Air Flotation Systems

Air Flotation systems for separating oil and solids from wastewater include  

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

Designs include rectangular, circular, cross flow and inclined parallel plate systems. 

Air flotation is one of the oldest methods for the removal of solids, oil grease and fibrous materials from wastewater. Suspended solids and oil grease removals as high as 99%+ can be attained with these processes. 

Air flotation is simply the production of microscopic air bubbles, which enhance the natural tendency of some materials to float by carrying wastewater contaminants to the surface of the tank for removal by mechanical skimming. Many commercially available units are packaged rectangular steel tank flotation systems shipped completely assembled and ready for simple piping and wiring on site. 

---

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

The bubble size in these cells tends to be the smallest (10 to 50 Im) as compared to the dissolved-air and dispersed-air flotation systems. Also, very httle turbulence is created by the bubble formation. Accordingly, this method is attractive for the separation of small particles and fragile floes. To date, electroflotation has been applied to effluent treatment and sludge thickening. However, because of their bubble generation capacity, these units are found to be economically attractive for small installations in the flow-rate range of 10 to 20 mVh. Electroflotation is not expected to be suitable for potable water treatment because of the possible heavy metal contamination that can arise due to the dissolution of the electrodes. 

The most common application of carbon adsorption in municipal water treatment is in the removal of taste and odor compounds. Figure 12 provides an example of a process flow diagram for a municipal water treatment plant. In this example water is pumped from the river into a flotation unit, which is used for the removal of suspended solids such as algae and particulate matter. Dissolved air is the injected under pressure into the basin. This action creates microbubbles which become attached to the suspended solids, causing them to float. This results in a layer of suspended solids on the surface of the water, which is removed using a mechanical skimming technique. Go back to Chapter 8 if you need to refresh your memory on air flotation systems. 

Krofta, M. and Wang, L.K., Design of Dissolved Air Flotation systems for Industrial Pretreatment and Municipal Water and Wastewater Treatment, National Meeting of American Institute of Chemical Engineers, Houston, Texas, March 1983. 

Wang, L.K. Wu, B.C. Treatment of groundwater by dissolved air flotation systems using sodium aluminate and lime as flotation aids. OCEESA J. 1984, 1 (5), 5-18 NTIS Report No. PB85-167229/AS. 

Krofta, M. Wang, L.K. Design of Dissolved Air Flotation Systems for Industrial Pretreatment and Municipal Wastewater Treatment - Design and Energy Considerations, AIChE National Conference, Houston, TX, NTIS-PB83-232868, 1983, 30 p. 

Combination Crossflow Pliue-Pack Coalescer and Induced-Air Flotation System. This in-scrics combination was offered with a recycled portion of the effluent from the plate coalescer being injected into the influent stream. Very little field experience is available on this design. Also, when the dissolved air breaks out of solution, turbulence is created. This can adversely affect the action of the plate-pack coalescer. Also, the system is very expensive it costs more than twice the other acceptable altcmauvcs. This system, therefore, was not accepted. 

Fig. 8. Piping arrangement of a bench-scale batch dispersed air flotation system.

The total incoming flow of feed can be treated with air under pressure, but it is more usual to introduce the air into a recycle stream of cleaned liquid and to allow the bubbles to develop in the flotation chamber at the point where the recycle stream enters. The feed stream is pretreated with chemicals, if necessary, prior to its introduction to the flotation chamber where it meets the bubble blanket. The fine particles in the supension are captured by the bubbles and are floated up to the surface where they form a thick scum which is removed by a continuous mechanical scraper. Some solids will settle fi om most feed streams and it is usual to provide for the withdrawal of accumulated silt fiom the base of the chamber. Figure 7.13 shows a conventional diflused air flotation system housed in a rectangular tank and Figure 7.14 a DAF unit which incorporates an inclined plate settler to capture by sedimentation those particles that escape flotation. 

The automated spreading process is performed on tables that can withstand the load of a fully equipped spreader. A table surface may be perforated and equipped with an air flotation system that lifts the lay while it is moved or with a vacuum system to hold the lay in place. 

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

Two air-saturation systems suited for use in water treatment are shown in Figure 15 (31). Such mechanisms faciHtate the release of air that generates much finer bubbles than mechanical air dispersion methods used in mineral flotation practice. 

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. 

Flotation is a physical process involving relative interaction of three phases solid, water, and air. An understanding of the wettability of the solid surface, physical surface, and chemical phenomena by which the flotation reagents act and the mechanical factors that determine particle-bubble attachment and removal of particle-laden bubbles, is helpful in designing and operating flotation systems successfully. 

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. 

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%). 

Sedimentation and dissolved air flotation are the most common clarification processes for removal of precipitates. Either sedimentation or flotation is often preceded by chemical coagulation or precipitation, which converts dissolved pollutants to a suspended form, and by flocculation, which enhances clarification by flocculating suspended solids into larger, more easily separating particles. Simple sedimentation normally requires a long retention time to adequately reduce the solids content. The detention time of dissolved air flotation, however, is much shorter. When chemicals are used, retention times are reduced and clarification removal efficiency of either sedimentation or flotation is increased. A properly operated clarification system is capable of efficient removal of suspended solids, metal hydroxides, and other wastewater impurities.10-12... 

Wang, L.K., Design and specification of Pittsfield Water Treatment System consisting of air flotation and sand filtration, Water Treatment, 6, 127-146, 1991. 

Krofta, M. and Wang, L.K., Development of innovative flotation-filtration systems for water treatment, part B dissolved-air flotation plants for small communities, Proc. American Water Works Association Water Reuse Symposium III, San Diego, CA, 3, 1238-1250, 1984. 

---