Modelling microbial population dynamics in multispecies biofilms including Anammox bacteria

A 1D mathematical model for analysis and prediction of microbial interactions within multispecies biofilms including Anammox pathway is presented. The model combines the related processes of organic carbon oxidation, denitrification, nitrification and Anammox and phenomena of substrate reaction and diffusion, biomass growth and advection, detachment. The biofilm growth process is governed by nonlinear hyperbolic PDEs and substrate dynamics are dominated by semilinear parabolic PDEs. It follows a complex system of PDEs on a free boundary domain. Equations are integrated numerically by using the method of characteristics as strongly suggested by the qualitative analysis of the free boundary value problem. Mass conservation equation plays an important role in checking the accuracy of simulations. The model has been applied to simulate Anammox competition and to evaluate the influence of substrate diffusion on microbial stratification. Specific scenarios are analyzed. The results reveal that in a thick multispecies biofilm, including heterotrophic, aerobic autotrophic nitrifying and Anammox bacteria, oxygen diffusion limitation determines the formation of both aerobic and anoxic microenvironments favouring interspecies competition. In contrast, oxygen excess causes a disturbance on microbial interactions leading to Anammox bacteria loss. The model predictions may help engineers or operators to have a better insight into biofilm dynamics in order to optimize process design or practical operation.