A kinetic model for dissolved air flotation in water and wastewater treatment

Abstract

A kinetic model for DAF is presented. The author's kinetic model consists of the equations for describing a process of bubble-floc collision and attachment in a mixing zone, and a rise velocity of bubble-floc agglomerates in a flotation tank. The attachment process is formulated on a population balance model with bubbles and flocs as a flocculation in a turbulent flow. The rise velocity of bubble-floc agglomerates is formulated with size of flocs and composition of flocs including the floc density function and attached bubble number. The experimental verification was carried out, using a batch flotation tested and a mini-plant with synthetic clay suspension and colored water. The results successfully verify the validity of the model. From a given condition such as floc size and attached bubble number, the rate and extent of removal by DAF can be readily assessed by the model. A single-collector collision model, often discussed in some occasions, seems to be not useful to describe the DAF process.

Comparing dissolved air flotation and conventional sedimentation to remove cyanobacterial cells of Microcystis aeruginosa

“…But large flocs are not necessary for flotation since bubbles and particles need to be attached, and particle-bubble agglomerate density should be reduced to less than water [21,45]. Literature agrees on the flocculation time reduction from 20 to 30 min in the conventional sequence (C/F/S) to 5-10 min in the DAF sequence (C/F/DAF) [3,20,22,23,25]. Edzwald et al [4] results showed that good flotation performance was achieved with either 8 or 16 min for flocculation, so shorter times 5-8 min should be adequate.…”

“…However, some authors disagree on the efficiency of these small size flocs in the C/F/DAF process. Fukushi et al [20] agreed that long flocculation is not needed, but they considered that larger flocs have a much higher collision efficiency, hence larger flocs should be prepared for effective DAF. Furthermore, Vlaski et al [24] concluded that the increase of the larger floc size fraction achieved by low flocculation G values resulted in relative floc density decrease, and in higher DAF efficiencies.…”

“…Therefore, there have been some studies regarding the optimisation of the operating conditions of the coagulation/flocculation prior to DAF, namely the flocculation velocity gradient (G) and time [3,[20][21][22][23][24][25].…”

Trajectory Analysis and Collision Efficiency during Microbubble Flotation

“…In dispersed air flotation the bubbles are on the order of 1 mm in diameter and the flow around the bubble can be modeled using potential flow theory. This paper considers the collision efficiency between a bubble and a particle in microbubble flotation with particular emphasis on dissolved air flotation used for potable water treatment where the particles to be removed are typically of the same size as the bubbles (1). In dissolved air flotation bubbles are produced by first saturating water with air at high pressure and then subsequently reducing the pressure which causes the supersaturated dissolved air to come out of solution in the form of microbubbles.…”

“…Finally, the resulting bubble-particle agglomerate must be positively buoyant and rise to the top of the flotation tank before bubble-particle detachment, or, in the case of a continuous throughput tank, before the bubble-particle agglomerate is carried out of the flotation tank in the effluent stream. The removal efficiency E of an isolated bubble-particle interaction is typically decomposed as (2) E ϭ E c E a ͑1 Ϫ E d ͒, [1] where E c represents the collision efficiency between a bubble and a particle, E a represents the probability that a given bubble-particle collision results in successful bubble-particle attachment, and E d represents the probability of bubble-particle detachment before the bubble-particle agglomerate rises to the foam layer. In this paper the focus is on modeling the close approach of the bubble and particle under the influence of gravity and interparticle forces and on calculating collision efficiencies.…”