Effects of floc and bubble size on the efficiency of the dissolved air flotation (DAF) process

Abstract

Dissolved air flotation (DAF) is a method for removing particles from water using micro bubbles instead of settlement. The process has proved to be successful and, since the 1960s, accepted as an alternative to the conventional sedimentation process for water and wastewater treatment. However, limited research into the process, especially the fundamental characteristics of bubbles and particles, has been carried out. The single collector collision model is not capable of determining the effects of particular characteristics, such as the size and surface charge of bubbles and particles. Han has published a set of modeling results after calculating the collision efficiency between bubbles and particles by trajectory analysis. His major conclusion was that collision efficiency is maximum when the bubbles and particles are nearly the same size but have opposite charge. However, experimental verification of this conclusion has not been carried out yet. This paper describes a new method for measuring the size of particles and bubbles developed using computational image analysis. DAF efficiency is influenced by the effect of the recycle ratio on various average floc sizes. The larger the recycle ratio, the higher the DAF efficiency at the same pressure and particle size. The treatment efficiency is also affected by the saturation pressure, because the bubble size and bubble volume concentration are controlled by the pressure. The highest efficiency is obtained when the floc size is larger than the bubble size. These results, namely that the highest collision efficiency occurs when the particles and bubbles are about the same size, are more in accordance with the trajectory model than with the white water collector model, which implies that the larger the particles, the higher is the collision efficiency.

Influência do tamanho de flocos e da concentração de ar dissolvido na eficiência da flotação

“…Com relação às técnicas atualmente empregadas na captura e análise de imagens de flocos e microbolhas de ar formados durante o processo de flotação, tem se verificado que ao longo dos últimos anos ocorreu um aumento significativo no número de pesquisas desenvolvidas, dada a importância que essas partículas exercem sobre a cinética do processo de flotação (BACHE & RASOOL, 2001;RODRIGUES & RUBIO, 2003;MORUZZI, 2005;LEE et al, 2007;HAN et al, 2007;KWAK et al, 2009;SOHN et al, 2009;OLIVEIRA et al, 2010).…”

Dissolved air flotation: A novel approach to recovery of organosolv lignin

“…Increased pressure results in a greater number of bubbles and smaller average bubble diameter. Image analysis has shown that average bubble size is 71 m using saturation pressure of 29 psi and 32.4 m using a saturation pressure of 73 psi [22]. Thus, higher pressure results in greater frequency of attachment and greater buoyancy.…”

Optimization of dissolved air flotation technique in harvesting microalgae from treated wastewater without flocculants addition

“…The significance of microalgae concentration is practically clear, since higher concentrations enhance the probability of collision between bubbles and microalgae cells, and eventually more harvesting. 51 In addition, higher microalgae concentrations facilitate the intrinsic agglomeration of small cells and increase the chance of bubble attachment (principally, addition of flocculants is for the same purpose). From the operational point of view, the microalgae concentration should be at the highest possible value.…”

“…It was demonstrated that floc size has a great impact on the algal removal efficiency, because the most efficient collision occurs when bubble and floc sizes are the same. 51 It was found that alum concentration of 60 mg/l was adequate to achieve the best floc size that needs to be equal to bubble size. 40 Saturator pressure was also reduced to 2 atm from 4 atm based on the usage of a commercial frother.…”