Simultaneous monitoring of flow patterns, and bubble, and plastics micro-particle characteristics in Dissolved Air Flotation (DAF)

Abstract

Dissolved air flotation (DAF) is an important water treatment process for removing suspended solids. Understanding the characteristic size and spatial distribution of both bubbles and suspended solids, including their hydrodynamics, within flotation tanks is key to optimising flotation performance. In this study, a continuously operating lab scale DAF tank was constructed to simultaneously monitor size distributions and motions of microbubbles (Ø = 67–86 ± 13–19 µm) and spherical polyethylene microplastic particles (Ø = 69.8 ± 5.3 µm) in unbuffered water at circumneutral pH without salt addition and with 0.5 g L−1 sodium chloride (NaCl) added. Microplastic particles were used for the first time in such a setup. Particle tracking showed different flow regimes in the separation zone, with short-circuiting flow paths varying at different flow rates. Within the range of flowrates employed, the bubble bed that formed was not prominent enough to promote stratified flow. The addition of NaCl greatly reduced microplastic particle counts within the separation zone during flotation. This new multi-factorial approach provides critical insights into the variations in DAF performance.