STEADY-STATE SOLUTION OF A 2-SPECIES BIOFILM PROBLEM

Through a thorough investigation of the boundary conditions for a gene ral two-species biofilm model, a simple and fast method for solving th e steady-state case is developed and presented. The methods used may b e extended to biofilm models in which more than two species are consid ered. Four different sets of boundary conditions are possible for the two-species biofilm model. Each set is shown to be asymptotically stab le. A biofilm model describing the competition between autotrophic and heterotrophic bacteria and a biofilm model considering only Nitrosomo nas and Nitrobacter are used for illustration. A parameter L(crit), cr itical film thickness for bacterial coexistence, is introduced from wh ich criteria on the bulk concentrations for coexistence are derived. F rom these criteria it is seen that the thinner the biofilm, the more r estrictive the conditions are for steady-state coexistence. For thin b iofilms there may, in many cases, be no point in considering more than one species in the biofilm model. Furthermore, the gradients of the b acterial concentrations are in many cases negligible in thin biofilms, and the biofilm may then be assumed to be homogeneous. The criteria o n the bulk concentrations together with the four sets of boundary cond itions provide the necessary information for a direct solution of the steady-state two-species biofilm model by means of an ordinary differe ntial and algebraic equation solver. (C) 1996 John Wiley & Sons, Inc.