Seok Dockko

Zeta potential measurement of bubbles in DAF process and its effect on the removal efficiency

Dissolved Air Flotation (DAF) process has proved its efficiency in water treatment process and has gained much interest, however, the research to investigate the process from the fundamental characteristics of bubbles and particles has been limited.In this research, the electrostatic nature of both bubble and particle/floc was focused. The zeta potential of bubbles is measured by a system composed of a measuring cell, microscope, video camera and a video image analyzer. The kaolin particle was measured by a commercial zeta meter. The zeta potential variation of bubbles and floc with pH are presented.The effects of several operational parameters in DAF process are investigated on a batch DAF reactor, microscope and video system. The effect of pH on the removal efficiency of DAF is presented. The effect of mixing time is presented. The effect of zeta potential of floc and bubbles with the addition of coagulant is presented. It is found that all the phenomena can be explained primarily from the electrostatic nature of bubble and particle and secondarily from the effect of particle size.The findings from this research will be helpful to understand and explain the process better and possibly can be used to modify and improve the DAF process. Dissolved air flotation, zeta potential, bubble, kaolin, removal efficiency, pH, mixing time.

Calculation of collision efficiency factor by trajectory analysis in dissolved air flotation

In order to calculate the collision efficiency factor of bubble and particle (αbp), trajectory analysis is performed. The formulation and equations are adopted from the trajectory analysis developed to calculate the collision efficiency factor in differential sedimentation (αos). The most recently developed hydrodynamic equation and interparticle force based on DLVO theory are included. The effect of each governing parameter is calculated. Although the inclusion of electric repulsion as a function of surface charge of particle and ionic strength of suspension is possible, only hydrodynamic and attraction forces are considered in this research to simulate the case when the bubble-particle system is destabilized. The collision efficiency factor of bubble and particle (αbp) is found to be functions of bubble diameter, particle diameter, Hamaker constant, and particle density. The result ofαbp shows similar trend with publishedαDs and experimental results.αbp increases as bubble size decreases at constant size ratio, as particle size ratio approaches unity and as Hamaker constant increases. There is only a slight effect of particle density onαbp.

Experimental verification of collision efficiency factor in DAF

The collision efficiency factor of bubble and particle αbp in dissolved air flotation (DAF) can be calculated theoretically by trajectory analysis, which takes into account both hydrodynamics and inter-particle forces. To determine the theoretically optimum particle size for any given bubble size, a collision efficiency diagram for DAF was developed where collision efficiency is contoured on a plane of particle and bubble sizes for different conditions of particle zeta potential. A set of experiments tested the validity of the suggested collision efficiency diagram, and examined whether pretreatment is important and why slight coagulant overdosing and shorter flocculation times are generally preferred in DAF, both current, accepted practices. Batch DAF reactors were used and kaolin samples were prepared from jar tests using different alum dosages and flocculation times. The particle size distribution, particle zeta potential, and turbidity removal in each experiment were measured, as were bubble size and zeta potential. The results agreed well with the predictions of the collision efficiency diagram and explained current practices. A collision efficiency diagram identifies the pretreatment goal, i.e., tailoring of the optimum characteristics required of particles (zeta potential and size) under existing operational bubble characteristic.