Dynamic Model of the Anaerobic Digestion Process

Abstract

The major problem associated with the anaerobic digestion process has been its poor record with respect to process stability as evidenced by the reports of failing digesters. A dynamic model, which predicts failure, is presented for the process. The key features of the model are: (1) The use of an inhibition function to relate volatile acid concentration and specific growth rate for the methane bacteria; and (2) consideration of the un-ionized acid as the growth-limiting substrate and inhibiting agent. The use of an inhibition function is necessary to obtain a model that will predict failure. Consideration of the un-ionized acid as the inhibiting agent resolves the conflict between volatile acid inhibition and pH inhibition because the concentration of un-ionized acid is a function of both total volatile acid concentration and pH. Experimental evidence and evidence from the bacteriological literature are presented in support of the model. Simulation studies, using a digital-analog simulation program, of both batch and continuous flow systems provide additional evidence by predicting results commonly observed in the field during the start-up, or failure, or recovery of anaerobic digesters.