Bubble-particle collision efficiency in rotating dissolved air flotation in paper-recycling wastewater treatment: experiments and modeling

Paper-recycling mills are serious environmental threats due to their high water consumption and highly polluted wastewater. Particles are the most common issue of paper-recycling wastewater, and their removal is of particular interest for recycling. Rotating dissolved air flotation (RDAF) is a system for separating particles, and the mixing zone of this system, where particles collide with air bubbles, is the most important part. As a widely used system in various industries, RDAF has rarely been investigated in the literature in terms of its optimization and efficiency improvement. In general, RDAF processes are basically the same as those of conventional dissolved air flotation (DAF) systems; thus the results of RDAF can be applied to other DAF systems. In this work, the mixing zone of a full-scale RDAF for the paper-recycling mill wastewater treatment was investigated to predict particle-bubble collision efficiency, consisting of both particle-bubble transport and attachment, and the diameter of the formed particles in different turbulence conditions. ANSYS CFX R18.0, mathematical modeling, and experimental analysis were simultaneously conducted in this research. Based on experimental operation, four scenarios including flow rates and the discharge condition of effluent into the mixing zone were studied. Bubbles with sizes of 60, 80, and 100 μm, the turbulences were calculated. The obtained particle-bubble collision efficiencies indicated the diameter of output particles ranged from 50 to 300 μm. Also, the static light scattering test was performed to determine the particle sizes. The modeling and experimental results both showed that the collision efficiency was higher with the production of larger particles when valves #3 and #4 were opened in the mixing zone.