Effect of media grain shape on physical capture of particles in a filter bed

Granular filters incorporated in stormwater conveyance systems can mitigate the impacts of non-point source stormwater by removing waterborne contaminants. Granular materials with a variety of grain shapes are commercially available for these filters. The shape of the media grains affects the filtration performance. Two mechanisms were investigated and demonstrate that using media grains that are less spherical improves filtration performance.; One-contact removal represents storage of particles on a single isolated media grain. A scalable "Small Pile Model" uses hyperbolic curves for the shape of the toe of slope and peak. The curves can be added by recognizing that the peak hyperbola requires a displacement correction at the toe of slope. The maximum size of the pile increases when the toe of slope is stable. This provides greater one-contact storage for non-spherical media grains as observed in filtration columns. This model is applicable to all granular piles, not just particle piles on filter bed media.; Three-contact removal occurs at the narrow openings (pore throats) along the pore tracks through the granular media. Three-dimensional measurements made from sectioned columns of granular media provide data to test a Stepwise Particle Movement Model. The model is conceptually similar to that of Silveira (1965) but modified to predict maximum particle storage in a filter bed. From this model, the minimum pore throat size of spherical grains was shown to adversely affect filter performance.; Both mechanisms show theoretically that angular grains store more particles than spherical. The theoretical predictions are consistent with results measured from filtration columns of various crushed and rounded granular media.