Bacterial deposition in porous media related to the clean bed collision efficiency and to substratum blocking by attached cells

Deposition of Pseudomonas putida mt2 and Rhodococcus strain C125 during transport through columns packed with Teflon grains was investigated. Deposition was analyzed in terms of the de an bed collision efficiency alpha(0) (the probability of a cell to attach upon reaching a cell-free substratum) and the surface area blocked by attached cells. Blocking was quantified by a blocking factor B, the ratio of the blocked area per cell to the geometric area of a cell. At an average interstitial fluid velocity of 200 mu m s(-1), alpha(0) is close to unity (0.83 +/- 0.01) for both strains, indicating that cell-solid interactions are almost completely favorable for deposition. Values for B of 1.6 +/- 0.1 and 12.0 +/- 0.8 were obtained for Ps. putida and Rhodococcus strain C125, respectively. This difference is consistent with differences in cell size and cell-cell repulsion, which were both smaller for Ps. putida than for Rhodococcus strain C125, At coverages close to saturation, multilayer adhesion and/or pore clogging occurs for the weakly blocking Pseudomonas cells but not for the strongly blocking Rhodococcus cells. The collision-blocking concept succesfully explains the breakthrough of Bacillus cells in coarse sand columns, as reported by Lindqvist and Enfield, for which adhesion is less favorable (alpha(0) = 0.10 +/- 0.01) and blocking is relatively strong (B = 5.8 +/- 0.8), The general conclusion is that deposition of microbes during their transport through coarse grain media is adequately described by the collision-blocking model in cases of strongly blocking cells or weakly blocking cells at low coverage conditions.