[1] Colloidal dispersion in porous media is a consequence of the different paths and velocities experienced by the colloids. We examined at the pore scale the effect of particle and pore size on colloid dispersion using water-saturated micromodels. The micromodels were produced with polydimethylsiloxane (PDMS), using a soft photolithography technique that allows creating transparent patterns that have dimensions in the range of those existing at the pore space. Four sizes of colloids were transported at several total pressure differences, and image analysis was used to determine particle trajectories, residence times, and dispersion coefficients through the micromodels. The magnitude of the dispersion at any given flow rate was found to be controlled by the pore-space geometry and the relative size of colloids with regards to pore channels. Dispersion coefficient and dispersivity decrease with increasing colloid size. Dispersivity is thus not just a function of pore geometry but depends on colloid characteristics. Because of their size, larger colloids travel in the center streamlines, leading to faster velocities, less detours, and thus lower range of transit times. These findings emphasize the role of particle and pore size on colloidal dispersion and have significant implications for predicting the movement of colloids through saturated porous media.

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[1] Colloid transport was studied at the pore scale in order to gain insight into the microscale processes governing particle removal. Monodisperse suspensions of colloids and water-saturated micromodels were employed. Experiments were carried out for different particle sizes, grain surface roughness, solution ionic strength, and flow rates. Straining and attachment were observed and measured by tracking the trajectory and fate of individual colloids using optical microscopy. Classical filtration theory proved appropriate for throat to colloid ratios (T/C) larger than 2.5 but did not take into account the possibility of straining that becomes an important capture mechanism for smaller T/C ratios. Spatially within the porous medium, straining occurred within the first 1-2 pore throats, while interception and attachment was seen from the inlet to the first 6-10 pore spaces, depending on particle size. Once a particle passed the initial region, the probability of attachment was very small. Colloid attachment increased with increasing solution ionic strength or decreasing flow rate, whereas straining was mainly independent of flow rate. Surface roughness of the grains also played a significant role in colloid capture, increasing collision efficiency by a factor of 2-3. The mechanisms of removal and the spatial distribution of colloid retention differed noticeably as a function of the T/C ratio. Micromodel visualizations clearly showed that physical straining and the effect of surface roughness should be taken into account when predicting the transport of colloids in saturated porous media.Citation: Auset, M., and A. A. Keller (2006), Pore-scale visualization of colloid straining and filtration in saturated porous media using micromodels, Water Resour. Res., 42, W12S02,

[1] Intermittent filtration through porous media used for water and wastewater treatment can achieve high pathogen and colloid removal efficiencies. To predict the removal of bacteria, the effects of cyclic infiltration and draining events (transient unsaturated flow) were investigated. Using physical micromodels, we visualized the intermittent transport of bacteria and other colloids in unsaturated porous media. Column experiments provided quantitative measurements of the phenomena observed at the pore scale. Tagged Escherichia coli and a conservative tracer (NaI) were introduced in an initial pulse into a 1.5 m sand column. Subsequent hydraulic flushes without tagged bacteria or tracer were repeated every 4 hours for the next 4 days, during which outflow concentrations were monitored. Breakthrough behavior between colloids and dissolved tracer differed significantly, reflecting the differences in transport processes. Advancement of the wetting front remobilized bacteria which were held in thin water films, attached to the airwater interface (AWI), or entrapped in stagnant pore water between gas bubbles. In contrast, the tracer was only remobilized by diffusion from immobile to mobile water. Remobilization led to successive concentration peaks of bacteria and tracer in the effluent but with significant temporal differences. Observations at the pore-scale indicated that the colloids were essentially irreversibly attached to the solid-water interface, which explained to some extent the high removal efficiency of microbes in the porous media. Straining, cluster filtration, cell lysis, protozoa grazing, and bacteriophage parasitism could also contribute to the removal efficiency of bacteria.Citation: Auset, M., A. A. Keller, F. Brissaud, and V. Lazarova (2005), Intermittent filtration of bacteria and colloids in porous media, Water Resour. Res., 41, W09408,

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