Air-dissolved flotation and

Clearly if the depth of the primary energy minimum and cohesive forces in the aggregate are very small, a detachment can take place. To overcome electrostatic repulsion is a critical task in microflotation systems of such type as well as in the transport stage. 

For submicron particles (for example, at a size of 0.1 - 0.3 pm) even the use of decimicron bubbles does not provide a sufficiently high collision efficiency. Particles of still smaller dimensions are not discussed here since their transport is influenced by Brownian diffusion. These difficulties are overcome by particle aggregation (cf Section 10.9). 

Two possibilities are presented here. Bubbles with a completely or sufficiently strongly retarded surface can be used at Reynolds numbers less than 40. The second possibility is to use millimeter-size bubbles with less than completely-retarded surfaces. It follows from the estimate (10.50) that the transport stage proceeds in both cases with approximately equal intensity. Each variant has its advantages and disadvantages. 

Appendix lOF Flotation with Centimicron and Millimeter Bubbles 

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It is difficult to produce centimicron bubbles. Flotation machines generate mainly larger size bubbles, and bubbles of smaller size are obtained in air-dissolved flotation and electroflotation. 

As pointed out in Section 10.8 and shown in Fig. 10.11, a remarkable polydispersity of bubbles is observed in air-dissolved flotation and electroflotation. Its degree is highly dependent on the conditions of electroflotation and microflotation. 

If a exceeds this value, a decrease of bubble size will only decrease the collision efficiency. Large bubbles with the size of the order of a 0.5mm are obtained in usual pneumatic dispersions formed in flotation machines. It is much more diflflcult to obtain bubbles 10 to 50 times smaller, which can be achieved by using completely different methods of bubble generation, electroflotation and air-dissolved flotation. Since a decrease of bubble dimensions is connected with substantial complication and a rise in price of the technology, it is necessary to forecast sufficiently accurately the required size of bubbles as a function of surfactant... 

Particle-bubble aggregates can be formed not only by collisions but also as a result of separation of bubbles from a solution (Klassen, 1973) in a so-called air-dissolved flotation process. Formation of floto-complexes is obtained in several ways as a result of collision of particles and bubbles (1), by ejection of gas and solution supersaturated with gas (air) to the particle surface (2), and by capture of rising bubbles by particle aggregates (3). 

If the polydispersity of bubbles generated in air-dissolved flotation or electroflotation is high, there is no need for additional introduction of centimicron bubbles. Optimal flow of two-stage flotation corresponds to the maximum attainable degree of monodispersity of bubbles. In this case the ratio between volume fractions of micro- and macrobubbles and collision efficiencies of the processes of particle capture by small bubbles and bubble coagulation must be such that the particle capture process outweighs the process of coalescence. 

Since a reduction in the bubble spectrum is connected to additional difficulties and polydispersity of bubbles in electroflotation and air-dissolved flotation cannot be therefore avoided, it is important to know the effect of polydispersity. This effect should not be too high since the process of particle capture should be ahead of the process of coalescence. It should also not be too low so large bubbles can capture small bubbles in a moderate time. Unfortunately, such a scheme which appears convincing at a first glance means a slow rate of coalescence of small bubbles. The coalescence can proceed faster than particle capture, so that the intensification of capture becomes very important. Hence it is necessary to combine introduction of small bubbles with aggregation of particles. 

Other plant-scale apphcations to pohution control include the flotation of suspended sewage particles oy depressurizing so as to release dissolved air [Jenkins, Scherfig, and Eckhoff, Applications of Adsorp-... 

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)]. 

This is a unit operation process where air bubbles, as gas, are used to remove solid or liquid particles from the liquid wastewater. The air bubbles are often trapped in the morphology of the suspended particles and as a result of buoyant forces, the particles move up and float on the surface where they are skimmed out. The common flotation methods include dissolved air, air flotation, vacuum flotation, and chemical additives.3... 

In the recycle flow pressurization system (Figure 27.10), a portion (15-50%) of the clarified effluent from the flotation chamber is recycled, pressurized, and semisaturated with air in the air dissolving tube. The recycled flow is mixed with the unpressurized main influent stream just before admission to the flotation chamber, with the result that the air bubbles come out of aqueous phase in contact with suspended particulate matter at the inlet compartment of the flotation chamber. The system is usually employed in applications where preliminary chemical addition and flocculation are necessary and ahead of flotation. It eliminates the problems with shearing the flocculated particles since only the clarified effluent passes through the pressurizing pump and the friction valve. It should be noted, however, that the increased hydraulic flow on the flotation chamber due to the flow recirculation must be taken into account in the flotation chamber design. 

Krofta, M. and Wang, L.K., Potable Water Pretreatment for Turbidity and Color Removal by Dissolved Air Flotation and Filtration for the Town of Lenox, Massachusetts, Krofta Engineering Corporation, Lenox, MA, Report No. KEC-10-81/3, 84pp., October 1981. 

Wang, L.K. Potable water treatment by dissolved air flotation and filtration. J. AWWA 1982, 74 (6), 304-310. 

Plant 000005 produces approximately 3.2 X 10" kkg/year (7.0 x 10 Ib/year) of isobutene-isopropene rubber. Wastewater generally consists of direct processes and MEC water. Contact wastewater flow rate is approximately 1040m /day (2.75 x 10 gpd), and noncontact water flows at about 327 m /day (8.64 x 10" gpd). Treatment consists of coagulation, flocculation, and dissolved air flotation, and the treated effluent becomes part of the noncontact cooling stream of the onsite refinery. 

Solvents, extender oils, and insoluble monomers are used throughout the rubber industry. In addition, miscellaneous oils are used to lubricate machinery. Laboratory analysis indicates the presence of oil and grease in the raw wastewater of these plants. Oil and grease entering the wastewater streams are removed by chemical coagulation, dissolved air flotation, and, to some extent, biological oxidation. 

Foams are dispersions of gas in a relatively small amount of liquid. When they are still on the surface of the which they were formed, they also are called froths. Bubbles range in size from about 50 fim to several mm. The data of Table 20.1 show densities of water/air foams to range from 0.8 to 24g/L. Some dissolved or finely divided substances may concentrate on the bubble surfaces. Beer froth, for instance, has been found to contain 73% protein and 10% water. Surface active substances attach themselves to dissolved materials and accumulate in the bubbles whose formation they facilitate and stabilize. Foam separation is most effective for removal of small contents of dissolved impurities. In the treatment of waste waters for instance, impurities may be reduced from a content measured in parts per million to one measured in parts per billion. High contents of suspended solids or liquids are removed selectively from suspension by a process of froth flotation. 

The objective of the work is to present an experiment-founded adsorption model for precipitate flotation. Batch precipitate flotation of CufOH) with dodecylbenzene sulphonate (DBS) as collector, was carried out both with dissolved (DAF) and dispersed (DIS) air. The processes were considered as a succession of the dynamic equilibria taking place at the gasliquid and solidliquid interfaces. Both flotation processes were expressed quantitatively in terms of surface concentrations of Cu(OH)2 and DBS per unit surface area of the air buble, as well as the ratio of the numbers of air bubbles and solid particles (B /P ). Also the maximal concentrations of both DBS and Cu(OH)2, recoverable under the given conditions were calculated. All these values were determined by following the Cu(OH)2 and DBS recovery. The 2 flotation techniques were compared in regard to their efficiency and mechanism. Finally, the results obtained were discussed in terms of the other models for the colloid particle adsorption at the air-water interface. 

Again, any type of technique can be used for generating gas bubbles in a nonfoaming adsorptive bubble separation system. The most effective bubble generation techniques for a nonfoaming system are dissolved air flotation and electrolytic flotation. The following are the process descriptions of selected nonfoaming processes. 

M. Krofta, L. K. Wang, L. L. Spencer, and J. Weber, Separation of algae from lake water by dissolved air flotation and sand filtration. Proceedings of the Water Quality and Public Health Conference, Worcester Polytechnic Institute, Worcester, MA, USA, pp. 103-110, 1983 (NTIS-PB83-219550). 

Dissolved air flotation (DAF) is the process of removing suspended solid, oils, and other contaminants via the use of air bubble flotation. Air is dissolved into water, mixed with the waste stream prior to being released from the solution and is in intimate contact with the contaminants. The small bubbles will attach to the floatable oils, increase their buoyancy, and reduce their specific gravity. In this system, a side stream of the oily waste is supersaturated under pressure with dissolved air so that the movement of the air bubbles will carry the floatables upward where they can be removed (46,51). 

An innovative potable filtration plant with a design capacity of 1.2 MGD has been reliably serving 10,000 residents and tourists in the town of Lenox, Massachusetts, USA, since July 1982. Its process system consists of chemical flocculation, dissolved air flotation, and automatic backwash and sand filtration. It substantially improves upon the conventional flocculation, sedimentation, and filtration system in performance, capability, operation, maintenance, and energy use (19 2). 

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