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

Abstract

Paper-recycling mills are serious environmental threats due to their high consumption of water and highly polluted wastewater. Particles are the most common issue of paper-recycling wastewater, and their removal is important 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 that has not been studied.In this work, the mixing zone of a full-scale RDAF in the paper-recycling mill wastewater treatment was investigated to predict particle-bubble collision efficiency 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 and the turbulences were calculated. The particle-bubble collision efficiencies indicated the output particles diameter ranging from 50 to 300 μm. Also, an experimental particle size test (SLS) was performed. 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.