Dynamic Model of the Anaerobic Digestion Process

by John F. Andrews,

Serial Information: Journal of the Sanitary Engineering Division, 1969, Vol. 95, Issue 1, Pg. 95-116

Document Type: Journal Paper


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.

Subject Headings: Anaerobic processes | Acids | Dynamic models | Failure analysis | Bacteria | pH | Flow simulation | Field tests

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