DYNAMICS OF DEEP-BED FILTRATION - VELOCITY, DEPTH, AND MEDIA

The dynamics of particle capture and head-loss development were studie d in laboratory filters over extended periods. Particle-size distribut ions and pressures were measured above the sand media and at three dep ths. 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 sedim entation effluent from a softening water-treatment plant. Ripening, th e improved removal efficiency with time, was extensive for small parti cles, but breakthrough, the decreased removal with time, was more sign ificant for intermediate and larger particles. Detachment of previousl y retained particles was demonstrated to contribute substantially to b reakoff; apparently, flocs are formed on the media surface by the sequ ential capture of particles and break off together. Theory suggests th at higher velocities and larger media sizes can be offset quantitative ly, with respect to particle removal, by proportionately greater depth s; data from this research support these ideas. Comparison with previo us experiments with glass spheres as the media but otherwise identical conditions showed that, for sand, initial removal was better, ripenin g was less extensive (perhaps because of initially better removal), an d detachment was more extensive.