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Dissolved Gas Flotation

  Flotation
Dissolved air flotation in solids-liquid separation is implemented through the introduction of micron-sized gas (usually air) bubbles into the liquid phase. As the micro bubbles attach to the solids, the resultant buoyant force of the combined solid-gas causes the matrix to rise to the surface where it is collected.




DAF Application

Clarification   Thickening
Where subnatant quality is the main performance factor, as in the case of refinery, meat packing, meat rendering, and other "oily" wastewater.   Where solids concentration is the main performance factor, as in the case of biological sludge and mining/metallurgical sludge.




Typical Applications

Algae Removal   Fiber Recovery
Textiles   Snack Foods
Rendering   Poultry Processing
Automotive   Oil Refinery
Pulp and Paper   Canning
Ballast Water Treatment   Municipal Wastewater
Water   Dairy Wastewater
Truck Wash   Bakery Wastewater




Dissolved Air Flotation (DAF)

Dissolved Air Flotation (DAF) is a method for separating and removing suspended solids or a liquid from wastewater. The driving force behind dissolved air flotation is the micron sized air bubbles. Typical bubble size is between 20 to 80 microns in diameter. Separation is accomplished by attaching micron sized air bubbles to the suspended solids particles thereby reducing the specific gravity of the agglomerate to less than that of water.

In a dissolved air flotation unit, air is dissolved under pressure in clean liquid, usually recycled water from the DAF unit. This pressurized stream is sent to the flotation (separation) tank where the pressure is released and combined with a raw feed stream for intimate interaction. The water becomes supersaturated as the pressure is released and the air dissolved at atmospheric pressure comes out of the solution in the form of micro bubbles.

Thousands of micro bubbles attach to the solids in the waste stream entering the distribution feed well. As the micro bubbles rise, they collide with the suspended solids and become attached. The net specific gravity of these agglomerates of particles and air are now less than that of water. The buoyant agglomerate and air mixture is driven to the surface where a sludge blanket forms above the water level, allowing water to drain from the sludge blanket. The clear effluent liquid is then withdrawn from the separation tank for disposal or reuse.



Pressurization System

The pressurization system is the heart of the DAF system. The unit of operation allows for a portion of the clear effluent (recycle) to be siphoned off by the recycle pump. The recycle flow is introduced in the saturation tank for the purpose of dissolving air into the recycle stream. Compressed air at 40 to 80 psi is also introduced to the saturation tank where the air and clear liquid are allowed to interact for the purpose of dissolving the air into the recycle stream.



Back Pressure Valve

Once the air has been dissolved into the recycle stream, the pressure must be maintained until the point of discharge into the flotation (separation) tank. The saturation pressure and flow rate are regulated by opening and closing the back pressure valve. In addition to flow rate and pressure regulation, the back pressure valve also provides a shearing action as the pressure is decreased to atmospheric pressure (14.7 psi), causing the air to come out of solution and form micro bubbles.



Solids Removal

Sludge thickening occurs within the blanket, which is generally 12 to 24 inches deep. The uppermost sludge layer will form a pudding like consistency. As the skimmer travels around the tank, the top layer of the solids is pushed up the sludge collection box. This operation insures that only the most concentrated solids are collected. Some solid particles with a high density, such as grit, will settle to the bottom of the flotation tank and should be periodically removed.

Sludge depth is controlled by increasing or decreasing the speed of the skimmer. By increasing the skimmer speed, the sludge depth will decrease. Conversely, by decreasing the skimmer speed, the sludge depth will increase.



Control Factors

Sludge concentration in a DAF unit typically ranges from 3 to 5 weight percent (wt%) solids. If operated properly, the DAF system can achieve at least 85 percent solids removal without chemical addition. Chemicals (i.e., polymer, coagulants) will improve solids removal greater than 90 percent; however, solids concentration may not improve significantly. Important sludge characteristics include sludge type, sludge age, and biological make-up.

Air to solids ratio is the quantity of air introduced into the recycle stream. This is a critical parameter to consider when operating the DAF unit. The more air dissolved in the recycle stream, the greater the number of micro bubbles released in the flotation tank. The more micro bubbles produced, the greater the probability of bubble particle contact. This in turn will allow the DAF operation to be more efficient.



Recycle Rate

The rate of the recycle (centrifugal) pumps are directly dependant on the pressure maintained within the saturation tank. The pressure within the saturation tank can be increased or decreased by opening or closing the back pressure valve. By increasing or decreasing the pressure, a change in recycle flow will result. The optimum recycle flow and retention tank pressure are determined through experimentation.

Up to now, Sciential has discussed a typical DAF system; however, new pump technology will allow for elimination of several DAF components, thus making installation and maintenance a snap. Please note that Sciential can provide both the conventional and alternative pump technology for dissolved air flotation.



DAF Anatomy

Conventional DAF Anatomy

(Click to see a larger version(in a new window).
  Pump Technology DAF Anatomy

(Click to see a larger version.(in a new window).




Factors Affecting Equipment Performance

The factors listed below must be considered equally, not individually. These factors have a synergistic effect on float solids concentration and solids capture.

Solids Loading Rate (lbs solids/hr/ft²)

Usual Solids Loading Rate (0.5 to 1.5)  
Solids Loading Rate greater than 1.5 gpm/ft2 usually causes float removal and solid capture problems.


Hydraulic Loading Rate (gpm/ft²)

Usual Hydraulic Loading Rate (0.5 to 1.5)  
Hydraulic Loading Rate greater than 1.5 gpm/ft2 cause large deviations from plug flow (turbulence), causing a reduction in float solids concentration and solids capture.


Air to Solids Ratio (lb air/lb solids)

Usual air to solids ratio (0.01 to 0.03)  
Air to solids ratio less than 0.01 usually does not supply enough air to effectively float solids (testing is recommended to quantify low air to solids ratio).
   
Air to solids ratio greater than 0.03 is often wasteful and can cause excessive bubbling at the inlet of the flotation (separation) tank which can reduce float solids concentration and solids capture.


Type of Solids

Grease, oil, scum  
Biological sludges
  - Aerobically Digested
  - Trickling Filter
  - Nitrified
  - Waste Activated Sludge


Chemical Conditioning

Oily Waste Clarification
  - Alum and/or lime and/or polymer
 
Sludge Concentration
  - Polymer dosages (3-10 lbs polymer/ton dry solids)
  - Testing should be conducted to determine the correct chemical dose and addition point


Sludge Volume Index (SVI)

Best results when SVI less than 150  
SVI greater than 250 will result in low solids concentration

Float solids concentration generally decreases with increasing SVI  
 




Bench DAF Test Apparatus

Scientialís bench-scale DAF kits are used to conduct tests on a variety of waste streams. Typical applications include:

Algae Removal   Fiber Recovery
Textiles   Snack Foods
Rendering   Poultry Processing
Automotive   Oil Refinery
Pulp and Paper   Canning
Ballast Water Treatment   Municipal Wastewater
Water   Dairy Wastewater
Truck Wash   Bakery Wastewater

The use of the bench unit has been utilized as a tool to optimize the full-scale DAF operation or determine the full-scale DAF unit. The use of the bench test unit will aid in determining the recycle rate (air to solids ratio), effluent clarity, and chemical dosage.


Click here for instructions.




Pilot Dissolved Air Flotation Unit

Scientialís pilot DAF unit features a revolutionary air saturation system that eliminates the need for ancillary equipment found in standard saturation systems (note the small footprint in the pictures to the right).


Unit Operation Of The Air Saturation System

Clarified wastewater from the DAF unit is recycled to the dissolved air flotation unit by an DAF Air Saturation System designed to operate at a pressure of 90 to 100 pounds per square inch (psi). Atmospheric air is dissolved into the recycle stream via the pump suction. Air flow to the pump is regulated by a needle valve rotameter.

As the air is introduced into the pump suction, a specially designed impeller shears the air into smaller air bubbles and under an increased pressure the air is dissolved into the clarified wastewater. This simplified unit of operation eliminates the need for an external air source (compressor), saturation tank, and air control panel, thus reducing space requirements, operation/maintenance, and capital equipment costs.


Equipment Operating & Design Parameters

Influent Flow Range: 5-10 gpm
Recycle Flow Rate: 1-10 gpm
Approximate Weight: 1,000 lbs.
Power Supply: <10 amps, 480 volt, 3 phase
Unit Dimensions: 3ft diameter


Shipment

Shipping arrangements for the equipment can be made upon request. All shipments are F.O.B. Monument, Colorado. Pilot equipment is skid mounted and can be shipped by flatbed truck.