Mechanics of adhesion-aggregation-detachment in water filtration

摘要

Access to drinking water is one of the grand challenge of the 21st century listed by the National Academy of Engineering. The world's water supplies are facing new threats; affordable, advanced technologies could make a difference for millions of people around the world. Lack of clean water is responsible for more deaths in the world than war. Natural cataclysm, manmade disasters, combined with the blooming of polluting industries in the Third World lead to inevitable invasion of pathogens and bacteria in fresh water supplies. It is therefore necessary to understand the mechanism of bacterial adhesion and detachment in water filtration. When bacteria rich water flows through percolated channels in a column packed with sand, the filtering efficiency, a, is influenced by ionic concentration, density and size of collector sand grains, temperature etc. Our previous work shows to be directly proportional to the pseudo Tabor parameters which depends on the range and magnitude of the intersurface force, size and elastic modulus of the particles. The classical colloidal filtration theory provides the standard model generally adopted by the environmental engineering community. One distinct prediction is that a is independent of the flow rate, i.e. no matter how fast the aqueous medium flows past the filter column, the proportion of bacteria adsorped on the sand surface remains constant. Our latest measurement and theoretical model, however, indicate otherwise. Based on typical Derjaguin-Landau-Vervey-Overbeek (DLVO) intersurface forces, the attachment and detachment of cylindrical bacteria depends significantly on the flow rate. By comparing the detachment torque due to the surface attraction and hydrodynamic torque due to the viscous shearing stress, the fate of a particle either staying on or detaching from a collector could be determined. Our new model accounts for the complex interplay of geometry, dimension, and elastic modulus of the particles, the first and second minima and intervening repulsive barrier of the DLVO interaction and the resulting adhesion energy, as well as the influence of ionic strength. The commercial multi-physics software COMSOL is adapted to account for the meridional and azimuthal distribution of adhered bacterial cells on a spherical collector sand grain. The numerical value of is determined from the first principles, and is shown to be consistent with our experimental measurement using a standard packed column test. The new model is capable to explain the underlying physics of water filtration in relation to bacterial adhesion.