Optimization of Solids Separation in Dissolved Air Flotation

Abstract Sizes of flocs were analyzed to identify characteristics of the particle size distribution optimal for separation by dissolved air flotation (DAF). Optical microscopes and two particle counters were used for floc sizing. A Brightwell Technologies particle counter was found to provide floc size measurements in agreement with improved microscopic methods. The particle counter provided distribution of flocs with sizes down to 1 micron (µm). This allowed for inclusion of flocs with size ranging from 5 to 1 µm, which were excluded from the analyses in the earlier study. Four alum dosages were applied: 15, 25, 40, and 60 mg/L. The turbidity and colour of the DAF effluent at alum dosages of 25, 40, and 60 mg/L were very similar. However, the analysis of the flocs in the treated effluent revealed that, at the alum dose of 60 mg/L, particle removal was the best. Therefore, this dosage was selected as optimal for the solid/liquid separation process. The average size of coagulation flocs at 60 mg/L was approximately 30 µm, and was equal to the estimated size of air bubbles produced by the saturator. Therefore, this study confirms the finding of the earlier work claiming that the optimum DAF performance is attained when the mean floc size and the bubble size are equal. Similar size of floc and bubble indicates that flocs act predominantly as nuclei for bubble formation. This finding contributes to the knowledge of mechanisms of floc air bubble attachment in DAF.

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Principles and applications of dissolved air flotation

Water Science & Technology ◽

10.2166/wst.1995.0512 ◽

1995 ◽

Vol 31 (3-4) ◽

pp. 1-23 ◽

Cited By ~ 58

Author(s):

James K. Edzwald

Keyword(s):

Bubble Formation ◽

Treatment Plant ◽

Drinking Water Treatment ◽

Plant Design ◽

Dissolved Air Flotation ◽

Floc Size ◽

Hydrophobic Particles ◽

Air Flotation ◽

The Impact ◽

Dissolved Air

Principles of dissolved air flotation (DAF) discussed include: bubble formation and size, bubble-particle interactions, measures of supplied air, and modeling of the reaction and clarification zones of the flotation tank. Favorable flotation conditions for bubble attachment or adhesion to particles requires a reduction in the charge of particles and production of hydrophobic particles or hydrophobic spots on particle surfaces. A conceptual model for the bubble-particle reaction zone based on the single collector collision efficiency is summarized and discussed. An alternative modeling approach is considered. Clarification or separation zone modeling is based on particle-bubble agglomerate rise velocities. The application of DAF in drinking water treatment is addressed beginning with summaries of design and operating parameters for several countries. DAF should not be considered as a separate process, but integrated into the design and operation of the overall treatment plant. This concept shows that flocculation ahead of DAF has different requirements regarding floc size and strength compared to sedimentation. The efficiency of DAF in removing particles and reducing particle loads to filters needs to be integrated into DAF plant design. The impact on filtration performance is illustrated. Finally, fundamental and applied research needs are addressed.

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Effects of Al3 + and hydraulic characteristics on the removal and behaviour of particles in dissolved air flotation

Water Science & Technology Water Supply ◽

10.2166/ws.2006.743 ◽

2006 ◽

Vol 6 (3) ◽

pp. 89-95

Author(s):

Jungsoo Mun ◽

Sungwon Park ◽

Mooyoung Han

Keyword(s):

Turbulent Flow ◽

Removal Efficiency ◽

Operating Conditions ◽

Point Of Zero Charge ◽

Hydraulic Characteristics ◽

Dissolved Air Flotation ◽

Floc Size ◽

Saturated Water ◽

Air Flotation ◽

Dissolved Air

The removal efficiency of the dissolved air flotation (DAF) process to separate particles from water and wastewater depends on the size and zeta potential of bubbles and particles, the solution and operating conditions, hydraulic characteristics, etc. The effects of aluminium ions and turbulent flow-produced when air-saturated water was spouted into the reactor in the DAF process, on removal and, particle behaviour were on investigated. When bubble size was similar to particle size (10–50 μm), the maximum removal efficiency was 92% in a Kaolin solution of 10−3 M Al3 + without pre-treatment for flocculation process, and, as time passed, the floc size was observed to increase at a pH of 8, which was the condition of high removal efficiency as seen through image analysis. When the air-saturated water was spouted into the reactor, the size of particle at p.z.c. (point of zero charge) seemed to increase to form a floc due to collision effects caused by turbulent flow. Consequently, floc formation by turbulent flow in the reactor seemed to positively affect removal efficiency.

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Dissolved air flotation model for drinking water treatment

Canadian Journal of Civil Engineering ◽

10.1139/l99-071 ◽

2000 ◽

Vol 27 (2) ◽

pp. 373-382 ◽

Cited By ~ 3

Author(s):

Ayman R Shawwa ◽

Daniel W Smith

Keyword(s):

Water Treatment ◽

Kinetic Model ◽

Contact Zone ◽

Removal Rate ◽

Particle Removal ◽

Dissolved Air Flotation ◽

First Order ◽

Air Flotation ◽

Particle Attachment ◽

Dissolved Air

In this study, a kinetic model that describes bubble-particle transport and attachment in the contact zone of dissolved air flotation (DAF) process is presented. The kinetic model, which is based on the assumption that the contact zone is analogous to a chemical reactor, describes the particle removal rate as a first-order reaction with respect to the concentration of particles. It identified important parameters, such as the bubble-particle attachment efficiency (αPB). The theoretical first-order particle removal rate constant (kP), based on the mathematical model, was determined by varying αPB from 0.1 to 1.0. On the other hand, the experimental kP value was determined by measuring the mean residence time, the degree of mixing of particles, and the particle removal efficiency of the contact zone by conducting pilot-scale DAF experiments at different hydraulic loading rates and recycle ratios. The experimentally determined first-order particle removal rate constant was equal to the theoretical kP value when the bubble-particle attachment efficiency (αPB) was in the range of 0.35 to 0.55, which is considered typical for water treatment applications. The kinetic model can be used to predict DAF removal efficiencies provided that αPB is determined for the system under investigation and that the operating conditions applied in this research are used. However, independent experiments are required to verify the applicability of the proposed model.Key words: algae, bubble, coagulation, dissolved air flotation, flocculation, kinetic model, particle size distribution, water treatment.

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The role of particle size and density in dissolved air flotation and sedimentation

Water Science & Technology ◽

10.2166/wst.1997.0114 ◽

1997 ◽

Vol 36 (4) ◽

pp. 177-189 ◽

Cited By ~ 5

Author(s):

A. Vlaški ◽

A. N. van Breemen ◽

G. J. Alaerts

Keyword(s):

Kinetic Modelling ◽

Single Cells ◽

Process Efficiency ◽

Laboratory Model ◽

Reservoir Water ◽

Dissolved Air Flotation ◽

Floc Size ◽

Air Flotation ◽

G Value ◽

Dissolved Air

Conventional (sedimentation) and advanced (dissolved air flotation) treatment were studied in the context of removal of the single cells form of the cyanobacteria Microcystis aeruginosa. This cyanobacterium species is recognised as an ideal surrogate for process removal efficiency assessment of particles of the problematic size range (3-10 m). The agglomeration (coagulation/flocculation) phase has been indicated as essential and determining the down-stream process efficiency, hence it is a prerequisite for process improvement. Relevant process parameters have been addressed on a laboratory (model water) and pilot plant (reservoir water) scale, including the influence of coagulant (FeCl3) dose, coagulation pH, flocculation time, energy input (G value), single stage versus tapered flocculation and application of cationic polymer as coagulant aid. The process efficiency was assessed as a function of the preceeding agglomeration (coagulation/flocculation) phase and the obtained particle (floc) size distributions. The particle (floc) size - density relationship was addressed in the context of more accurate process kinetic modelling.

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Solid/liquid separation behavior of alum and polyaluminium chloride coagulation flocs

Water Science & Technology Water Supply ◽

10.2166/ws.2006.010 ◽

2006 ◽

Vol 6 (1) ◽

pp. 87-93 ◽

Cited By ~ 1

Author(s):

B. Gorczyca ◽

G. Zhang

Keyword(s):

Liquid Separation ◽

Dissolved Air Flotation ◽

Gravity Settling ◽

Mean Size ◽

Air Flotation ◽

Solid Liquid ◽

Solid Liquid Separation ◽

Separation Behavior ◽

Polyaluminium Chloride ◽

Dissolved Air

Properties of alum and polyaluminium chloride (PACl) flocs were analyzed in order to explain solid/liquid separation behavior of these aggregates in dissolved air flotation and gravity settling. PACl flocs settle better and are less sensitive to changes in water temperature than alum flocs. Therefore, PACl flocs may be more suited for gravity separation, especially in cold waters, and alum flocs may be preferred for flotation. At an optimum coagulant dose for dissolved air flotation the logarithmic mean size of alum flocs was close to the size of the air bubbles (30 μm) and the proportion of flocs smaller than 20 μm was about 30.5%.

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The influence of floc size and hydraulic detention time on the performance of a dissolved air flotation (DAF) pilot unit in the light of a mathematical model

Bioprocess and Biosystems Engineering ◽

10.1007/s00449-014-1221-6 ◽

2014 ◽

Vol 37 (12) ◽

pp. 2445-2452 ◽

Cited By ~ 6

Author(s):

R. B. Moruzzi ◽

M. A. P. Reali

Keyword(s):

Mathematical Model ◽

Dissolved Air Flotation ◽

Detention Time ◽

Floc Size ◽

Pilot Unit ◽

Air Flotation ◽

Dissolved Air

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

Water Science & Technology ◽

10.2166/wst.2007.779 ◽

2007 ◽

Vol 56 (10) ◽

pp. 109-115 ◽

Cited By ~ 21

Author(s):

Mooyoung Han ◽

Tschung-il Kim ◽

Jinho Kim

Keyword(s):

Bubble Size ◽

Saturation Pressure ◽

Collision Efficiency ◽

Water And Wastewater Treatment ◽

Dissolved Air Flotation ◽

Floc Size ◽

Recycle Ratio ◽

Air Flotation ◽

The 1960S ◽

Dissolved Air

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.

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