimation of biomass.