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141g

Particle and Bacterial Deposition: Role of Gravity

Gexin Chen, Yongsuk Hong, and Sharon Walker. Chemical & Environmental Engineering, University of California, Riverside, Bourns Hall B365, 900 University Avenue, Riverside, CA 92521

The role of gravitational forces on the deposition of 0.5, 1.1, 1.83 μm carboxylate-modified polystyrene latex microspheres and bacterium Escherichia coli D21g has been evaluated using a parallel plate flow chamber system. This experimental system utilized an inverted and an upright optical microscopes attached with image-capturing devices to directly observe and determine the deposition kinetics onto glass surfaces located at the top and bottom of the flow chamber where gravity was observed to minimize and enhance colloid deposition, respectively. Deposition kinetics was quantified at two ionic strengths (0.01 and 0.1M KCl) under electrostatically unfavorable and favorable attachment conditions and at two fluid velocities (0.13 mm/s and 6.6 mm/s), simulating the range of velocities from groundwater flow to rapid granular filtration. Comparing the particle deposition kinetics onto the top and bottom surfaces under identical flowing exposure time, fluid chemistries and hydrodynamic conditions, results showed that significant differences were observed between two surfaces, suggesting that gravity was a significant driving force for the initial stages of colloidal deposition. The only exception to this was 0.5 μm particles at the high fluid velocity conditions, at which deposition onto top and bottom surfaces were virtually identical. To further understand the influence of gravitational forces on particle deposition in the paral lel plate flow chamber system, the experimental results were compared with both bacterial deposition rates and theoretical calculations of particle deposition on surfaces for diffusion-limited transport or by sedimentation. Finally, numerical simulations were conducted to further interpret and confirm our experimental data. The results of the experimental and theoretical studies will presented and the implications for groundwater quality and industrial applications will be discussed.