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Dissolved Air Flotation Theory of Operation

Background
The implementation of DAF systems around the world appears to be steadily developing, with applications becoming more varied in nature. Applications employing DAF systems range from potable water treatment to industrial effluent treatment as well as sludge (e.g. activated sludge) thickening. The use and development of DAF has particularly been expanding in the industrial field with a variety of applications currently being employed. DAF systems are commonly applied to remove suspended solids, fats, oils and greases and the associated BOD and COD from a variety of waters and wastewaters.

A key application of DAF units includes the removal of free and emulsified hydrocarbons from petrochemical and similar wastewaters upstream of biological processes. This has been done in an effort to prevent toxic or inhibitory materials from hindering the biological processes downstream of the DAF unit. Other industrial applications of DAF systems include the treatment of concentrated fish farming wastewaters, the pre-treatment of food and meat processing effluents and the treatment of effluents generated by the pulp and paper industries. In certain circumstances, DAF can also be a substitute for gravity settlement of solids generated by a biological treatment process.

Advantages linked with DAF systems include the fact that they are high-rate processes when compared with more traditional gravity-based settlement systems. This means that a reduction in space requirements can be achieved, and in terms of sludge thickening, a thicker sludge can be produced. Additionally, DAF systems offer the operator some degree of flexibility, subject to design, with regard to the system's operating parameters.

Basic Operation
DAF is a purely physical process which operates based on a reasonably simple design philosophy. Incoming effluent may require pre-treatment as necessary, e.g. the addition of chemical coagulant(s) and/or flocculent(s) may be required with associated mixing and coagulation/flocculation stages. Adjustment of pH may also be a consideration to ensure optimum conditions for coagulation and flocculation.

DAF systems may be designed for pressurization and air dissolution of the total flow or, more commonly, the incoming effluent enters the flotation vessel where it comes into contact with a portion of recycled, treated effluent (sometimes termed whitewater). The percentage of the total effluent flow into which air is dissolved under pressure and subsequently recycled will be determined by several factors. Increasing the pressure within the vessel where the air is being dissolved ensures that a higher concentration of air dissolves into the liquid phase than is possible at atmospheric pressure. Once this portion of saturated effluent enters the flotation tank, the pressure is released back to atmospheric pressure. This immediately results in the recycled flow becoming supersaturated, resulting in the generation of micro-bubbles as the dissolved air comes back out of solution. These bubbles attach to, and form within, the solids or chemical flocs entering the vessel, causing them to float to the surface where they are retained and subsequently removed by a mechanical skimmer.

In the case of rectangular flotation tanks, the skimmer mechanism consists of a series of paddles or 'flights' which run on a belt or chain and skim just below the surface of the tank removing the 'float' into a trough for further treatment or, in some instances, recovery of materials. The alternative of circular DAF tanks may incorporate rotating skimmer blades feeding a 'float' trough or involve use of a circulating, revolving scoop. In cases where some gross solids may be present and there is risk of gradual accumulation of sludge build-up on the flotation tank floor, the design may also incorporate a floor scraper.

There are limits to what can efficiently be removed by applying flotation technology. It would seem a logical step therefore to apply DAF systems to effluents where the solids present are of approximately neutral or perhaps even positive buoyancy so that the bubbles produced are working with gravity rather than against it. Under these circumstances DAF systems would, on first approximation, appear to be a process worthwhile of consideration should standard sedimentation systems not provide the required removal of contaminants.

Key Design Parameters
Inevitably the design details for any given effluent treatment system will be dependent on a number of specific factors. There are, however, several key design parameters which are commonly applied when considering and assessing the design of a DAF system. The parameters listed below are, where applicable, accompanied by design figures. These figures are provided as an indication of the range of figures one may encounter.

The basic principles of operation of the dissolved air flotation system are evolved from:

  1. Henry's Law

  2. Nucleus Theory

  3. Stokes Law

Henry's Law
W. Henry discovered in the year 1803 that the amount of gas absorbed by a liquid is directly proportional to the pressure of the gas.  As a result, the partial pressure of a gas in equilibrium with a solution is equal to a constant times its concentration in the solution, or:

P = CX

Where:

P = Partial pressure of a gas in equilibrium with a solution

C = Constant

X = Concentration in the solution

The constant "C" is different for each system and varies with temperature.  Simply stated, at a constant temperature, the greater the pressure the more air can be absorbed into the water.  As an example, if you double the pressure on a liquid, you double the solubility of the solution.  At atmospheric pressure and 68° F, theoretically only 0.2 lbs of air are soluble in 1,000 gallons of water.  However, at 40 PSI pressure and 68° F, theoretically 0.755 lbs of air are soluble in 1,000 gallons of water.  In addition, at 65 PSI and 68° F, theoretically 1.1 lbs of air are soluble in 1,000 gallons of water.  Therefore, if air is injected into a fluid under pressure, the fluid will absorb more of the air than if the fluid were not under pressure.  Conversely, as the fluid pressure is relieved, under proper hydraulic conditions, the air comes out of solution in minute bubbles or molecular form.  This regularly occurs in carbonated beverages.  Before a carbonated beverage is opened the pressure of gas is not visually apparent; however, after the cap is removed with the subsequent loss (or equalization) of pressure, the gas burst from solution and rises to the surface in bubble form.

Nucleus Theory
The second primary principle of operation of the dissolved air flotation system is the Nucleus Theory which is that a gas coming out of a solution from a liquid will preferentially form a bubble on a finite nucleus.  In other words, molecules tend to attach themselves to a nucleus (the contaminant in waste water).  In seconds, a sufficient number of air molecules have collected to form "life preservers" around contaminant nuclei and float the contaminant to the water's surface.

Stokes Law
The combining of the sufficient number of air molecules with the contaminants (solids) to form the "life preservers" results in the combined air/solids mass having a specific gravity which is less than the liquid.  Therefore, the solids that would eventually settle or perhaps remain in suspension float to the top of the flotation cell where they can be easily removed from the top of the flotation cell. 

In 1845, an English mathematician named George Stokes first described the physical relationship that governs the settling solid particles in a liquid (Stokes' Law, 1845). This same relationship also governs the rising of light liquid droplets within a different, heavier liquid. This function, simply stated is:

terminal, fall or settling velocity

terminal, fall or settling velocity

acceleration of gravity

acceleration of gravity

particle diameter

particle diameter

density of medium

density of medium (e.g. water, air, oil)

particle density

particle density

viscosity of medium

viscosity of medium

Where:

  1. viscosity of medium (μ) 

  2. fall, settling or terminal velocity (Vt) 

  3. acceleration of gravity (g) 

  4. density of particle (ρp)

  5. density of medium (ρm) 

  6. particle diameter (d)

A negative velocity is referred to as the particle (or droplet) rise velocity.

Assumptions Stokes made in this calculation are:

  1. Particles are spherical

  2. Particles are the same size

  3. Flow is laminar, both horizontally and vertically. Laminar flow in this context is equal to a Reynolds number less than 500.

The variables are the viscosity of the continuous liquid, specific gravity difference between the continuous liquid and the particle, and the particle size.

The rise rate of oil droplets is also governed by Stokes' Law. If the droplet size, specific gravity, and viscosity of the continuous liquid are known, the rise rate may be calculated.

Calculation of rise rate by this method is a gross simplification of actual field conditions because oil droplets are not all the same size, and they tend to coalesce into larger droplets. Furthermore, inevitable turbulence within a separator makes an orderly rise of very small droplets impossible.

Droplets will rise following Stokes' Law so long as laminar flow conditions prevail. When the particle size exceeds that which causes a rise rate greater than the velocity of laminar flow, the flow around the droplet (as they rise) begins to be turbulent. Particles of this size and larger do not rise as rapidly as would be expected from calculations based on Stokes' Law because of the hydrodynamic drag. They do, however, rise very quickly in relationship to smaller droplets, and will be removed by a properly designed separator.

Very small particles, such as those of 10 microns (micrometers) and less in diameter, do not rise according to Stokes’ Law (or hardly at all) because the random motion of the molecules of the water is sufficient to overcome the force of gravity and therefore they move in random directions. This random motion is known as Brownian Motion. Fortunately, the volume of a droplet decreases according to the cube of the diameter, so these very small droplets tend to contain very little oil by volume.  And unless there are large, large quantities of very small droplets (such as would be present with an emulsion or created by using a centrifugal pump to pump the water) they contain negligible amounts of oil.

Rate of Rise Theory
The separation process can be accomplished and enhanced in a variety of ways and with a variety of equipment configurations.  One common way to improve separation without increasing the need for floor space is to install a multiple plate pack that will create many separation chambers in one vessel, each with a shallow depth.  This is done by adding a series of appropriately spaced plates.  The flow is distributed through the plates and the rate of rise of the droplet is applied to the application.  The advantages of multiple plates is that surface area is increased without requiring additional floor space.

The most efficient oil/water separators are designed to exploit Stokes' Law and the rate of rise for a given droplet.  In order for a particle to be removed according to Stokes' Law, the separator must conform to several critical design criteria:

  1. Laminar flow conditions must be achieved (Reynolds “Re” number less than 500) in order to allow a droplet to rise.

  2. Hydraulic flow path must distribute influent AND effluent flow in such a way as to ensure complete utilization of the coalescing surface area, in order to take full advantage of the plate pack coalescing surface area. Design of the flow distribution must be such as to prevent any hydraulic short circuiting of the plate pack, which would be detrimental.

  3. Horizontal flow-through velocities in the separator must not exceed 3 feet per minute, or 15 times the rate of rise of the droplets - which ever is smaller - per the American Petroleum Institute’s Publication 421 of February 1990.

  4. Coalescing surface area must not become clogged during use, which would adversely alter flow characteristics, possibly creating hydraulic short circuiting and increasing the “Re” number past 500.

  5. If inclined parallel plates are used, they must be at the proper angle of repose to allow solids to settle in a liquid medium (ideally 55-60 degrees from horizontal) , and they must be smooth enough to allow the unhindered migration of a solid particle to the bottom of the plate pack and an oil droplet to the top of the plate pack, where they will exit the waste stream.

There are several important factors to consider in efficient parallel plate oil/water separator design.  As stated earlier, the parallel plate must have a smooth surface in order to allow unhindered migration of the droplet to the top of the pack and solid particles to the bottom.  Another enhancement is to use cross corrugated plates.  Corrugated plates provide additional coalescing surface area, within the same volume, in the form of crests and valleys that aid in the migration of the droplet to the top of the pack.  As the droplets impinge on the crests and valleys and begin to migrate toward the top of the plate pack, they will coalesce with other droplets, thus creating larger droplets with increased mass which will improve their rate of rise.

Hydro-Flo Technologies employs parallel plates in a most efficient manner.  We use ultra-smooth surface, cross corrugated plates, that are arranged at a 60 degree angle of inclination. This promotes self- flushing and efficient droplet agglomeration, which improves the migration of droplets toward the top of the plate pack, and sludge to the bottom of the plate pack and out of the waste stream into the sludge settling chamber.  Our influent and effluent flow distribution systems are carefully designed to ensure efficient, even and complete usage of the entire plate pack and to prevent short circuiting.  In summary, our parallel plate oil/water separators are thoughtfully designed to ensure reliability and performance.  We also offer custom design services to meet specific requirements.

Example of Henry's Law and Nucleus Theory Working Together
A procedure referred to as a "pressure baum" provides a good example of Henry's Law and the Nucleus Theory working together as they do in the Dissolved Air Flotation System.  A sample of effluent is poured in a vessel and air is pumped in until 40 PSIG, or higher, is achieved.  The combined gas (air), liquid and solids are agitated for approximately one minute.  At this time the solution is released to an open vessel and gas bubbles are created in the solution separation.  The bubbles and solids are combined and float to the surface, due to their reduced combined specific gravity.  This is exactly what happens, in simplified terms, in Dissolved Air Flotation Waste Water Treatment Systems. 

Air:Solids Ratio
The Air:Solids (A:S) ratio may be reported as a volume:mass ratio or a mass:mass ratio and will be application specific. To give an idea of the range of A:S ratios commonly applied, typical values range between 0.005 – 0.06 ml/mg which, at 20oC and atmospheric pressure (say 1.0133 bar) is equivalent to 0.006 mg – 0.072 mg of air per mg of solids to be removed.

Hydraulic Loading Rate
The DAF hydraulic loading rate is a measurement of the volume of effluent applied per unit effective surface area per unit time. This results in process design figures expressed as equivalent up-flow velocities with units of m/h. This figure should be application specific but as a general guide the figures which should be expected would be between 2 m/h and 10 m/h. A key consideration with regard to this design parameter is whether the loading rate includes the recycled volume as well as the influent wastewater volume being applied per unit area of the system.

Typical Solids Loadings
Solids loadings are normally given in units of mass per unit area per unit time (kg/m2.h). Typical figures encountered range from around 2 kg/m2.h up to15 kg/m2.h, although again the design will be application specific, depending on the nature of the solids to be removed and the extent to which chemical aids are used.

Recycle Ratio
The recycle ratio is determined as the fraction of the final effluent produced which is returned and saturated under pressure prior to entering the flotation vessel where the pressure is subsequently released and the bubbles are generated. The recycle ratio can vary immensely with recycle ratios being typically 15-50% for water and wastewater treatment application. However, for activated sludge flotation thickening, up to 150-200% recycle rates have been applied. Air dissolution rates are proportional to absolute pressure (i.e. system gauge pressure + atmospheric pressure) in accordance with Henry' Law of partial pressures of gases adjacent to liquids. Thus, for a given application, the higher the operating pressure of the air/water saturation vessel, the lower the required percentage recycle – and vice-versa. Operating pressures can therefore vary widely but are typically in the range 3-7 barg.

Saturation of Effluent
The production of saturated water from which the micro-bubbles are generated is normally achieved in two ways. The first which is common to potable water treatment involves passing the required flow of treated effluent through a packed bed system which is pressurized using a pump which is often a centrifugal pump. In systems where solids are likely to be encountered, e.g. sludge treatment, the saturation vessel is likely to be empty to prevent the fouling of any packing materials. The percentage of saturation which can be achieved will depend on the design of the system but, with good design, saturation efficiencies of up to 80-95% can be expected.

Flow Regime
To ensure that DAF systems operate as designed it is important to ensure that the system does not encounter sudden changes in the flow regime. For this reason some form of flow balancing or regulation is recommended to ensure a consistent flow rate. Another consideration is to develop a flow path through the flotation tank which ensures the maximum removal of solids via their entrainment in the air micro-bubbles generated.

Summary
DAF units can be applied successfully to a range of different waters, wastewaters and sludges and demonstrate certain specific advantages over more conventional solids removal processes. DAF plants can be designed to be small, compact and robust systems with a high rate of operation. DAF systems are capable of coping with reasonable variations in influent water quality, and to some extent variations in flow. Disadvantages of DAF systems may include the increased service and maintenance costs when compared with traditional sedimentation systems and the increased operating costs due to the energy requirements of the system.

Key References
Kiuru, H and Vahala, R (Editors). Dissolved Air Flotation in Water and Waste Water Treatment. Wat. Sci. Tech. 43 (8).

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