Floc morphology and cyclic shearing recovery: comparison of alum and polyaluminum chloride coagulants |
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Authors: | McCurdy Kevin Carlson Kenneth Gregory Dean |
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Affiliation: | Department of Civil Engineering, Colorado State University, Fort Collins, CO 80523-1372, USA. |
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Abstract: | This study investigated the floc formation and re-aggregation potential for alum, polyaluminum chloride (PACl) and a blend of these coagulants. Bench-scale testing included floc morphology characterization for well-developed floc, post-shear floc, and non-settleable (filter influent) floc. Different applications of coagulants were observed to produce non-settleable floc that was morphologically different. The alum treatment had a decrease in average floc size from coagulated to non-settleable conditions, whereas the PACl and PACl/alum treatments resulted in similar sized floc between these processes. Zeta potential distribution measurements showed that the alum treatment resulted in a negative shift from coagulated to non-settleable conditions whereas the PACl and PACl/alum treatments had no significant shift. A photometric dispersion analyzer (PDA) was employed to compare the differences between coagulant treatments with respect to shear induced aggregate breakup and recovery. The PDA allowed a dynamic monitoring of initial floc aggregation and measured the degree of recovery from cyclic shearing. The degree of recovery from shearing was greatest for the PACl/alum treatment likely as a result of increased collision efficiency due to more effective charge neutralization. |
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Keywords: | Drinking water Floc PACl Alum Morphology Shear strength |
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