Colloidal and Bacterial Deposition: Role of Gravity

The role of gravitational force oil the deposition of 0.5, 1.1, and 1.8 mu m carboxylate-modified polystyrene latex (CML) microspheres and bacterium Burkholderia cepacia G4g has been evaluated using a parallel plate now chamber system. This experimental System utilized all inverted and an upright optical microscope 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. Deposition kinetics was quantified at 10 mM KCI under electrostatically unfavorable and favorable attachment conditions and at two now rates (0.06 and 3 mL/min), simulating the range of flow velocities from groundwater to rapid granular filtration. Comparing the particle deposition kinetics on the top and bottom surfaces under identical flowing exposure time, fluid chemistries, and hydrodynamic conditions, results showed that significant differences were observed between the two Surfaces, Suggesting that gravity was a significant driving force for the initial stages of deposition of particles that were larger than 1 mu m size. Simulation results utilizing a particle trajectory model confirmed these experimental observations. This was further supported by additional deposition experiments with 1.1 mu m microspheres suspended in a deuterium oxide (D2O)/water mixture (heavy water) where the density of colloid and the Suspending heavy water were effectively the same. Under this condition, deposition rates were observed to be identical between the top and bottom Surfaces. Results from normal and heavy water solutions indicated that the greater deposition of colloidal particles larger than 1 mu m oil the bottom in normal water solutions is due to gravity. Finally, the experimental results were compared with deposition studies using smaller 0.5 mu m colloids as well as some theoretical calculations of expected rates of particle deposition.