Dissolved air flotation

Dissolved air flotation (DAF) is a water treatment process that clarifies wastewaters (or other waters) by the removal of suspended matter such as oil or solids. The removal is achieved by dissolving air in the water or wastewater under pressure and then releasing the air at atmospheric pressure in a flotation tank or basin. The released air forms tiny bubbles which adhere to the suspended matter causing the suspended matter to float to the surface of the water where it may then be removed by a skimming device.^[1][2][3]

Dissolved air flotation is very widely used in treating the industrial wastewater effluents from oil refineries, petrochemical and chemical plants, natural gas processing plants, paper mills, general water treatment and similar industrial facilities. A very similar process known as induced gas flotation is also used for wastewater treatment. Froth flotation is commonly used in the processing of mineral ores.

In the oil industry, dissolved gas flotation (DGF) units do not use air as the flotation medium due to the explosion risk. Natural gas is used instead to create the bubbles.

Process description

A typical dissolved air flotation unit (DAF)

DAF unit with a capacity of 20 m³/h , visible also: flocculant preparation station and pipe flocculator

Modern DAF units using parallel plate technology are quite compact.
Picture shows a 225 m³/h DAF.

The feed water to the DAF float tank is often (but not always) dosed with a coagulant (such as ferric chloride or aluminum sulfate) to flocculate the suspended matter.

A portion of the clarified effluent water leaving the DAF tank is pumped into a small pressure vessel (called the air drum) into which compressed air is also introduced. This results in saturating the pressurized effluent water with air. The air-saturated water stream is recycled to the front of the float tank and flows through a pressure reduction valve just as it enters the front of the float tank, which results in the air being released in the form of tiny bubbles. The bubbles adhere to the suspended matter, causing the suspended matter to float to the surface and form a froth layer which is then removed by a skimmer. The froth-free water exits the float tank as the clarified effluent from the DAF unit.^[1]

Some DAF unit designs utilize parallel plate packing material, lamellas, to provide more separation surface and therefore to enhance the separation efficiency of the unit.

DAF can be categorized by Circular (more efficient) and Rectangular (more residence time). The former type requires just 3 minutes and the example is Wockoliver DAF system, whereas Rectangular requires 20 to 30 minutes the typical example can be syskill DAF system. One of the bigger advantage of Circular is its Spiral Scoop.

See also

• Industrial water treatment
• Wastewater
• Industrial wastewater treatment
• API oil-water separator
• Flotation process
• Induced Gas Flotation

References

1. ^ ^{a b} Beychok, Milton R. (1967). Aqueous Wastes from Petroleum and Petrochemical Plants (1st ed.). John Wiley & Sons. LCCN 67019834. 
2. ^ Lawrence K. Wang, Yung-Tse Hung, Howard H. Lo and Constantine Yapijakis (2004). Handbook of Industrial and Hazardous Wastes Treatment (2nd ed.). CRC Press. ISBN 0-8247-4114-5. 
3. ^ Kiuru, H.; Vahala, R., eds (2000). "Dissolved air flotation in water and waste water treatment". International conference on DAF in water and waste water treatment No. 4, Helsinki, Finland. IWA Publishing, London. ISBN 1-900222-81-7. 

External links

• Treatment and Disposal of Ship-Generated Solid and Liquid Wastes (REMPEC Regional Marine Pollution Emergency Response Centre for the Mediterranean Sea, Project MED.B4.4100.97.0415.8, April 2004)
• http://www.wockoliver.com/daf.html http://www.wockoliver.com/daf.html]