WO2008147050A1

WO2008147050A1 - Apparatus and method for generating micro bubbles

Info

Publication number: WO2008147050A1
Application number: PCT/KR2008/002431
Authority: WO
Inventors: Jai Sub Park; Kwan Woo Lee
Original assignee: G & G Korea Co., Ltd.
Priority date: 2007-05-29
Filing date: 2008-04-29
Publication date: 2008-12-04
Also published as: KR100845785B1; CN101541690A

Abstract

Disclosed is an apparatus for generating a large quantity of micro-bubbles. The micro-bubble generating apparatus includes: a pump for inhaling and mixing a gas and a liquid; a mixing chamber for remixing the gas and the liquid pressurized and fed from the pump, and a nozzle for discharging the mixture of the gas and liquid, which is formed by remixing the gas and the liquid in the mixing chamber. The mixing chamber is provided with one or more partitions, each of which is formed with one or more holes, the mixture of the liquid and gas flowing through the holes. According to the present invention, a large quantity of micro-bubbles according to a dissolved air floating (DAF) process with low costs by employing the mixing chamber modified as described above.

Description

APPARATUS AND METHOD FOR GENERATING

MICRO BUBBLES

Technical Field

[1] The present invention relates to an apparatus and method for generating micro- bubbles, and more particularly to a micro-bubble generating apparatus capable of generating a large quantity of micro-bubbles through a DAF (dissolved Air Flotation) process with a pump and a modified pressure tank, and a method of generating micro- bubbles using the same. Background Art

[2] As well-known in the art, the term micro-bubbles generally means bubbles with a diameter not exceeding 50 /M. Generally, micro-bubbles are generated through a dissolution-under-pressure process. In the present invention, micro-bubbles are generated through a dissolved air flotation (DAF) process.

[3] The dissolved air flotation (DAF) process is a water treatment method which makes air be sufficiently dissolved in water under high pressure, and introduces the water with the dissolved air into raw water to be treated, so that the super-saturated air is decompressed again in the raw water, thereby forming micro-bubbles in the raw water. The micro-bubbles are attached to floes in the raw water, whereby the floc-bubble aggregates rapidly float from underwater sites to the water surface. The floes are relatively large aggregates formed by solid particles coagulated with each other with the aid of coagulant under a suspension condition.

[4] The characteristics of such a DAF process involve a small required area (short process time) as compared to an existing sedimentation reservoir, an improved removal efficiency of aquatic plants, such as Diatomaceae and Blue-green algae, saving of coagulant (about 10 to 25% is saved), low water content of sludge (the water content is in the range of about 95% to 97%), etc.

[5] The DAF process offers several advantages in sedimentation for water treatment, including a decrease in the amount of coagulant used and volume of sediment (sludge) produced. In addition, because high hydraulic loading rate can be used for solid-liquid separation, a floating chamber smaller than that required for sedimentation can be used, reducing the construction costs (Edzwald and Walsh, 1992).

[6] It is also known that the residual coagulant concentration of treated water is low as compared to that in the process of sedimentation, and the quality of treated water is excellent, even at water temperatures below 4⁰C (AWWA, 1999). Due to its many benefits, the DAF process has been increasingly expanded despite circulation and saturator systems, power costs, and somewhat complicated operating conditions. As the DAF process is now widely used, there is a need for improved designs and more optimal operating conditions. Several researchers have carried out various experiments and modeling studies.

[7] According to recent developments in DAF theory and practice, bubble size is one of the most important parameters in the DAF process (Edzwald, 1995). The size of produced bubbles can be easily measured through image analysis and by particle counters (Han et al., 2002c). It has been found that the effect of pressure on bubble size is nonlinear. The bubble size decreases with an increase of pressure, but does not decrease at a pressure above 3.5 atm (De Rijk et al., 1994; Han et al., 2002a). Fbwever, there is still some denunciation about the optimum size of bubbles.

[8] Han (2001) and Han et al. (2001) indicate that the removal efficiency is highest when bubbles and particles have similar sizes and are oppositely charged. Fbwever, some manufacturers argue that smaller bubbles have higher efficiency (Rubio et al., 2002).

[9] In the DAF process, the smaller bubble size increases the collision efficiency between bubbles and floes at the contact zone, and the larger bubble size increases separation efficiency as the floc-bubble aggregate forming rate increases. Fbwever, in the case of small bubbles, the number of bubbles attached to the aggregates is increased due to the high collision efficiency, and the increased small bubbles exhibit rising rates as high as large bubbles rising rates. Therefore, the attachment of the small bubbles to the floes can be promoted, which in turn can increase the efficiency of water treatment.

[10] In conclusion, the generation of small bubbles is costly but favorable for improved efficiency of a DAF process. Fbwever, the high energy cost and sophisticated operation associated with the generation of small bubbles have limited the adoption of such a DAF system.

[11] The size of bubbles generated in a DAF process is an important factor in terms of process efficiency. High energy and cost are required for generating bubbles. The average size of bubbles currently generated in a DAF system is about 30 μm. According to experimental results and modeling results, the bubble size represents the size range of particles to be removed. That is, the smaller the bubble size, the smaller the particle size that can be removed.

[12] Referring to FIG. 1, a conventional DAF system 1 includes a high pressure water circulation pump 2 capable of being employed as a recycle pump, an air compressor Ia, and a pressure tank 3 in which bubbles are generated, the bubble size being changed by changing the air volume flowing into the tank, and by changing the pressure within the tank. In FIG. 1, "Ib" indicates a discharge valve, and "3a" indicates a pressure gage.

[13] Fbwever, the conventional bubble generator is complicated in construction, and is difficult to operate. In addition, because a compressor should be provided as an essential component, noise and vibration occurring by the operation of the compressor are high, and the manufacturing costs are greatly increased. Disclosure of Invention Technical Problem

[14] Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art, and the present invention is to provide a micro- bubble generating apparatus with a very simple construction, which can be easily operated, and which is capable of generating a large quantity of micro-bubbles through a DAF process with small costs, and a method of generating micro-bubbles using the micro-bubble generating apparatus. Technical Solution

[15] In order to accomplish the above-mentioned objects, according to a first aspect of the present invention, there is provided an apparatus for generating micro-bubbles including: a pump for inhaling and mixing a gas and a liquid; a mixing chamber for remixing the gas and the liquid pressurized and fed from the pump; and a nozzle for discharging the mixture of the gas and liquid, which is formed by remixing the gas and liquid in the mixing chamber, the apparatus being adapted to generate micro-bubbles by adjusting the volume of the gas and the pressure of the mixing chamber.

[16] According to another aspect of the present invention, there is provided a method of generating micro-bubbles including the steps of: separately inhaling a gas and a liquid into a pump; making the gas and the liquid pass through the pump so that they are crushed into pieces and mixed with each other; pressurizing and feeding the mixture of the gas and liquid mixed in the pump to a mixing chamber with the pump; developing a predetermined level of pressure within the mixing chamber, the gas and the liquid being remixed in the mixing chamber; and discharging the mixture of the gas and liquid remixed in the mixing chamber to the outside.

[17] According to a preferred embodiment of the present invention, the mixture of the gas and liqiid remixed in the mixing chamber is discharged through a nozzle, preferably a porous nozzle capable of being opened and closed, which is provided at one side of the mixing chamber.

[18] Preferably, the mixing chamber is provided with one or more partitions, each of which is formed with one or more holes, the mixture of the liquid and gas flowing through the holes.

[19] According to a modified embodiment, the mixing chamber is formed in a dual chamber structure with a top-closed outer chamber and a top-opened inner chamber, the inner chamber being spaced from the wall of the outer chamber, and wherein an inlet pipeline connected with an inlet of the mixing chamber extends into the inside of the inner chamber to a position adjacent to the bottom of the inner chamber, and an outlet pipeline connected with an outlet of the mixing chamber extends along a space formed between the inner chamber and the outer chamber to a position adjacent to the bottom of the outer chamber. The mixture of the gas and the liquid mixed in the pump is introduced into the inner chamber, and the mixture overflowing the inner chamber is supplied to the nozzle side from the outer chamber through a pipeline.

[20] In addition, an intake valve is provided in front of the pump for adjusting the volume of the gas flowing into the pump, and the size distribution of the micro-bubbles can be adjusted through the opening side of the intake valve and the level of the pressure developed in the mixing chamber.

[21] Preferably, the gas is air, oxygen or ozone, and the liquid is water.

[22] According to the present invention, a large quantity of micro-bubbles can be generated according to a dissolved air floating (DAF) process with low costs by employing a mixing chamber modified as described above.

Advantageous Effects

[23] The specific advantages of the present invention are as follows:

[24] (1) The inventive micro-bubble generating apparatus has a much simpler structure than a conventional DAF bubble generating system. A pump performs all the functions of a recycle pump, a compressor, and a pressure tank. Moreover, the pressure within the mixing chamber and the air volume inhaled into the pump can be adjusted by a simple procedure.

[25] (2) Using the inventive micro-bubble generating apparatus, an average bubble size of less than 34 μm can be achieved, depending on the pressure within the mixing chamber and the inhaled air volume. Changes in inhaled air volume, pressure, etc. determine the bubble size. According to the present invention, a very small bubble size, e.g. 22 μm or less, can be achieved by allowing air and water to be mixed well in the inner structure of the mixing chamber, and allowing the pressurized water to move rapidly. [26] (3) The above-mentioned experimental results and modeling results indicate that bubble size represents the size range of particles to be moved. That is, the smaller the bubble size, the less the particle range to be moved. Therefore, the inventive micro- bubble generating apparatus is capable of generating desired micro-bubbles in the range of 20 μm to 100 μm, and contributes to the improved efficiency of the DAF process.

Brief Description of the Drawings [27] The above and other objects, features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: [28] FIG. 1 is a view showing a schematic construction of a micro-bubble generating apparatus of the prior art; [29] FIG. 2 is a conceptual view of a micro-bubble generating apparatus according to the present invention; [30] FIG. 3 is a view showing a schematic construction of the micro-bubble generating apparatus according to FIG. 2; [31] FIG. 4 is a simplified view showing a preferred embodiment of the pressure tank according to FIG. 3; [32] FIG. 5 is a perspective view showing a modified embodiment of the pressure tank according to FIG. 3; [33] FIG. 6 is a vertical cross-sectional view showing the inner structure of the pressure tank according to FIG. 5;

[34] FIG. 7 is a view showing a sensor of an online particle counter in detail;

[35] FIGs. 8 and 9 are graphs showing a bubble size distribution and average bubble size as a function of pressure within a bubble generator; [36] FIGs. 10 and 11 are graphs showing bubble size distributions as a function of the opening dimension of an air intake valve at 4 atm; [37] FIG. 12 is a graph showing a bubble size distribution obtained by the inventive micro-bubble generating apparatus and a conventional saturator type DAF system; [38] FIG. 13 is a view showing an inner structure of a mixing chamber;

[39] FIG. 14 is a conceptual view showing pressure loss caused while fluid is moving from one end to the other end of a nozzle; [40] FIG. 15 is a diagram of trajectory analysis-collision efficiency; and [41] FIG. 16 is a graph showing a relationship between floe size and residual turbidity at

5 atm. Best Mode for Carrying Out the Invention

[42] Hereinafter, the basic principle and construction of the present invention will be described with reference to accompanying drawings.

[43] FIG. 2 shows a conceptual construction of a micro-bubble generating apparatus. The inventive micro-bubble generating apparatus 1 essentially includes a pump 2 for inhaling and mixing gas and liquid, and a mixing chamber 3' for re-mixing the gas and liquid pressure-fed from the pump 2'.

[44] Pipelines 4 and 5 extend from the inlet and outlet of the mixing chamber 3' respectively, and the pump 2 is connected to the inlet pipeline 4. In addition, two pipelines 6 and 7 are branched from the inlet side of the pump 2'. When the pump 2 is operated, the liquid is introduced into the pump 2 through the pipeline 5, and the gas is introduced into the pump 2 through the pipeline 7. An intake valve 7 a is provided on the pipeline 7 so as to control the volume of the gas to be supplied to the pump 2'. In addition, a nozzle 8 for discharging the mixture of the re-mixed gas and liquid is provided at the outlet side of the mixing chamber 3'.

[45] From the above-mentioned construction, the inner pressure of the mixing chamber is controlled by adjusting the pressure by the pump 2 and the opening extent of the nozzle 8, and optionally by modifying the inner construction of the mixing chamber 3'. At the same time, a large amount of micro-bubbles is generated within the mixing chamber 3 and discharged through the outlet side pipeline 5. In FIG. 2, reference numeral 3'a indicates a pressure gage, and 3'b indicates a discharge port.

[46] Meanwhile, according to the present invention, the gas is preferably the air in the atmosphere, oxygen or ozone, and the liquid is preferably water.

[47] Next, the most preferred embodiments of the present invention will be described. In the embodiments below, air, oxygen or ozone may be selectively used as the gas, and water may be used as the liquid.

[48]

[49] Example 1

[50] FIG. 3 is a view showing an outlined construction of a micro-bubble generating apparatus. As shown in the drawing, the inventive micro-bubble generating apparatus essentially includes a pump 10 and a pressure tank 20 corresponding to the mixing chamber 3' of FIG. 2.

[51] The outlet side of the pump 10 is connected to the inlet side of the pressure tank 20 through a pipeline (water pipeline 11), and pipelines (water pipelines 12 and 13) extend from the inlet side of the pump 10 and the outlet side 20 of the pressure tank 20, respectively, and arrive at the inside of a water tank 15.

[52] According to the present invention, a flow control valve 14 and a check valve 16 may be provided between the water inlet part 12a of the water pipeline 12 and the pump 10 so as to control the supplying of water introduced from the water tank 15. In addition, an intake pipeline 17 for inhaling the air in the atmosphere or the like may be connected between the water inlet part 12a of the water pipeline 12 and the pump 10. In the present invention, the intake pipeline 17 may be preferably connected between the flow control valve 14 and the check valve 16 as shown in FIG. 3.

[53] The intake pipeline 17 extends to the outside of the water tank, and is provided with a flow meter 18 and a three-way valve 19. A first branch tube 17a extends to one side of the three-way valve 19 so as to allow the inflow of the air in the atmosphere, and a second branch tube 17b extends to the other side of the three-way valve 19 and is connected to an oxygen generator or an ozone generator 30 (hereinafter, to be referred to as oxygen/ozone generator). The first and second branch tubes 17a and 17b can be respectively communicated with the intake pipeline 17 depending on the opening direction of the three-way valve 19. As a result, the supplying of the air in the atmosphere or oxygen or ozone generated by the oxygen/ozone generator 30 can be selectively performed according to the opening direction of the three-way valve 19.

[54] Referring to FIG. 3, an intake valve 19a is provided on the second branch tube 17b.

The intake valve 19a is provided so as to control the volume of oxygen or ozone supplied to the pump 16. Of course, the intake valve 19a may be provided on the intake pipeline 17 so as to control the volume of air supplied through the first branch tube 17a from the atmosphere as well, although that is not shown in the drawing. The end of the water pipeline 13 extending from the pressure tank 20, i.e. the water outlet part, is provided with a nozzle 13a so as to discharge and control the mixture of the gas and liquid including micro-bubbles. For this purpose, the nozzle 13a is preferably formed in a porous type and in a construction to be capable of being opened and closed.

[55] In FIG. 3, reference numeral "20a" indicates a positive pressure gage, "20b" indicates a negative pressure gage, and "20c" indicates a safety valve.

[56] FIG. 4 shows an outlined inner structure of an embodiment of the pressure tank show in FIG. 3.

[57] As shown in the drawing, the pressure tank 20 includes an inlet 21 for introducing the mixture of water and air or oxygen (or ozone), which is pressurized and fed from the pump 10, and an outlet 22 for discharging bubbles generated within the pressure tank 20.

[58] The pressure tank 20 has an inner space with a predetermined volume, and one or more partitions 23, 24, 25 and 26 are provided across the inner space, thereby interconnecting opposite inner walls of the pressure tank 20. It is preferable if two or more partitions 23, 24, 25 and 26 are provided and equally spaced from each other as shown in the drawing. Each of the partitions 23, 24, 25 and 26 is formed with holes 23a, 23b; 24a; 25a, 25b; and 26a, such as orifices. In addition, the diameters and number of the holes 23a, 23b; 24a; 25a, 25b; and 26a formed through the respective partitions 23, 24, 25 and 26 may be optionally determined depending on the required pressure for the mixture passing through the holes.

[59] In FIG. 4, reference numeral " 13a" indicates a nozzle (see FIG. 3).

[60] With the above-mentioned structure, while flowing into the pressure tank 20 and escaping from the pressure tank 20, the mixture of water and air or oxygen (or ozone) rapidly passes the holes 23a, 23b; 24a; 25a, 25b; and 26 formed through the respective partitions 23, 24, 25 and 26.

[61] Especially, the pressurized mixture has a higher flow rate as it approaches the outlet

22. In addition, the pressure of the fluid is rapidly reduced at each of the partitions as the fluid approaches the outlet 22 in contrast to the flow rate. The rapid reduction of pressure within a short period of time like this will cause the generation of bubbles smaller than critical /M-size bubbles generated in a DAF process, and hence the generation of more micro-bubbles.

[62] Now, a description will be made in terms of a procedure in which the mixture of gas and air pressurized and fed through the pump from the structure of the inventive micro-bubble generating apparatus as mentioned above suffers from changing while passing through the pressure tank.

[63] The supplying of water and air (oxygen or ozone) to the pressure tank 20 is performed by operating the pump 10. As the pump 10 is operated, the water is carried to the inside of the pump along the second water pipeline 12. At the same time, the air in the atmosphere or oxygen (or ozone) generated by the oxygen/ozone generator 30 is carried and mixed with the water in the pump 10. That is, the supplying of air and oxygen (or ozone) is selectively performed according to the opened/closed direction of the three-way valve 19, wherein, when the second branch tube 17b is closed by the three-way valve 19, the air in the atmosphere is supplied, and when the first branch tube 17a is closed by the three-way valve 19, the oxygen generated in the oxygen/ ozone generator 30 is supplied.

[64] The air (oxygen or ozone) carried to the pump 10 together with the water is primarily crushed into pieces by an impeller (not shown) rotating within the pump 10, thereby generating bubbles in the state dissolved in the water, and pressurized and fed to the inside of the pressure tank 20 through the first water pipeline 11. The mixture of the water and air (oxygen or ozone) introduced into the pressure tank 20 through the inlet 21 flows through the holes 23a, 23b, 24a, 24b, 25a, 25b, and 26a formed through the respective partitions 23, 24, 25 and 26. As a result, high turbulent mixing zones occur at the downstream sides of the respective holes 23a, 23b, 24a, 24b, 25a, 25b, and 26a.

[65] The turbulent flow in these zones causes severe pressure fluctuation. Snce the pressure of jet stream is low, negative pressure zones occur. Under these conditions, bubbles are generated each time an abrupt pressure drop occurs. In particular, as shown in FIG. 4, the more partitions the mixture of the water and air (oxygen or ozone) passes, the smaller the bubbles that are generated.

[66]

[67] Example 2

[68] FIG. 5 is a perspective view showing a modified embodiment of the pressure tank according to FIG. 3, and FIG. 6 is a vertical cross-sectional view showing the inner structure of the pressure tank.

[69] As shown in the drawings, the pressure tank 20' is formed in a dual-chamber structure with an inner chamber 27 and an outer chamber 28. The outer chamber 28 is provided in a top-sealed construction, and the inner chamber 27 is provided in a top- opened construction, and spaced from the inner walls of the outer chamber 28. The outer chamber 28 is provided with, on its wall, an inlet 21' and an outlet 22' which are connected to the water pipelines 11 and 13 (see FIG. 3), respectively. From the inlet 21' and the outlet 22', pipelines 20'a and 20'b extend into the pressure tank 20' respectively.

[70] Preferably, the inlet pipeline 20'a extending from the inlet 21' into the inner chamber

27 extends to an area adjacent to the bottom of the inner chamber 27, and the outlet pipeline 20'b extending from the outlet 22' extends to an area adjacent to the bottom of the outer chamber 28 through a space between the inner chamber 27 and the outer chamber 28.

[71] Now, a description will be made in terms of the change the mixture of gas and liquid pressurized and fed from the modified embodiment of the pressure tank by the pump suffers while it is passing the pressure tank.

[72] The air (oxygen or ozone) carried together with the water from the construction of

FIG. 3 by the pump 10 is primarily crushed into pieces by the impeller (not shown) rotating within the pump 10, thereby generating bubbles in the state dissolved in the water, and is carried into the pressure tank 20' through the first water pipeline 11.

[73] The mixture W of the water and air (oxygen or ozone) introduced through the inlet

21' of the pressure tank 20 is supplied into the inner chamber 27 through the inlet pipeline 20'a, and the mixture overflowing from the inner chamber 27 fills the inside of the outer chamber 28, that is, the space between the outer chamber 28 and the inner chamber 27.

[74] At this time, the air which is not fully dissolved in the water is separated from the water and fills the inner top space of the pressure tank 20', and the mixture is continuously charged into the outer chamber 28 through the inlet pipeline 20'a. During this process, high pressure is applied to the mixture in the pressure tank 20', and the air dissolved in the water remains highly pressurized.

[75] Meanwhile, the pressurized mixture rises along the outlet pipeline 20'b with an increased rate, and is discharged along a third water pipeline 13 extending to the outside of the pressure tank 20'. The flow rate of the mixture is increased as it approaches a nozzle 13a (see FIG. 3) positioned at the end of the water pipeline 13, whereby it has the highest flow rate at the instant it passes the nozzle 13a.

[76] In contrast to the flow rate of the pressurized mixture, the pressure of the mixture drops rapidly as the mixture approaches the nozzle 13a, whereby the mixture has the lowest pressure at the instant it passes through the nozzle 13a. During this process, micro-bubbles occur from the dissolved air in the mixture.

[77] Due to the rapid pressure drop within a short period of time, the mixture of the water and air (or oxygen or ozone) generates bubbles smaller than the critical /M-size bubbles generated in the DAF process.

[78]

[79] Experimental Method

[80] As described above, according to the present invention, there is provided a bubble generating apparatus capable of generating micro-bubbles by a substantially simple system. In addition, there is also provided a bubble generator capable of generating micro-bubbles while satisfying a low energy requirement. Moreover, the inventors found that bubbles with all desired average sizes in the range of 20/M to μm can be generated by changing the operation parameters of the bubble generator and the inner structure of the mixing chamber (pressure tank).

[81] Now, the significance of developing the present invention will be discussed while presenting some experimental results of generation of micro-bubbles according to the present invention.

[82] The micro-bubble sizes may be measured through image analysis as follows: the micro-bubble sizes are visually measured through the most simple and widely used method. This method has problems of a complicated experimental instrument and long term measurement, despite of high accuracy in measuring each bubble. Therefore, this method has a limitation in reliably measuring a large number of bubbles with different sizes.

[83] Another method is to measure the rising rates of bubbles and then to calculate the sizes of bubbles according to Stake's Law. However, because the sizes of the bubbles are not uniform and the rising rates of plural bubbles are different from the rising rate of a single bubble, no equation can be used for estimating the size distribution of the bubbles from the rising rates.

[84] In order to overcome the above-mentioned problems, the measurement was performed according to the method of Han et al. (2002b) in the present invention. In the present invention, online particle counters (Chemtrac Model PC 2400 D, USA) was used for measuring a bubble size. The instrument provides seven (7) adjustable channels for measuring a size range. In making the present invention, two identical particle counters are used for increasing the number of the channels so as to improve the accuracy of measurement (see detailed view of a sensor of an online particle counter shown in FIG. 7).

[85] Referring to FIG. 7, laser beams are projected to a detector through a sensor (for holding a sample). When passing the sample in the sensor, the beams are scattered by bubbles, thereby being darkened. Such scattering and darkening of the beams reduce the intensity of beams arriving at the detector. As the intensity of the laser beams is reduced, voltage pulses are generated. Here, the number of pulses indicates the number of bubbles, and the height of each pulse indicates the size of a specific bubble.

[86] In order to minimize the possible combination of bubbles prior to arriving at the sensor, a straight tube with a length retained as short as possible was used. The sampling flow rate was set to 100 m# as recommended by the manufacturers. In this method, distillated and deionized water was used because bubbles and particles are indistinguishable from each other, and in order to reduce the interference of the particles. Although the measuring range of the particle counters was set as 2 μm to 100 μm with an increment of 10 /M, the results from the first channel (2 /M to 10 /M) were disregarded. Under each set of requirements, three distribution data sets were obtained, and average values were used for analysis.

[87]

[88] Experimental Results and Discuss

[89] The inventive micro-bubble generating apparatus includes a pump, a pressure pump

(mixing chamber), an intake pipeline (air inhaling valve), and a nozzle. The principle of generating bubbles is to simultaneously inhale air and water.

[90] The water and air separately inhaled into the pump are mixed under high pressure, whereby the air is dissolved in the water according to Henry's Law. In addition, remixing occurs within the mixing chamber. Snce pressurized water is discharged through the nozzle under the atmosphere, micro-bubbles are generated due to a drop in pressure.

[91] The substantial difference between the inventive bubble generator and a conventional

DAF system is that the inventive pump is capable of performing the functions of a recycle pump, a compressor, and a pressure tank. Therefore, there is a merit in that the DAF process can be performed with a simpler system (see FIGs. 1 and 2).

[92] The pressure of the mixing chamber (pressure tank) is measured, and the bubble size and the number of bubbles are determined after ejection from the nozzle. By changing pressure from the pump, air volume flowing into the pump, the inner structure of the mixing chamber, and a hydraulic characteristic, it is possible to change the bubble size. The sizes of the bubbles generated from the bubble generator are comparable with those of the bubbles generated from the DAF experimental system.

[93] In order to determine the effects of the change of the pressure within the mixing chamber, the air volume flowing into the pump, and the shape (inner structure) of the mixing chamber on the bubble size, an investigation was carried out.

[94]

[95] (1) Effect of pressure within the mixing chamber

[96] The bubble size in the DAF process is significantly affected by the pressure difference before and after the nozzle, and the shape of the nozzle (AWWA, 1999). The higher the pressure, the smaller the bubble size. It is known that at 4 to 6 atm, the bubble size is generally about 10 to 100 μm (40 μm on average) (Edzwald, 1995). The bubble size distribution and average bubble size from the inventive bubble generator are shown in FIGs. 8 and 9) (bubble size distribution and average bubble size as a function of the pressure inside the bubble generator). [97] The pressure inside the mixing chamber can be adjusted by the pump. As the pressure increases, the size distribution is reduced. This means that a greater number of smaller bubbles are generated. The bubble size distributions at 5 atm and 6 atm are almost identical. The average bubble size was smaller at the higher pressure; at 5 atm, it was not less than 34 μm. Several researchers indicated that beyond a certain level of pressure, average bubble size was not continuously reduced (De Rijk et al., 1994; Han et al., 2002a; Rykaart and Haarhoff, 1995). The above results can be explained on the basis of their findings.

[98] The average bubble sizes and peak points of bubble size distributions resulting from the inventive bubble generator and a DAF process are comparatively shown in Table 1. It can be seen that, as compared to the bubbles generated through the DAF process, the bubbles generated from the inventive bubble generator are somewhat larger in average bubble size at all pressures.

[99] Table 1 [Table 1] [Table ]

[100] [101] Fbwever, in view of the peak point of the bubble size distribution, at 5 atm and 6 atm, the average sizes of bubbles generated by the inventive bubble generator are large, but the peak points are small. That is, the DAF system did not show a change in bubble size at a pressure above 4 atm, whereas the inventive generator did not show a change in bubble size at a pressure above 5 atm and generated bubbles smaller than those generated by the DAF system.

[102] [103] (2) The air volume flowing into the pump [104] In order to determine how the air volume flowing into the pump affects on the bubble size, the bubble size was measured after the dimensions of the opening of the air inhaling valve installed at the front end of the pump were changed to 0.99, 1.98 and 2.97 mm (the opening ratios were 1:8; 2:8 and 3:8, respectively). The pressure in the mixing chamber was kept constant at 4 atm by adjusting the revolutions per minute of the pump. [105] FIGs. 10 and 11 show the bubble size distribution and the cumulative distribution according to the opening dimensions of the inhaling valve, in particular the bubble size distribution according to the opening dimensions of the inhaling valve at 4 atm. The

2 opening dimension of 0.99 mm (opening ratio 1:8) generates smaller bubbles (10 μm

2 to 40 μm), but the opening dimension of 2.97 mm (opening ratio 3:8) generates relatively larger bubbles (50 μm to 100 μm). If the valve is opened further so as to increase the air volume, the bubble size is increased and the number of generated

2 bubbles is reduced. In addition, when the opening dimension is smaller than 3.96 mm (opening ratio 4:8), no pressure is created with in the mixing chamber. This is probably because the over inhalation of air leads to a lower average density of fluid within the pump, thereby resulting in an abrupt drop in efficiency and a decreased capability to

2 inhale air. When the opening dimension is smaller than 0.99 mm (opening ratio 1:8), the bubble size and generation were constant. In the inventive bubble generator, the inhaled air volume is an important operating factor in generating micro-bubbles. Therefore, it is important that the appropriation of the air inhaling valve is secured in order to generate a small range of micro-bubbles.

[106]

[107] (3) Effect of the inner structure of the mixing chamber and hydraulic characteristics

[108] Table 2 shows the average bubble size and peak point of bubble size distribution in the DAF process and before (BGl) and after (BG2) the inner structure of the mixing chamber is changed at a fixed pressure (5 atm). FIG. 12 shows the bubble size distribution achieved with the inventive bubble generator (BGl, BG2) compared with the saturator type DAF process. The results of bubble generation after changing the inner structure of the mixing chamber in the inventive bubble generator are shown in FIGs. 10 and 11. In the case of BG2, by changing the inner structure at the fixed pressure (5 atm), a smaller average bubble size and peak point as compared to those of the bubbles generated in the DAF process were obtained. That is, bubbles smaller than the critical size (29 μm) were generated by changing the inner structure of the mixing chamber; hence, a larger number of small micro-bubbles could be generated as compared to those generated in the DAF process.

[109]

[110] Table 2 [Table 2]

[Table ]

Average bubble sizes of the inventive bubble generator and the saturator type DAF

[111] [112] In generating bubble through the DAF process, if circulating water is super-saturated and then suffers a rapid drop in pressure from the atmospheric pressure, micro-bubbles are generated. De Rijk et al. (1994) explains this based on the Bernoulli Equation below (see the inner structure of the mixing chamber of FIG. 13, and the pressure loss occurring while moving from one end to the other end of the nozzle of FIG. 14).

[113] P + l/2pv² = P + l/2pv² (Bernoulli Equation), o o k k [114] As v » v , k 0

[115] P = P - l/2pv² k 0 k [116] where P and v are the pressure and velocity before the narrow area of the nozzle, re- o o spectively, and P and v are the pressure and velocity at the narrow area of the nozzle, k k respectively.

[117] When pressurized water passes through a nozzle, the flow rate (v )is very high but the pressure(P ) is very low. Behind the orifice of the nozzle, a high turbulent mixing k zone is formed. The turbulence in this zone causes severe pressure fluctuation, and because the pressure in the jet stream is low (v is very high), negative pressure may k occur. Under these conditions, when a sudden drop in pressure occurs, bubbles are generated (the principle of generating bubbles).

[118] The reason for a change in average bubble size caused after changing the inner size of the mixing chamber, and for the generation of a larger number of micro-bubbles smaller than those generated in the DAF process, can be ascribed to this rapid drop in pressure.

[119] The inner structure of BG2 is provided with an assembly of narrowly spaced partitions, and halls, such as orifices, are formed through the partitions. Pressurized water passes rapidly through the orifice plate until it flows into the mixing chamber and then flows out the mixing chamber, and the pressurized water will have a higher flow rate as it approaches the nozzle. This means that pressure decreases at each partition. A very rapid drop in pressure for a short time period leads to the generation of bubbles smaller than the bubbles of critical size generated in the DAF processes as well as a larger number of small bubbles. Fbw quickly such a sudden drop in pressure occurs at a certain pressure is a key factor in the generation of bubbles smaller than the critical size (29 μm).

[120]

[121] Relationship between bubble size and particle removal efficiency

[122] Modeling studies including Edzwald's single-collector model, Tambo's heterogeneous condensation, and Han's analysis of trajectories were carried out in order to determine the relationship between bubbles and particles in a contact zone.

[123] In Han's model, particles larger than 1 μm had an increased collision efficiency as the particles size increased, and showed the highest efficiency when the bubble size was the same as the particle size. In addition, the efficiency was slightly decreased when the particle size was greater than the bubble size. However, when the particle density was the same as the actual floes (1.2 g/cm ), there was no decreases in efficiency, as shown in FIG. 15. Further, in the case of lower floe density (1.01 g/cm ), collision efficiency did not decrease (see trajectory analysis-collision efficiency (bubble size: 35 μm).

[124] FIG. 16 shows the relationship between floe size and residual turbidity at 5 atm. The efficiency is gradually improved to a certain point. From the certain point, the efficiency remains unchanged at a recycle rate of 4% and 10%, even if the floe size is increased. The point showing no change is about 30 μm, which is similar to the bubble size of 29 μm at 5 atm. The similar trend was evident under the following conditions: 2 atm, 3 atm and 4 atm, at a recycle ratio of 4% and 10%. This means that the results presented in FIG. 16 correspond with Han's modeling.

[125] Therefore, the inventive bubble generator can adjust the average bubble size to 20 μm, and can treat even small particles in consideration of the above modeling, and according to the results of batch tests, the inventive bubble generator has an advantage in that it is not necessary to increase its size in a floc-formation process.

[126] In the DAF process, bubble size is considered as a significantly important parameter. Although much effort has been directed at making bubble size smaller, bubbles with an average size of less than 30 μm have not been yet obtained. According to the present invention, there is provided a new bubble generator with a relatively simple system so as to adapt bubble size to a predetermined object. In addition, by changing operating conditions and the inner structure of the missing chamber, bubbles of desired sizes can be selectively made.

[127] The inventive micro-bubble generating apparatus has an advantage in terms of treating lighter particles as compared to a conventional sedimentation process.

[128] In addition, the present invention can be applied to various fields beyond drinking- water and waste-water treatment. For example, the inventive method can be employed in various problematic areas, in which pretreatment is too costly or impossible, such as the removal of algae from lakes or oceans, pharmaceutical processes, for which the addition of chemicals is prohibited, or the like.

[129] In addition, the present invention can be employed in an aeration bath which uses bubbles generated in bath water. The aeration bath is a bathing method which generates small bubbles at the bottom of the bath with a motor and uses the bubbles so as to accomplish the same effect as a massage. Supersonic waves, which are produced when bubbles are generated, apply relaxing and contracting stimuli to skin, thereby providing hypothermic effects. In particular, if the diameters of bubbles are not more than 1 mm, the bath is referred to as a supersonic bath, which has treatment effects for myalgia, skin care, sequelae of head trauma, etc. In addition, because positive air ions are bonded to the micro-bubbles floating on the water surface, negative air ions increase adjacent to the water surface, wherein the negative air ions have a sedation effect for setting a person's heart at rest and relieving stress.

[130] The inventive micro-bubble generating apparatus intermittently inhales air to be mixed with bath water together with pulse waves by an electromagnetic valve which is rapidly opened and closed, whereby the bubbles of the mixture of air injected to a bath can be formed as micro-bubbles. Therefore, by the micro-bubbles, bath water can be alkalized, and supersonic waves and negative ions are produced, whereby the bath water can be softened.

[131] In addition, because micro-bubbles are capable of penetrating into skin pores as well as rapidly floating, fat and impurities accumulated deep in skin pores of a bathing person can be extracted to the outside of the skin, and oxygen energy can be supplied deep in the skin. As a result, it is possible to obtain a skin care effect, such as skin- cleaning, removal of horny layer, skin moisture, skin- whitening, increase of skin elasticity, etc., and an effect of treating skin diseases, such as atopic dermatitis, acne, tinea pedis, chickenpox, etc. In addition, it is also possible to obtain other effects, such as fatigue recovery, recovery from hangover, increase in body temperature, improvement of blood circulation, treatment of insomnia, etc. [132] In addition, the present invention generates negative ions naturally with the aid of micro-bubbles, rather than using electric friction, whereby it can be expected that the metabolism of a bathing person can be facilitated.

[133] Moreover, because the entirety of the micro-bubble generating apparatus including a structure and elements for generating micro-bubbles can be miniaturized and simplified, the handling and management operations of the micro-bubble generating apparatus, such as installing, moving and using the apparatus, can be easily performed. Industrial Applicability

[134] As described above, the inventive micro-bubble generating apparatus, which includes a pump and a pressure tank as principal constituents, is capable of mixing air and water well, thereby generating bubbles with an average size in the range of 20 μm to 100 μm, with a very simple structure and low costs.

[135] (1) The inventive micro-bubble generating apparatus has a much simpler structure than a conventional DAF bubble generating system. A pump performs all the functions of a recycle pump, a compressor, and a pressure tank. Moreover, the pressure within the mixing chamber and the bubble volume inhaled into the pump can be adjusted by a simple procedure.

[136] (2) Using the inventive micro-bubble generating apparatus, an average bubble size of less than 34 μm can be achieved, depending on the pressure within the mixing chamber and the inhaled air volume. Changes in inhaled air volume, pressure, etc. determine the bubble size. According to the present invention, a very small bubble size, e.g. 22 μm or less can be achieved by allowing air and water to be mixed well in the inner structure of the mixing chamber, and allowing the pressurized water to move rapidly.

[137] (3) The above-mentioned experimental results and modeling results indicate that bubble size represents the size range of particles to be moved. That is, the smaller the bubble size, the less the particle range to be moved. Therefore, the inventive micro- bubble generating apparatus is capable of generating desired micro-bubbles in the range of 20 μm to 100 μm, and contributes to the improved efficiency of the DAF process.

[138] Meanwhile, although air or oxygen (or ozone) and water are mixed so as to generate micro-bubbles in the embodiments described above, the present invention is not limited to this, and all gases capable of being dissolved in a liquid can be employed in the present invention, including air or oxygen (ozone). In addition, any liquid, including water, can be employed in the present invention depending on the use of the present invention, if the liquid is capable of dissolving the gases. Therefore, although preferred embodiments of the present invention have been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims

[1] An apparatus for generating micro-bubbles comprising: a pump for inhaling and mixing a gas and a liquid; and a mixing chamber for remixing the gas and the liquid pressurized and fed from the pump, the apparatus being adapted to generate micro-bubbles by adjusting the volume of the gas and the pressure of the mixing chamber.

[2] The apparatus as claimed in claim 1, further comprising a nozzle for discharging the mixture of the gas and liquid, which is formed by remixing the gas and liquid in the mixing chamber.

[3] The apparatus as claimed in claim 2, wherein the nozzle is a porous nozzle capable of being opened and closed.

[4] The apparatus as claimed in claim 1, wherein the mixing chamber is provided with one or more partitions, each of which is formed with one or more holes, the mixture of the liquid and gas flowing through the holes.

[5] The apparatus as claimed in claim 1, wherein the mixing chamber is formed in a dual chamber structure with a top-closed outer chamber and a top-opened inner chamber, the inner chamber being spaced from the wall of the outer chamber, and wherein an inlet pipeline connected with an inlet of the mixing chamber extends into the inside of the inner chamber to a position adjacent to the bottom of the inner chamber, and an outlet pipeline connected with an outlet of the mixing chamber extends along a space formed between the inner chamber and the outer chamber to a position adjacent to the bottom of the outer chamber.

[6] The apparatus as claimed in claim 1, further comprising an intake valve provided in front of the pump for adjusting the volume of the gas flowing into the pump.

[7] The apparatus as claimed in claim 1, wherein the outlet side of the pump is connected with the inlet side of the mixing chamber through a first liquid pipeline, second and third liquid pipelines extending into the inside of a liquid tank from the inlet side of the pump and the outlet side of the mixing chamber, respectively, first and third liquid pipelines being connected to an inlet and an outlet of the mixing chamber, respectively, and an intake pipeline for introducing a gas in the atmosphere being connected to the second liquid pipeline, and wherein a three-way valve is provided on the intake pipeline, one side of the three-way valve being connected to a first branch tube communicating with the atmosphere, and the other side of the three-way valve being connected to a gas generator through a second branch tube, the first and second branch tubes being selectively comminicated with each other depending on the opening direction of the three-way valve.

[8] The apparatus as claimed in claim 7, further comprising a flow control valve and a check valve provided between a liquid inlet part of the second liquid pipeline and the pump so as to control the supplying of liquid flowing into the pump from the liquid tank.

[9] The apparatus as claimed in claim 8, wherein the intake pipeline is connected to the second liquid pipeline between the flow control valve and the check valve.

[10] The apparatus as claimed in claim 1, wherein the gas is air, oxygen or ozone.

[11] The apparatus as claimed in claim 1, wherein the liquid is water.

[12] A method of generating micro-bubbles comprising the steps of: separately inhaling a gas and a liquid with a pump; making the gas and the liquid pass through the pump so that they are crushed into pieces and mixed with each other; pressurizing and feeding the gas and the liquid mixed in the pump to a mixing chamber with the pump; developing a predetermined level of pressure within the mixing chamber, the gas and the liquid being remixed in the mixing chamber; and discharging the mixture of the gas and liquid remixed in the mixing chamber to the outside.

[13] The method as claimed in claim 12, wherein the mixture of the gas and liquid is discharged through a nozzle provide at one side of the mixing chamber.

[14] The method as claimed in claim 13, wherein the nozzle is a porous nozzle capable of being opened and closed.

[15] The method as claimed in claim 12, wherein the mixing chamber is provided with one or more partitions, each of which is formed with one or more holes, the mixture of the liquid and gas flowing through the holes.

[16] The method as claimed in claim 12, wherein the mixing chamber is formed in a dual chamber structure with a top-closed outer chamber and a top-opened inner chamber, the inner chamber being spaced from the wall of the outer chamber, and wherein an inlet pipeline connected with an inlet of the mixing chamber extends into the inside of the inner chamber to a position adjacent to the bottom of the inner chamber, and an outlet pipeline connected with an outlet of the mixing chamber extends along a space formed between the inner chamber and the outer chamber to a position adjacent to the bottom of the outer chamber.

[17] The method as claimed in claim 12, wherein an intake valve is provided in front of the pump for adjusting the volume of the gas flowing into the pump.

[18] The method as claimed in claim 12, wherein the gas is air, oxygen or ozone.

[19] The method as claimed in claim 12, wherein the liquid is water.