JP5435545B2 - Desalting method - Google Patents

Description

  The present invention relates to a technique for separating impurities from a liquid containing impurities using membrane separation, and more particularly to a desalination treatment method suitable for desalination of seawater or brine (low-concentration brine).

  Conventionally, a membrane separation method using a filtration membrane is widely known as a method for selectively separating a specific substance from a liquid containing impurities. Such filtration membranes include microfiltration membranes with a pore size of approximately 10 μm to 5 nm, ultrafiltration membranes with a pore size of 200 nm to 2 nm, and reverse osmosis membranes with a pore size of 2 nm or less. . As a material for these filtration membranes, cellulose acetate and aromatic polyamide are generally used. Of these, the reverse osmosis membrane (RO membrane) is a membrane that allows water to pass through but does not allow impurities other than water, such as ions and salts, to pass through. It is widely used for desalting to obtain fresh water. Among the reverse osmosis membranes, those with pore sizes of 1 to 2 nm and ions and salts blocking rate of approximately 70% or less are also called nanofilters or NF membranes. The same.

  In the desalination treatment using a reverse osmosis membrane, a pressure higher than the osmotic pressure of raw water (referred to as “operation pressure”) is applied to raw water (for example, seawater) and water that are in osmotic equilibrium across the reverse osmosis membrane. By adding from the raw water side, water molecules in the raw water are transferred to the water side. The difference between the operating pressure and the osmotic pressure of the raw water is the “effective pressure”.

  Salts that cannot permeate the reverse osmosis membrane stay in the vicinity of the membrane surface, and the salt concentration in the vicinity of the membrane surface increases. If this is retained as it is, the osmotic pressure on the raw water side will rise as much as possible and filtration will not be possible, so it is necessary to continuously discharge water in which salts and impurities are concentrated (referred to as “concentrated water”). Therefore, in the reverse osmosis method, the whole amount of raw water cannot be filtered out.

  In the reverse osmosis method, as the salt concentration of the raw water is higher and the concentrated water is reduced, it is necessary to apply a higher operating pressure to the raw water for filtration. For example, from a seawater with an average salt concentration of 3.5%, fresh water with a salt concentration of 0.01% that meets Japanese drinking water standards is 40% water recovery (the remaining 60% is discarded as concentrated water). In the recent technical level, an operation pressure of about 5.5 to 6.5 MPa is required.

  In order to apply a high operating pressure to the raw water, a large amount of power energy is required to operate the high-pressure pump, and this particularly increases the processing cost. In reality, it is said that in the desalination treatment of seawater by the reverse osmosis method, about half of the treatment cost is occupied by the electricity usage fee for operating the high-pressure pump.

  Therefore, for example, in Patent Documents 1 to 3, module units containing reverse osmosis membranes are provided in multiple stages and connected in series, and the concentrated water obtained from the previous reverse osmosis membrane module unit is further boosted to reverse the latter stage. There has been proposed a technique for saving the operating energy of the pump and reducing the processing cost by supplying it to the osmotic membrane module unit.

  In Patent Document 4, the raw water near the membrane surface is agitated by mixing fine bubbles in the raw water upstream of the reverse osmosis membrane module unit, and the suspended substance adheres to the membrane surface by the agitation action. There has been proposed a technique for preventing membrane suspension or removing suspended substances adhering to the membrane surface to improve membrane treatment efficiency.

JP-A-9-276663 Japanese Unexamined Patent Publication No. 2000-056163 JP 2001-252659 A Japanese Utility Model Publication No. 58-53203

  However, if the reverse osmosis membrane module units are arranged in multiple stages as described in Patent Documents 1 to 3, the entire processing apparatus is inevitably increased in size and complexity. Further, even if the reverse osmosis membrane module unit is arranged in multiple stages, a larger operating pressure (about 7 to 9 MPa in the embodiment described in the above document) acts on the subsequent module unit. Problems such as the pressure-resistant burden of the reverse osmosis membrane module unit pointed out cannot be solved sufficiently.

  According to the verification experiment conducted by the present applicants, the membrane treatment method for generating fine bubbles in the raw water described in Patent Document 4 also improves the adhesion of suspended substances on the membrane surface to some extent due to the stirring action of the fine bubbles. However, it is difficult to say that cost improvement effects such as an increase in permeate flow rate and a saving in pump operation energy due to a decrease in operating pressure can be obtained to a satisfactory degree.

  Therefore, the present invention provides a more efficient desalting treatment method that can achieve a membrane treatment efficiency higher than that of conventional membranes even at a low operating pressure, and further aims to further reduce the treatment cost.

  It is known that fine bubbles existing in water have a negative charge on the surface and attract positively charged ions to the surroundings. When the present inventors pay attention to this phenomenon and generate microbubbles in the raw water for membrane separation, ions having a positive charge gather around the microbubbles to partially increase the ion concentration. It has been found that the ion concentration is relatively lowered at a position away from the fine bubbles, and the low concentration portion contributes to lowering the osmotic pressure in the vicinity of the filtration membrane. And it verified that this effect became so large that the surface potential of a microbubble deviated from plus or minus zero.

That is, the desalination treatment method of the present invention generates microbubbles composed of microbubbles having a diameter of several tens of μm or less or nanobubbles having a diameter of 1 μm or less in the raw water containing salts, and the raw water containing the microbubbles is converted into a reverse osmosis membrane. in membrane separation desalted method water Ru obtained by, as the zeta potential in the vicinity of the surface of the fine air bubbles become -40mV~-100mV, lower the osmotic pressure of the raw water to the fine air bubbles by negatively charged It is characterized by making it.

  The surface potential of the microbubbles is confirmed by measuring and calculating the zeta potential using electrophoresis or the like. The zeta potential is the potential at the “slip surface” (boundary between the immobile layer and the diffusion layer) in the electric double layer formed around the fine particles dispersed in water. Since the layer of water that moves with the bubbles is extremely thin, there is no particular problem in discussing the electric characteristics of the bubble surface with the zeta potential value.

  When this zeta potential is induced in a range of −10 mV to −150 mV (more preferably −40 mV to −100 mV), which is particularly negatively charged, positively charged ions are efficiently attracted in the vicinity of the fine bubbles, As a result, the portion where the ion concentration is relatively lowered preferentially permeates the filtration membrane. Thus, in the entire system, a substantially sufficient effective pressure acts on the membrane surface even at a lower operating pressure than before, and a fresh water recovery rate equal to or higher than that of the conventional one is obtained, or at the same operating pressure as that of the conventional one. It is possible to obtain a fresh water recovery rate.

The zeta potential of the fine bubbles varies depending on the quality of the raw water, and is greatly affected by the hydrogen ion concentration index (pH) of the raw water. Therefore, in the desalting method of the present invention, the hydrogen ion concentration index of the raw water is adjusted to the range of pH 8 to pH 12 as a method of charging the zeta potential in the vicinity of the surface of the fine bubbles to be −40 mV to −100 mV. Is a further feature.

  As a means for adjusting the hydrogen ion concentration index of the raw water to a range of pH 8 to pH 12, a method of adding a known general pH adjuster can be used within a range that does not adversely affect the water quality and the filtration membrane. The membrane treatment efficiency can be increased as the pH value is increased, but the alkali treatment of the collected fresh water may be troublesome. Therefore, in practical terms, the adjustment range of the hydrogen ion concentration index is about pH 8 to pH 10. Is preferred. In addition, as pH treatment, it is possible to bring the pH close to neutral by continuing to generate fine bubbles, so this method may be used for neutralization of water after recovery.

Further, the desalting method of the present invention is a bubble generating means suitable for negatively charging fine bubbles, connecting a gas supply pipe to a main pipe disposed in the raw water, and a flow rate of the raw water fed into the main pipe Using the negative pressure generated by the gas, gas is mixed into the raw water in the main pipe from the gas supply pipe, and the gas mass in the gas-liquid mixture is sheared at the gas-liquid interface by the flow path change in the main pipe A bubble generating device configured to discharge fine bubbles from a plurality of slits formed in the body pipe while being crushed by the above is recommended.

  By using such a bubble generating device, it becomes easier to charge the zeta potential of the fine bubbles to the minus side. Also, according to this bubble generating device, compared with other types of bubble generating devices such as a swirling flow method, a bubble discharge site (a slit in this device, a nozzle in other devices) Since the pressure loss when passing through becomes small, fine bubbles can be generated efficiently with low power. As a result, it is relatively easy to generate fine bubbles at a high concentration, and as a result, pump operation energy can be saved.

The desalinization treatment method of the present invention configured as described above is a finely charged negative charge such that the zeta potential is -10 mV to -150 mV, more preferably -40 mV to -100 mV , in the raw water containing salts. When bubbles are generated and the raw water containing the fine bubbles is separated by filtration membrane, positively charged ions are attracted around the fine bubbles and the osmotic pressure near the membrane surface is substantially reduced. Is to be used. As a result, the effective effective pressure in the vicinity of the membrane surface increases, and a fresh water recovery rate equal to or higher than that of the conventional level can be obtained at a lower operating pressure than the conventional level. It becomes possible to get. In addition, since the effective effective pressure increases, an effect of improving the processing cost such as saving of pump operation energy and reduction of the pressure-resistant load of the filtration membrane can be obtained.

  Such a desalination treatment method can increase the effective effective pressure just by generating fine bubbles having a negative charge in the raw water just before the separation of the filtration membrane, so that the stored raw water is added. Even in a conventional general filtration membrane separation device configured to pass through the membrane module under pressure, it can be economically implemented simply by adding a bubble generating means upstream of the filtration membrane separation step.

It is a schematic diagram which shows the structure of the experimental apparatus used for verification of this invention. It is the schematic which shows the internal structure of the pressure vessel in the experimental apparatus of FIG. It is principal part sectional drawing of the bubble production | generation apparatus accommodated in a pressure vessel. It is the schematic which shows the example of the method of measuring the zeta potential of a microbubble. It is a graph which shows the time change of the transmittance | permeability in the verification experiment of this invention. It is a graph which shows the relationship between pH and zeta potential in the verification experiment of this invention. It is a graph which shows the relationship between the difference in pH and the transmittance | permeability in the verification experiment of this invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  First, a reverse osmosis membrane (RO membrane or NF membrane) is particularly suitable as a filtration membrane used in the desalting treatment method of the present invention. Further, the fine bubbles preferably used in the desalting method of the present invention are microbubbles having a diameter of several tens of μm or less, or nanobubbles having a smaller diameter (1 μm or less) than the microbubbles. If the diameter of the bubbles is several tens of μm or less, it is more preferable because it remains in the raw water for a long time without rising and defoaming in a short time.

  FIG. 1 shows a schematic configuration of an experimental apparatus used for verification of the present invention. The experimental apparatus includes a pressure vessel 1 in which a bubble generating device 6 described later is housed, a pressurization control unit 2 that applies pressure to the pressure vessel 1 using nitrogen gas, and a high-pressure pump 3 connected to one end of the pressure vessel 1. A membrane module unit 4 connected to the other end of the pressure vessel 1 and a stirrer (stirring device) 5 for preventing an increase in salt concentration near the filtration membrane.

  FIG. 2 shows a schematic configuration inside the pressure vessel 1. FIG. 3 shows a cross section of the main part of the bubble generating device 6 accommodated in the pressure vessel 1. The illustrated bubble generating device 6 includes a main body pipe 62 connected to a raw water supply pipe 61 that feeds pressurized raw water (treated water) through the high-pressure pump 3, and a gas supply pipe 63 connected to the main body pipe 62. A plurality of slits 64 formed in the main body pipe 62 on the downstream side of the gas supply pipe 63, and a collision wall 65 for closing the end of the main body pipe 62 on the further downstream side of the slit 64. Yes. The main body pipe 62 is arranged in a state of being submerged in the raw water. The gas supply pipe 63 is connected near the upstream end of the main body pipe 62 in a direction substantially orthogonal to the main body pipe 62. The distal end of the gas supply pipe 63 has entered the vicinity of the center of the main body pipe 62, and a gas discharge hole 66 that opens toward the downstream side of the main body pipe 62 is formed on the side surface of the gas supply pipe 63. The gas to be fed (in the exemplary embodiment, nitrogen but air may be used) is discharged from the gas discharge hole 66 into the main body pipe 62. As the shape of the gas discharge hole 66, a slit shape as shown in the figure, a round hole shape, a rectangular hole shape, or a shape in which the tube end portion is simply opened without being blocked can be appropriately selected.

  The slits 64 formed in the main body pipe 62 serve as discharge portions for fine bubbles generated in the main body pipe 62. In the illustrated embodiment, ten slits 64 having an opening width that covers substantially half of the peripheral surface of the main body pipe 62 are arranged at substantially equal intervals along the length direction of the main body pipe 62 and parallel to each other. Is formed.

  The basic operation of the bubble generating device 6 configured as described above is as follows. That is, the raw water fed in a pressurized state from the raw water supply pipe 61 into the main body pipe 62 generates a negative pressure due to a change in flow velocity near the tip of the gas supply pipe 63. Due to this negative pressure, gas is self-primedly introduced into the raw water from the gas discharge hole 66 formed at the tip of the gas supply pipe 63. The raw water mixed with gas (gas-liquid mixture) collides with a collision wall 65 that blocks the downstream end of the main body pipe 62, and the gas mass is pulverized by the impact and flow velocity change. It is discharged out of the pipe 62.

  The main pipe 62 has a two-phase flow due to the negative pressure, and the two-phase flow is divided into two flows when reaching the slit 64 region. One is a flow in the downstream direction, and the other is a flow to the slit 64. By the branching of this flow, the bubbles are subjected to shearing force and are refined. Further, the bubbles are made finer by the narrowing effect due to the slit 64 itself and the shearing force due to the velocity gradient between the flow after passing through the slit 64 and the flow immediately after being discharged from the slit 64.

  In the above configuration, the slit 64 may be formed so as to be orthogonal to the axis of the main body pipe 62. However, if it is slightly inclined as compared to the orthogonal state, the shearing action due to the branching of the water flow is strengthened and the bubbles are made finer. Is advantageous. Specifically, the inclination angle θ on the upstream side of the slit 64 with respect to the axis of the main body pipe 62 may be set to 30 ° to 80 °, more preferably about 50 ° to 70 °. The number of the slits 64 may be appropriately increased or decreased while observing the generation state of the bubbles, but the total opening area of the slits 64 is about 0.5 to 3.0 times the effective sectional area of the main body pipe 62, more preferably. It may be set to about 0.8 to 2.0 times. It is also advantageous to reduce the thickness of the main body pipe 62 in order to make the bubbles finer. Furthermore, by making the slit 64 open to the lower side of the main body pipe 62, the generated fine bubbles are easily diffused widely in the raw water.

  By using such a bubble generating device 6, it is possible to efficiently generate fine bubbles and to easily charge the zeta potential of the fine bubbles to the minus side. In addition, according to the bubble generating device 6, it is possible to suppress the pressure loss at the bubble discharge portion as compared with other types of bubble generating devices such as a swirling flow method.

  Next, a method for measuring and calculating the zeta potential of fine bubbles using electrophoresis will be described. FIG. 4 is a schematic view showing an example of the measurement method.

  The bubble generating device 6 is arranged in the water tank 7 to generate fine bubbles in the raw water in the water tank 7, and the raw water containing the fine bubbles is guided to the electrophoresis cell 8. In the electrophoresis cell 8, two electrodes 81, 82 made of thin silver plates are arranged to face each other, and a constant voltage generator 9 is connected to each electrode 81, 82, and every certain time (for example, 1 second). Then, a DC voltage of about 150 V, in which the polarity is reversed in a pulsed manner, is applied between the electrodes 81 and 82. The charged fine bubbles move horizontally in this electric field. The movement is photographed by the high speed camera 101, and the photographed image is processed by the personal computer 102 to measure the bubble diameter and the moving speed. The zeta potential is calculated from the horizontal movement speed measured in this way, according to the Smoluchowski equation.

  The outline of the verification experiment conducted using the above experimental apparatus is as follows.

  Salt water having a concentration of 1% was used as raw water, and the pressure vessel 1 containing the salt water was pressurized with nitrogen gas. The high pressure pump 3 was driven to generate fine bubbles in the salt water. And the weight and salt concentration of fresh water collected every 3 minutes were measured simultaneously, and the measurement was repeated 5 times continuously for a total of 15 minutes.

  The time change of the transmittance is shown in FIG. Here, the “permeability” is defined as the ratio of the weight of fresh water collected in a state where fine bubbles are generated to the weight of fresh water collected in a state where fine bubbles are not generated. In other words, the transmittance is a value indicating how much the film processing efficiency is substantially increased by the action of the fine bubbles.

  When the experiment was conducted with a salt concentration of 1% in raw water, a pressure of 0.95 MPa, and a pH = 7, the transmittance increased to 140% or more for the first 3 minutes, and then gradually increased to about 153%. .

  Subsequently, in order to examine the effect of the surface potential (zeta potential) of the fine bubbles on the transmittance, the membrane filtration was performed in the same manner as described above while changing the zeta potential of the fine bubbles. The change in zeta potential due to the difference in pH is shown in FIG.

  HCl and NaOH were used as the pH adjuster. It can be seen that the microbubbles have a high negative charge when the pH is high, and a positive charge when the pH is low. When pH = 11, the zeta potential is around −80 mV, whereas when pH = 3, the zeta potential is around 0 mV. Using the characteristics of the fine bubbles, it was examined whether the transmittance was affected when pH = 3 and when pH = 11. FIG. 7 shows the change in transmittance over time due to the difference in pH. The filtration conditions are the same as in the case of pH = 7, the salt concentration is 1%, the pressure is 0.95 MPa, and the transmittance is the ratio with respect to the case where fine bubbles are not generated with the same pH.

  When pH = 3, the transmittance was low from the beginning and increased with time, but remained at a maximum of 133%. On the other hand, in the case of pH = 11, the transmittance exceeded 180% from the beginning and increased to about 211% at the maximum. Compared with the transmittance in the case of pH = 7 shown in FIG. 5, it is a remarkably large value, and clearly the transmittance increases as the pH value becomes higher on the alkali side. That is, the higher the surface potential of the fine bubbles is on the minus side, the higher the transmittance is. Therefore, it can be said that the transmittance is affected by the surface potential (zeta potential) of the fine bubbles.

  From the above experiment, when microbubbles are generated in the raw water, the membrane treatment efficiency is improved as compared with the case where the microbubbles are not generated. It was confirmed that the effect of improving the efficiency was further increased.

  In addition to seawater desalination, the desalination treatment method of the present invention removes impurities other than water from natural water such as lake water, river water, rainwater, and mixed solutions of various inorganic salts, for industrial and agricultural use. It can be widely used for techniques for obtaining fresh water such as drinking.

DESCRIPTION OF SYMBOLS 1 Pressure vessel 3 High pressure pump 6 Bubble generator 62 Main body pipe 63 Gas supply pipe 64 Slit 65 Collision wall

Claims (2)

Raw water to thereby produce microbubbles consisting diameter of several tens μm or less of microbubbles or diameter 1μm following nanobubbles, desalted raw water containing the fine bubbles to membrane separation by reverse osmosis membrane Ru obtain a water containing salts In the processing method,
By adjusting the hydrogen ion concentration index of the raw water to a range of pH 8 to pH 12, the fine bubbles are negatively charged so that the zeta potential in the vicinity of the surface becomes −40 mV to −100 mV, and the osmotic pressure of the raw water A desalinization method characterized by lowering the odor. The desalinization processing method according to claim 1,
A method for generating fine bubbles in the raw water is to connect the gas supply pipe to the main pipe arranged in the raw water, and to use the negative pressure generated by the flow rate of the raw water fed into the main pipe from the gas supply pipe to the main pipe. Gas is mixed into the raw water in the gas, and the gas lump in the gas-liquid mixture is crushed by the shearing effect at the gas-liquid interface due to the flow path change in the main pipe, and from the multiple slits formed in the main pipe A desalting method which is to release fine bubbles.

Images