Particle visualisation - a tool for determination of rise velocities

Particles from a post denitrifying Kaldnes Moving Bed™ Process were studied using an optical borescope, a visualisation technique that provides opportunities for both qualitative and quantitative studies of the separation mechanism in Dissolved Air Flotation (DAF). Rise rates for particle/bubble aggregates were estimated showing great variability. Two groups of aggregates were distinguished; relatively small flocs (<100 μm) with single bubbles attached rising comparatively slowly and large flocs (>100 μm) with several bubbles attached rising very fast. The high rise rates for large aggregates are discussed, possibly explained and suggested as the reason for the effective separation of large particles noticed in previous studies. Removal efficiency of different size categories of particles in DAF were investigated on the basis of particle size analysis indicating increased separation efficiency with increasing particle size.