Shape factor analysis of granular filter media and its effects on settling velocity and stratification post-backwash

摘要

Research on irregularly-shaped granular filter media has been sparse. Previous studies attempted to identify a common shape factor and incorporate it into settling velocity models for spherical media. This common shape factor was frequently called sphericity. However, the use of a sphericity factor results in inaccurate calculations of the actual particle diameter and surface area due to the irregular nature of particles. Through this research, the shape of granular filter media, anthracite specifically, was analyzed in order to better understand the controlling factors of shape as it relates to settling velocity and stratification post-backwash. Media grains were measured utilizing a three- dimensional, perpendicular axis approach and tested in a zero-flow settling column. It was found that the smallest dimension is the strongest predictor of settling velocity. This is because the drag surface area, or the perimeter surface area of the particle falling parallel to the direction of the fall, changes as this smallest dimension changes altering the drag forces on the falling particle. Rectangular aluminum bars were employed as model particles to better understand the results seen in irregular-shaped anthracite and confirmed the relationship between the smallest dimension (or height) and settling velocity.;Additionally, it was shown that anthracite does not stratify in the same manner as sand following backwash. Instead, there was significant evidence to support that the settling velocity of anthracite is strongly influenced by shape variations instead of a single size measurement, whereas the settling velocity of sand was more strongly controlled by size since there was significantly less variation in the shape of these grains. When comparing stratification of anthracite to sand, anthracite only showed stratification of 10-15% of the total number of grains in the filter column while sand showed 91% stratification.