Effect of salinity on air dissolution, size distribution of microbubbles, and hydrodynamics of a dissolved air flotation (DAF) system

Abstract

In the present study, air dissolution, size distribution and hydrodynamics of microbubbles in a dissolved air flotation system were experimentally investigated. The amount of dissolved air was calculated by Henry’s laws of gas dissolution and compared with measured values from the liquid displacement technique. The measured air concentrations were 23.2 and 10.1 mg/l for freshwater and saline water (30 g/L), respectively, which were much lower than those calculated by Henry’s equations. Shadow imaging technique was applied to measure the size distributions of microbubbles for saline and freshwater. Whilst the size of microbubbles for freshwater were between 40–250 μm, the corresponding values for the saline water were smaller and varied between 20–160 μm. Particle image velocimetry (PIV) was also applied to map flow patterns and velocities of microbubbles for saline and freshwater. The results indicated that the flow patterns were mostly affected by the flow itself rather than the salinity and the presence of microbubbles since there was not a noticeable change between single-phase (flow without microbubbles) and two-phase flows. While the range of the velocities was between 0–64 mm/s for the freshwater, it did not exceed 48 mm/s for the saline water.