Dissolved air flotation unit

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. 

In addition, oily sludge from a wastewater treatment facility that results from treating sour wastewaters may be a hazardous waste (unless recycled in the refining process). These include API separator sludge, primary treatment sludge, sludge from various gravitational separation units, and float from dissolved air flotation units. 

Treatment Method. Oils and Fats Recovery. The oils and fats recovery system adopted obviously depends on the local circumstances. Typically, the first stage of pretreatment is the use of a physical process to recover the free oils and fats. The most commonly used physical separation process for the removal of free oils and fats are fat traps, tilted-plate separators, and dissolved air flotation units. In addition, centrifuge and electroflotation systems are occasionally used (73). 

Liquid waste streams containing an insoluble liquid can arise from extraction processes, from steam ejectors operating on solvent distillation systems, or from the loss of heat exchange fluid from a heat exchanger. These should be phase-separated before final disposal measures are undertaken. A simple settler, or a unit such as an American Petroleum Institute (API) separator can be used to accomplish this step. Coupling the initial separator to an entrained or dissolved air flotation unit can reduce the concentration of residual organics further [75]. The recovered organics can be recycled via a further cleanup if required, and the water phase more safely discarded. 

FIGURE 18.9 Operating details of an American Petroleum Institute separator, followed by a dissolved air flotation unit for oil-water separation. 

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 introduc-... 

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... 

A common modification of this process is dissolved air flotation (DAF), in which air under pressure is injected into the wastewater. DAF units are more efficient than conventional flotation clarifiers because more air is introduced into the wastewater, thereby removing more solids.43 5-59... 

Section 27.3 introduces the improved activated sludge systems involving the use of either a DAF clarifier or a dissolved air flotation-filtration (DAFF) clarifier as the secondary flotation clarification unit. 

When separating low-density solid particles or oil droplets from water, the most common method used is dissolved-air flotation. A typical arrangement is shown in Figure 8.12b. This shows some of the effluent water from the unit being recycled, and air being dissolved in the recycle under pressure. The pressure of the recycle is then reduced, releasing the air from solution as a mist of fine bubbles. This is then mixed with the incoming feed that enters the cell. Low-density material floats to the surface with the assistance of the air bubbles and is removed. 

Dissolved air flotation in combination with flocculation can reduce oil content in refinery wastewater to levels approaching oil solubility [40]. According to Katz [41], DAF plus chemical aids for flocculation can be expected to reduce BOD and COD by 30-50% and to reduce total oil to the range 5-25 mg/L. Table 17 shows some data for oil removal from refinery wastewater [27]. Removal efficiencies range from 70 to 90%. The accepted design overflow rates for DAF units are between 60 and 120 L/min per square meter (1.5-3.0 gpm/sq ft) [17]. 

Dissolved air flotation equipment is available from a number of manufacturers. Packaged units of steel construction are available with capacities to 7.6 cu m/min (2000 gpm). The essential elements of the DAF system are the pressurizing pump, air injection facilities, pressurization tank or contact vessel, back-pressure regulating device, and the flotation chamber [40]. 

At the present time, Cummins has decided to install a pretreatment system using dissolved air flotation as the primary treatment followed by an anthracite/sand backwashable filter and an activated carbon polishing filter. The effluent from the filters then goes to the RO unit. The RO will remove the last traces of the most soluble oils and organics, it is Cummins intention to have the effluent from the filters at a turbidity of less than kS JTU s. This should keep the RO units from fouling and will allow the RO to be used with minimal cleaning. 

Like all unit chemical processes, the RO is usually not capable of standing entirely by Itself. The important thing to remember is that a system must be developed to give a complete product. In the case of Cummins, the complete system includes dissolved air flotation, anthracite/sand filtration, activated carbon filtration and RO. In the case of Whitestone Chemical, the complete system includes RO and a cleaning regimen to maintain the RO at the required permeate flow rate. Time will tell which of these alternatives has the best economics. The economics and the proper system using RO are both dependent on the problem that requi res a solution. 

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