Validation of a simple kinetic modelling approach for agro-industrial waste anaerobic digesters

Abstract Anaerobic co-digestion of agroindustrial wastes constitutes a promising option for the stabilisation of residues producing biogas. A mixture of bovine ruminal content, tannery carving fat and activated sludge purge was considered for this work. Biodegradability tests for individual wastes and for the mixture of wastes were performed in batch and also in continuous conditions in a completely mixed reactor. The continuous reactor was operated at 30-day average residence times with a load volume of 3.0 g VS/L d, reaching a VS removal efficiency of 66%, and yielding 0.38  L CH 4 /kg VS added and 0.58  L CH 4 /kg VS removed . A first order kinetic model with time delay was used to describe the behaviour of the whole process. The determination of parameters was performed in both systems using direct search methods. Monte Carlo methods were used in order to determine the range of parameters. Ultimate methane yield of 90 ± 6% was obtained from batch tests and 88 ± 9% in the continuous reactor. The kinetic rate constant in the reactor (0.205 ± 0.013 d −1 ) was twice the value obtained in batch conditions (0.089 ± 0.010 d −1 ), showing that microbial adaptations occurred in the continuous mode.