Role of Short-Range Forces in Particle Detachment during Filter Backwashing

Abstract

Considerable research on the process of filter backwashing with fluidization has been done during the last two decades, but there is little fundamental understanding of the mechanisms of particle detachment from filter media. The overall goal of the research reported in this study is to gain a fundamental understanding of the mechanisms by which colloidal-sized particles detach from larger grains. The theoretical part of this study has focused on calculating the interaction forces between a single particle and a collector surface coated with a deposited layer. The interaction forces that would control particle detachment include van der Waals attractive forces, electrical double-layer interactive forces, Born repulsive forces, and structural forces. The experimental studies were performed on laboratory scale model systems with well-defined spherical particle suspensions. Latex particles (5 μm) were filtered in a packed bed column of glass beads. The detachment of particles during filter backwashing under fluidization conditions was studied in relation to the chemical characteristics of the backwash water. The experimental results indicated that higher pH and lower ionic strength enhance detachment during backwashing. Theoretical force calculations based on Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory alone could not explain detachment behavior. Inclusion of short-range forces in the calculations qualitatively explains the experimental results.