KINETIC-MODEL FOR SUBSTRATE UTILIZATION AND METHANE PRODUCTION IN THE ANAEROBIC-DIGESTION OF ORGANIC FEEDS

A kinetic model for anaerobic digestion of organic substrates is presented. Two complementary kinetic equations, one each for substrate utilization and methane production, are derived. The model equations take into account extracellular hydrolysis of complex substrates and consider the hydrolyzed products as the limiting substrates for cell growth and product formation according to Monod kinetics. The equations are applied to the experimental data on the anaerobic digestion of a variety of feeds reported in the literature with reasonable agreement of the predicted and observed values. When substrate hydrolysis is poor and rate limiting, the equations are reduced to a Contois-type equation. On the other hand, if no hydrolysis of the substrate is involved as in the case of simple and soluble substrate, the equations take the form of a Monod-type relationship. Thus, the equations represent a generalized kinetic expression for fermentation relating the Monod and Contois equations, which hold good in two extreme cases.