Dissolved air flotation, or DAF, is a way to clean water by getting rid of things like oil or solids that are suspended in the water. This is done by putting air under pressure into the water or wastewater and then letting the air out in a flotation tank basin, where the pressure is back to normal. The released air forms tiny bubbles that stick to the suspended matter and lift it to the water's surface, where it can be removed by a skimming device.
Dissolved air flotation is a very common way for oil refineries, petrochemical and chemical plants, natural gas processing plants, paper mills, general water treatment, and other industrial facilities to clean their waste water. The same kind of process, called "induced gas flotation," is also used to clean up wastewater. Flotation with foam is often used to process mineral ores.
A typical unit for dissolved air flotation (DAF) A 20 m3/h DAF unit with a flocculant preparation station and a pipe flocculator can also be seen.
Modern DAF trucks that use parallel plate technology aren't very big.
A picture of a 225 m3/h DAF is shown.
The water that goes into the DAF float tank is often (but not always) dosed with a coagulant, like ferric chloride or aluminium sulphate, to make the colloidal particles stick together, and/or a flocculant, which makes the particles stick together in bigger groups.
Some of the water that comes out of the DAF tank after it has been cleaned is pumped into a small pressure vessel called the air drum. Compressed air is also put into the air drum. This causes a lot of air to get into the pressurised effluent water. The air-filled water stream is recirculated to the front of the float tank. As it enters the front of the float tank, it flows through a pressure reduction valve, which lets the air out in the form of tiny bubbles. At places where particles start to stick together, called nucleation sites, bubbles form and stick to the particles. As more bubbles form, the lift from the bubbles eventually beats the force of gravity. This makes the things in the water rise to the surface, where they form a layer of foam that is then taken away by a skimmer. The effluent from the DAF unit is the foam-free water that comes out of the float tank.
Some DAF unit designs use parallel plate packing material (such as lamellas) to give the unit more separation surface and, as a result, improve its ability to separate.
Circular DAF systems are better at what they do than rectangular ones (more residence time). The first one takes only 3 minutes. One circular DAF system is called "Zero speed," and it works best when the water is still and quiet. An Easyfloat 2K DAF system is a good example of this. It takes 20 to 30 minutes to make the rectangular type. The spiral scoop is one of the best things about the round type.
Additionally, DAF can be used to cleanse wastewater and filter out contaminants like oil and sediment. Typically, this method is used to clean up after paper mills, factories, petrochemical and chemical industries, and even natural gas processing plants.
Mineral and mining wastes are being treated by flotation on an ever-increasing scale. Users of the method point to its efficacy in removing elements like heavy metals or oil pollutants, as well as its reduced sludge formation. The same DAF system can be used to reuse both the process water and the raw materials.
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