MICROBIAL CLOGGING OF SATURATED SOILS AND AQUIFER MATERIALS - EVALUATION OF MATHEMATICAL-MODELS

Bacterial reductions of the saturated hydraulic conductivity of natural porous media appear to be caused by a wide range of mechanisms, few of which have been carefully studied. Nevertheless, a number of mathematical models have been developed in recent years to describe the microbial clogging process, based on the assumption that bacterial cells form impermeable biofilms uniformly covering pore walls. In the present study, two independent sets of experimental data available in the literature are used to test the existing bioclogging models. To broaden the scope of the assessment, an additional model, initially developed to describe the deep filtration of suspended colloids, is also included in the comparisons. Analysis of the experimental data reveals a clear relationship between the texture of a porous medium and the ability of a given level of biomass to reduce its saturated hydraulic conductivity; at equal biomass, clogging is much more pronounced in fine-textured materials than in coarse-textured ones. In addition, the results of the model comparisons suggest that none of the existing models can predict satisfactorily the saturated hydraulic conductivity reductions observed in fine sands, whereas they fare somewhat better in coarser materials. It is argued that this inadequacy of existing models is due to the continuous biofilm assumption on which they are founded. Indeed, a simplistic model that assumes the biomass to be distributed as plugs instead of as continuous biofilms produces quantitatively much improved predictions of the saturated hydraulic conductivity reductions. Reference is made to the consequences of this observation in terms of future research.