Intermittent filtration of bacteria and colloids in porous media

Abstract

[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,

Occurrence and Persistence of Bacterial Pathogens and Indicator Organisms in Beach Sand along the California Coast

“…During wetting, indicators and pathogens were mobilized in the microcosms, perhaps via thin film expansion, air-water interface scouring, and shear mobilization (4,18), suggesting that a fraction of these organisms are capable of being transported to adjacent waters. These mobilized organisms may enter the water column directly (over-beach transport) (74,77) or may be transported through the unsaturated zone of the beach to the water table (through-beach transport) (7).…”

Removal of bacterial indicators and pathogens from domestic wastewater by the multi-soil-layering (MSL) system

“…The adsorption phenomenon is expected to occur at the soil mixture layers in the MSL system. Adsorption is mainly controlled by the surface characteristics of the porous medium, the HLR, the pH, the water content and the surface characteristics of bacterial cells (Auset et al 2005). Filtration and adsorption are the first mechanisms of bacterial removal, followed by other elimination mechanisms such as microbial degradation and the natural death of bacteria.…”

Review on subsurface colloids and colloid-associated contaminant transport in saturated porous media

“…A comprehensive amount of literature exists on the topic of colloid mobilization and transport in model and natural systems [7,10,14,22,24,27,41,87,88]. Considerable advances have been made on the prediction of colloidal transport using experimental laboratory and field studies as well as numerical models [87,89,90].…”

“…Considerable advances have been made on the prediction of colloidal transport using experimental laboratory and field studies as well as numerical models [87,89,90]. Laboratory migration experiments [88,90,91] have demonstrated that colloids can migrate with natural flow velocities, although compared to dissolved tracers, their migration is much more sensitive to flow velocity and flow path geometry [92].…”