Dissolved-Air Flotation: Theory and Practice

Abstract

This paper* presents a physical-mathematical model for dissolved-air flotation (DAF). The flotation model is an important aid for gaining insight into the effect of design parameters such as bubble diameter, particle size, bubble-volume concentration, contact time, temperature and surface load. The results with the flotation model were compared with the treatment results of existing Dutch flotation plants. The model distinguishes two separate mechanisms: the contact or filtration process and the separation process. The contact process describes the collision between a bubble and a particle on the analogy of the filtration theory. The efficiency of the filtration zone depends mainly on the bubble-volume concentration, contact time and temperature. During the separation process the bubble-particle agglomerates move upward and form a floating layer on the surface of the water. The floating velocity determines mainly the efficiency of the separation zone and appears to be dependent on especially the particle size. Larger particles result in higher floating velocities, through which a high surface load is possible.