MATHEMATICAL-MODELING OF AN ANAEROBIC BUTYRATE DEGRADING CONSORTIA - PREDICTING THEIR RESPONSE TO ORGANIC OVERLOADS

The steady-state and dynamic response of a butyrate-using methanogenic consortia has been modeled as three sequential bacterial growth reactions. The model simulates the concentration of fermentation intermediates (acetate and hydrogen), incorporating their inhibition of butyrate oxidation and stimulation of methanogenesis. The hydrogen-producing acetogenic conversion of butyrate is modeled as a product-inhibited or reversible, modified Monod reaction, while hydrogen-using and acetoclastic methanogenesis are simple Monod reactions. The growth yield coefficient for each growth reaction is variable and dependent on the Gibbs free energy of the energy reaction, which in turn depends on the time-varying concentrations of reactants and products. These substrate utilization and growth reactions are incorporated into mass balances for a completely mixed, fluidized bed reactor, resulting in a series of coupled ordinary differential equations that were solved with numerical integration software. The model was calibrated with experimental data from a high-rate laboratory reactor; a single aet of biological kinetic parameters was found to successfully reproduce steady-state, step-loading, and pulse-loading results involving butyrate, acetate, and hydrogen or formate substrates. The response for the consortia was also simulated for a wide range of steady-state and step-loading conditions to better understand the interactions in this microbial consortia that lead to process stability or instability.