MODELING BACTERIAL DETACHMENT DURING TRANSPORT THROUGH POROUS-MEDIA AS A RESIDENCE-TIME-DEPENDENT PROCESS

Bacterial transport through porous media was modeled using detachment functions that incorporate the dependence of detachment rate on bacter ial residence time on the collector. Model parameters and the relative merit of alternative forms for the detachment function were evaluated on the basis of comparisons between model simulations and experimenta lly derived bacterial breakthrough and elution curves. Only detachment functions that provided an initial period in which bacteria were rapi dly released, followed by slow bacterial detachment, were able to repr oduce the elution portion of the breakthrough curves. In optimal simul ations, 90% of the bacteria that were captured by the porous medium de tached within 1 min of attachment. Experiments involving saturated flo w through columns packed with sand indicated that the time to achieve complete breakthrough was inversely related to the influent bacterial concentration. On this basis and because of the relatively slow approa ch to breakthrough that was typically observed in transport experiment s, it was hypothesized that the experimental medium contained a number of preferred attachment sites that must be essentially filled before breakthrough is achieved. Only when such (irreversible) sorption sites were included in the model formulations was it possible to produce tr ansport simulations that matched both the breakthrough and elution por tions of the empirically derived curves. It is concluded that both a t ime-dependent detachment function and a degree of sorption site hetero geneity are required to describe bacterial attachment and detachment d uring transport as observed in our laboratory.