Dissolved-air flotation

Dissolved air flotation (DAL) is another process used to remove oil and grease constituents. In this process, groundwater (or some fraction thereof) is saturated 

FIGURE 5.2 Illustration of dissolved air flotation in which pressurized water supersaturated with air is released into the bottom of a tank of water producing an abundance of small bubbles giving the water a white or milky appearance. Flocculated particles that entrain air bubbles are buoyant and rise to the top where the 

These systems can be used for removal of suspended fine particles from the effluents. 

Air is dissolved into water under pressure. (The solubility of air is more under pressure.) This is now mixed with the incoming waste stream, and the pressure is released in a separate chamber where by the dissolved air comes out of solution, producing a large number of very fine bubbles. 

The air bubbles get attached to the suspended solids making them lighter and hence making them float to the surface. A skimmer at the surface removes these floating solids, thus cleaning the incoming effluent. It is then discharged from the system. 

A small fraction of the outgoing treated effluent is taken and again saturated with air. This is recycled, mixed with the wastewater influent, and injected into the separation chamber where the pressure is again released to continue the process. 

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Disposable products Disposal systems Disproportionation Dissociation Dissociation constant Dissolved air flotation... 

Two main operational variables that differentiate the flotation of finely dispersed coUoids and precipitates in water treatment from the flotation of minerals is the need for quiescent pulp conditions (low turbulence) and the need for very fine bubble sizes in the former. This is accompHshed by the use of electroflotation and dissolved air flotation instead of mechanically generated bubbles which is common in mineral flotation practice. Electroflotation is a technique where fine gas bubbles (hydrogen and oxygen) are generated in the pulp by the appHcation of electricity to electrodes. These very fine bubbles are more suited to the flotation of very fine particles encountered in water treatment. Its industrial usage is not widespread. Dissolved air flotation is similar to vacuum flotation. Air-saturated slurries are subjected to vacuum for the generation of bubbles. The process finds limited appHcation in water treatment and in paper pulp effluent purification. The need to mn it batchwise renders it less versatile. 

Dissolved Air Flotation. Dissolved air flotation (DAF) is used to separate suspended soflds and oil and grease from aqueous streams and to concentrate or thicken sludges. Air bubbles carry or float these materials to the surface where they can be removed. The air bubbles are formed by pressurizing either the influent wastewater or a portion of the effluent in the presence of air. When the pressurized stream enters the flotation tank which is at atmospheric pressure, the dissolved air comes out of solution as tiny, microscopic bubbles. Dissolved air flotation is used in many wastewater treatment systems, but in the United States it is perhaps best known with respect to hazardous waste because it is associated with the Hsted waste, K048, DAF flotation soflds from petroleum refining wastewaters. Of course, the process itself is not what is hazardous, but the materials it helps to remove from refining wastewaters. 

Low concentrations of oil can be removed by dissolved air flotation (DAF). In this process, an effluent recycle is pressurized in the presence of excess air, causing additional air to go into solution, in accordance with Henry s Law. When this water is discharged to the inlet chamber of the flotation unit at close to atmospheric pressure, the dissolved air comes out of solution in the form of tiny air bubbles which attach themselves to and become enmeshed in suspended solids and oil globules. The primary design criteria is the air/solids ratio, which is defined as the mass of air released divided by the mass of solids fed. Sufficient air must be released to capture the solids in the influent wastewater. The performance of DAF for the treatment of several... 

Fig. 4. Schematic of a pressurized dissolved air flotation system, (a) Influent pressurization (b) recycle pressurization.

In dissolved air flotation, air bubbles float the sludge, which is then removed by a scraper. It is generally appHcable to large volumes of waste-activated sludge. 

FIG. 14-97 The Flotator dissolved-air flotation thickener. (Process Kngineets, Inc., a division ofEtmco Corp., now Envitotech Corporation.)... 

In contrast to vacuum flotation, dissolved-air flotation units can be operated on a continuous basis by the application of pressure. This consists of pressurizing and aerating the process stream and introducing it into the flotation vessel that is maintained at the atmospheric pressure. The reduction of pressure results in the formation of fine air bubbles and the collection of fine particulates to be floated and removed as sludge. 

Figure 19-72 illustrates a dissolved-air flotation plant flowsheet for water treatment. The flowsheet shows that the incoming raw water is... 

FIG. 19-72 Schematic diagram of a recycle dissolved-air flotation plant for water treatment. 

The dissolved-air flotation process is most commonly used for sewage and potable water treatment. It is also gaining popularity for the treatment of slaughterhouse, poultry processing, seafood processing, soap, and food processing wastes (Zoubulis et. al., 1991). 

Dissolved Air Flotation ODispersed Air Flotation Induced Air Flotation O Diffused Air Flotation... 

The following are useful references on dissolved air flotation. This bibliography... 

Vrablik, E. R. Fundamental principles of dissolved air flotation of industrial wastes. Industrial Waste Conference Proceedings. 14th, 1959, Purdue University, Ann Arbor, USA, 743-779. 

UltrafiUration is a preferred alternative to the conventional systems of chemical flocculation and coagulation followed by dissolved air flotation. Ultraflltration provides lower capital equipment, installation, and operating costs. 

Dissolved Air Flotation Effective for WAS Relatively high power consumption... 

DAF Dissolved air flotation - one of many designs for waste treatment. 

Other plant-scale applications to pollution control include the flotation of suspended sewage particles by depressurizing so as to release dissolved air [Jenkins, Scherfig, and Eckhoff, Applications of Adsorptive Bubble Separation Techniques to Wastewater Treatment, in Lemlich (ed.). Adsorptive Bubble Separation Techniques, Academic, New York, 1972, chap. 14 and Richter, Internat. Chem. Eng, 16,614 (1976)]. Dissolved-air flotation is also employed in treating waste-water from pulp and paper mills [Coertze, Prog. Water TechnoL, 10, 449(1978) and Severeid, TAPPl 62(2), 61, 1979]. In addition, there is the flotation, with electrolytically released bubbles [Chambers and Cottrell, Chem. Eng, 83(16), 95 (1976)], of oily iron dust [Ellwood, Chem. Eng, 75(16), 82 (1968)] and of a variety of wastes from surface-treatment processes at the maintenance and overhaul base of an airline [Roth and Ferguson, Desalination, 23, 49 (1977)]. 

It should be noted that dissolved air flotation (DAF) is a more effective process for clarification.8-10 As shown in Appendix D, with an additional step of chromium reduction, the secondary treatment system effectively removed chromium (over 99%), copper (89%), cadmium (64%), lead (67%), and zinc (77%). 

Most of the pollutants may be effectively removed by precipitation of metal hydroxides or carbonates using a reaction with lime, sodium hydroxide, or sodium carbonate. For some, improved removals are provided by the use of sodium sulfide or ferrous sulfide to precipitate the pollutants as sulfide compounds with very low solubilities. After soluble metals are precipitated as insoluble floes, one of the water-solid separators (such as dissolved air flotation, sedimentation, centrifugation, membrane filtration, and so on) can be used for floes removal.911 The effectiveness of pollutant removal by several different precipitation methods is summarized in Tables 5.15-5.17. 

API stands for the American Petroleum Institute and TEB stands for Thermal Emulsion Breaking. Figures 5.1 and 5.2 show two typical types of oil-water separators, gravimetric and parallel plate.7 A dissolved air flotation (DAF) clarifier is commonly used for polishing the effluent from an oil-water separator.8"1013... 

Dissolved Air Flotation Equipment—Annual 2005-2006 Buyer s Guide, Water Waste Digest, 45, 73-76, 2005. 

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