Limits To Growth And What Keeps A Biofilm Finite

Two of the factors, shear erosion and diffusive mass transfer, which limit the growth of heterogeneous biofilms are considered. For permeable beds of particulates, with a regulated throughflow, equating shear induced erosion and biofilm growth, leads to estimates of biofilm thickness and activity which conform with experimental measurements. In the more open environments of pipes and channels, increased thickness of biofilm is not directly balanced by increased cell erosion from the biofilm surface. However increasing thickness leads to growth limitations as diffusion limits the rate of mass transfer to cells deep in the film. For heterogeneous biofilms , consisting of complex clusters intersected by channels, mass transfer into the biofilm is by a combination of advective flow in the channels and diffusive transfer in clusters. In this paper we have considered mass transfer into simplified cluster forms, that is cylinders and hemispheres. Using the concept of critical dimension we have explored some of the implications of these simplified structures. We discuss the limitation to this approach as fluid shear alters the form of these simplified clusters. The viscoelastic properties of the biofilm clusters are being investigated and should allow better prediction of the effect of lateral shear on simple forms. The advection in biofilm channels and the related mass transfer processes needs further investigation.