Dynamics of Deep-Bed Filtration: Velocity, Depth, and Media

Abstract

The dynamics of particle capture and head-loss development were studied in laboratory filters over extended periods. Particle-size distributions and pressures were measured above the sand media and at three depths. Conditions included filtration velocities of 1.8 and 5.5 mm/s and mean media sizes of 0.39, 0.78, and 1.85 mm. The suspension was sedimentation effluent from a softening water-treatment plant. Ripening, the improved removal efficiency with time, was extensive for small particles, but breakthrough, the decreased removal with time, was more significant for intermediate and larger particles. Detachment of previously retained particles was demonstrated to contribute substantially to breakoff; apparently, flocs are formed on the media surface by the sequential capture of particles and break off together. Theory suggests that higher velocities and larger media sizes can be offset quantitatively, with respect to particle removal, by proportionately greater depths; data from this research support these ideas. Comparison with previous experiments with glass spheres as the media but otherwise identical conditions showed that, for sand, initial removal was better, ripening was less extensive (perhaps because of initially better removal), and detachment was more extensive.