Modelling and simulation of anaerobic stratified biofilm for methane production and prediction of multiple steady states

Abstract

The present work deals with modelling of anaerobic metabolism in a stratified biofilm consisting of two distinct layers of microorganisms in a fixed film reactor. The model is applied to steady state conditions and is based on substrate utilization kinetics and mass transport. The model assumes that the substrate is first metabolized by the outer acidogenic layer and the subsequent products are metabolized by the inner layer of methanogens to produce finally methane and carbon dioxide. The model considers slab geometry of the biofilm, Monod kinetics for growth rate of acidogens and the inhibition effect of volatile fatty acids on growth rate of methanogens. The diffusion equations are solved simultaneously using a modified form of the technique of orthogonal collocation on finite elements to obtain concentration profiles and the rate of methane production per unit volume of the biofilm is determined. Computer simulations are carried out to study the effect of various parameters on the rate of methane production and to investigate the existence of multiple steady states. The simulation shows that for a range of parameters values three steady states exist. It is shown that the upper steady state with highest rate of methane production may lead to unsatisfactory reactor performance.