Shape Factor Analysis of Granular Filter Media and Its Effects on Settling Velocity and Stratification Post-Backwash
1 online resource (117 pages) : PDF
University of North Carolina at Charlotte
Research on irregularly-shaped granular filter media has been sparse. Previousstudies attempted to identify a common shape factor and incorporate it into settlingvelocity models for spherical media. This common shape factor was frequently calledsphericity. However, the use of a sphericity factor results in inaccurate calculations of theactual particle diameter and surface area due to the irregular nature of particles. Throughthis research, the shape of granular filter media, anthracite specifically, was analyzed inorder to better understand the controlling factors of shape as it relates to settling velocityand stratification post-backwash. Media grains were measured utilizing a three-dimensional, perpendicular axis approach and tested in a zero-flow settling column. Itwas found that the smallest dimension is the strongest predictor of settling velocity. Thisis because the drag surface area, or the perimeter surface area of the particle fallingparallel to the direction of the fall, changes as this smallest dimension changes alteringthe drag forces on the falling particle. Rectangular aluminum bars were employed asmodel particles to better understand the results seen in irregular-shaped anthracite andconfirmed the relationship between the smallest dimension (or height) and settlingvelocity.Additionally, it was shown that anthracite does not stratify in the same manner assand following backwash. Instead, there was significant evidence to support that thesettling velocity of anthracite is strongly influenced by shape variations instead of asingle size measurement, whereas the settling velocity of sand was more stronglycontrolled by size since there was significantly less variation in the shape of these grains.When comparing stratification of anthracite to sand, anthracite only showed stratificationof 10-15% of the total number of grains in the filter column while sand showed 91%stratification.
ANTHRACITEBACKWASHFILTER MEDIASHAPE FACTORSTRATIFICATION
Bowen, JamesKeen, Olya
Thesis (M.S.)--University of North Carolina at Charlotte, 2016.
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