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Dissolved air flotation (DAF) is a
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 coagulate the colloidal particles and/or a flocculant to conglomerate the particles into bigger clusters.
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. Bubbles form at nucleation sites on the surface of the suspended particles, adhering to the particles. As more bubbles form, the lift from the bubbles eventually overcomes the force of gravity. This causes the suspended matter to float to the surface where it forms 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.
Some DAF unit designs utilize parallel plate packing material (e.g. lamellas) to provide more separation surface and therefore to enhance the separation efficiency of the unit.
DAF systems can be categorized as circular (more efficient) and rectangular (more residence time). The former type requires just 3 minutes. A particular circular DAF system is called "Zero speed", allowing quite water status then highest performances; a typical example is an Easyfloat 2K DAF system. The rectangular type requires 20 to 30 minutes. One of the bigger advantages of the circular type is its spiral scoop.
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) {{Separation processes Flotation processes Water treatment Waste treatment technology
water treatment
Water treatment is any process that improves the quality of water to make it appropriate for a specific end-use. The end use may be drinking, industrial water supply, irrigation, river flow maintenance, water recreation or many other uses, inc ...
process that clarifies wastewater
Wastewater is water generated after the use of freshwater, raw water, drinking water or saline water in a variety of deliberate applications or processes. Another definition of wastewater is "Used water from any combination of domestic, industrial ...
s (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
Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, ...
in a flotation tank 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.
Dissolved air flotation is very widely used in treating the industrial wastewater effluents from oil refineries
An oil refinery or petroleum refinery is an industrial process plant where petroleum (crude oil) is transformed and refined into useful products such as gasoline (petrol), diesel fuel, asphalt base, fuel oils, heating oil, kerosene, lique ...
, petrochemical
Petrochemicals (sometimes abbreviated as petchems) are the chemical products obtained from petroleum by refining. Some chemical compounds made from petroleum are also obtained from other fossil fuels, such as coal or natural gas, or renewable so ...
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
Froth flotation is a process for selectively separating hydrophobic materials from hydrophilic. This is used in mineral processing, paper recycling and waste-water treatment industries. Historically this was first used in the mining industry, wher ...
'' is commonly used in the processing of mineral ores.
In the oil industry
The petroleum industry, also known as the oil industry or the oil patch, includes the global processes of exploration, extraction, refining, transportation (often by oil tankers and pipelines), and marketing of petroleum products. The larges ...
, ''dissolved gas flotation
Dissolved gas flotation (DGF) systems are used for a variety of applications throughout the world. The process floats solids, oils and other contaminants to the surface of liquids. Once on the surface these contaminants are skimmed off and removed ...
'' (DGF) units do not use air as the flotation medium due to the explosion risk. Nitrogen gas
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at seventh ...
is used instead to create the bubbles.
Process description
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 coagulate the colloidal particles and/or a flocculant to conglomerate the particles into bigger clusters.
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. Bubbles form at nucleation sites on the surface of the suspended particles, adhering to the particles. As more bubbles form, the lift from the bubbles eventually overcomes the force of gravity. This causes the suspended matter to float to the surface where it forms 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.
Some DAF unit designs utilize parallel plate packing material (e.g. lamellas) to provide more separation surface and therefore to enhance the separation efficiency of the unit.
DAF systems can be categorized as circular (more efficient) and rectangular (more residence time). The former type requires just 3 minutes. A particular circular DAF system is called "Zero speed", allowing quite water status then highest performances; a typical example is an Easyfloat 2K DAF system. The rectangular type requires 20 to 30 minutes. One of the bigger advantages of the circular type is its spiral scoop.
Drinking water treatment
Drinking water
Drinking water is water that is used in drink or food preparation; potable water is water that is safe to be used as drinking water. The amount of drinking water required to maintain good health varies, and depends on physical activity level, a ...
supplies that are particularly vulnerable to unicellular algal bloom
An algal bloom or algae bloom is a rapid increase or accumulation in the population of algae in freshwater or marine water systems. It is often recognized by the discoloration in the water from the algae's pigments. The term ''algae'' encompass ...
s, and supplies with low turbidity
Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality.
Fluids ...
and high colour
Color (American English) or colour (British English) is the visual perceptual property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Color categories and physical specifications of color are assoc ...
often employ DAF. After coagulation and flocculation processes, water flows to DAF tanks where air diffusers on the tank bottom create fine bubbles that attach to floc resulting in a floating mass of concentrated floc. The floating floc blanket is removed from the surface and clarified water is withdrawn from the bottom of the DAF tank.Edzwald, James K., ed. (2011). ''Water Quality and Treatment.'' 6th Edition. New York:McGraw-Hill. pp. 9.46.
See also
*API oil-water separator
An application programming interface (API) is a way for two or more computer programs to communicate with each other. It is a type of software interface, offering a service to other pieces of software. A document or standard that describes how ...
* Flotation process
* Industrial wastewater treatment
*Industrial water treatment
There are many uses of water in industry and, in most cases, the used water also needs treatment to render it fit for re-use or disposal. Raw water entering an industrial plant often needs treatment to meet tight quality specifications to be of use ...
*List of waste-water treatment technologies
This page consists of a list of wastewater treatment technologies:
See also
*Agricultural wastewater treatment
* Industrial wastewater treatment
*List of solid waste treatment technologies
* Waste treatment technologies
*Water purification
*Se ...
* Microflotation
References
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) {{Separation processes Flotation processes Water treatment Waste treatment technology