SCALING BACTERIAL FILTRATION-RATES IN DIFFERENT-SIZED POROUS-MEDIA

Aquifer sediments contain a wide distribution of particle sizes, but o nly a single collector diameter (d) can be used in a filtration equati on. To establish a method for selecting a characteristic d when media are composed of different sized particles, we measured bacterial reten tion in columns packed with either crushed quartz sand (separated into three different size ranges) or borosilicate glass beads. The best me thods for choosing d were those that produced nearly constant collisio n efficiencies (alpha's). Characteristic diameters included: d(10) (10 % of all particles were smaller), d(90) (90% of all particles were sma ller), d(a) (arithmetic mean), and d(g) (geometric mean), where all di ameters were calculated using number, area, and volume size distributi ons. Bacterial alpha's decreased in proportion to the distance travele d in the packed bed, and were scaled by the number of bacteria-sedimen t collisions using a dimensionless collision number (xi). These Compar isons indicated that characteristic diameters based on the smaller par ticles (d(a) and d(g) using number distributions, and d(10) using a vo lume distribution) most accurately described bacterial transport in th e different-sized porous media.