Dissolved Air Floatation
Flotation is a process that involves producing small air bubbles in the water being treated. The bubbles then attach to flocs particles produced by coagulation (Link?) and flocculation. The resulting bubble-flocs agglomerates are buoyant and so float to the surface of the water where they accumulate as a floating layer of sludge, known as float, before being removed.
DAF is a relatively modern process, so much so that many water treatment plant operators have yet to realize how appropriate it might be for them. DAF has two basic origins. Firstly, it was a process that was initially developed in Scandinavia in the 1950’s to treat paper manufacturing effluent, but it was then realized in the early 1960’s that DAF would be appropriate for potable water and wastewater treatment.
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.
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. Nitrogen gas is used instead to create the bubbles.
Process, counter flows
With a mechanical Floculator
A typical dissolved air flotation unit (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, but often is also done by a special peripheral or turbine pump, eliminating the necessity of compressed air supply. 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.
|GreeNfloat DAF – 100 m3/hr||GreeNfloat DAF – 1 m3/hr|
Henry’s law is used to determine the solubility of a gas in water under chemical concepts.
It is important to note here that Henry’s law applies to individual gases, and so for air, which is a mixture of gases, one has to calculate solubility for each gas and then add them to obtain the solubility of air in water. Nitrogen and oxygen account for over 99 percent of the composition of air. Thus for practical reasons, nitrogen is set at 79.1 percent (slightly higher than actual to account for other minor gases) and oxygen is set at 20.1 percent. The solubility concentrations of nitrogen (Cs,N), oxygen (Cs,O), and air (Cs,air) are given in Table 9-2 for various temperatures. As shown, the solubility of air is a little more than 20 mg/L for warm waters increasing to about 30 mg/L for cold water.