DESCRIPTION AERATION APPARATUS, SEAWATER FLUE GAS DESULPHURIZATION APPARATUS INCLUDING THE SAME, AND METHOD FOR OPERATING AERATION APPARATUS Field [0001] The present invention relates to wastewater treatment in a flue gas desulphurization apparatus used in a power plant such as a coal, crude oil, or heavy oil combustion power plant. In particular, the invention relates to an aeration apparatus for aeration used for decarboxylation (aeration) of wastewater (used seawater) from a flue gas desulphurization apparatus for desulphurization using a seawater method. The invention also relates to a seawater flue gas desulphurization apparatus including the aeration apparatus and to a method for operating the aeration apparatus. Background [0002] In conventional power plants that use coal, crude oil, and the like as fuel, combustion flue gas (hereinafter referred to as "flue gas") discharged from a boiler is emitted to the air after sulfur oxides (SOx) such as sulfur dioxide (SO2) contained in the flue gas are removed. Known examples of the desulphurization method used in a flue gas desulphurization apparatus for the above desulphurization treatment include a limestone-gypsum method, spray dryer method, and seawater method. [0003] In a flue gas desulphurization apparatus that uses the seawater method (hereinafter referred to as a "seawater flun gas desulphurization apparatus"), its deaulphui: I/at Ion mo I: hod uses seawator as .111 absorbent. In this metlioi.l, nuawaLer and flue gaa from u bullur are supplied to the inside of a desulfurizer (absorber) having a vertical tubular shape such as a vertical substantially cylindrical shape, and the flue gas is brought into gas-liquid contact with the seawater used as the absorbent in a wet process to remove sulfur oxides. The seawater (used seawater) used as the absorbent for desulphurization in the desulfurizer flows through, for example, a long water passage having an open upper section (Seawater Oxidation Treatment System: SOTS) and is then discharged. In the long water passage, the seawater is decarbonated (exposed to air) by aeration that uses fine air bubbles ejected from an aeration apparatus disposed on the bottom surface of the water passage (Patent documents 1 to 3). Citation List Patent Literature [0004] Patent Literature 1: Japanese Patent Application Laid-open No. 2006-055779 Patent Literature 2: Japanese Patent Application Laid-open No. 2009-028570 Patent Literature 3: Japanese Patent Application Laid-open No. 2009-028572 Summary Technical Problem [0005] Aeration nozzles used in the aeration apparatus each have a large number of small slits formed in a rubber-made diffuser membrane that covers a base. Such aeration nozzles are generally referred to as "diffuser nozzles". These aeration nozzles can eject many fine air bubbles of substantially equal size from the slits with the aid of the pressure of the air supplied to the nozzles. [0006] When aeration is continuously performed in seawater using the above aeration nozzles, precipitates such as calcium sulfate in the seawater are deposited on the wall tnnl (iiioH ul the slits of tho

|i|.y nl aLr at predetermined Inl.orvals when supplying air I hroutjh discharge unit, thumby preventing clogging. [0017] Advantageously, the method further includes: feeding water to an air supply pipe, the feeding being performed independently or at the same time when temporarily stopping or increasing the supply of air. Advantageous Effects of Invention [0018] According to the present invention, it is possible to remove precipitates generated in the slits of the diffuser membranes of the aeration apparatus. Brief Description of Drawings [0019] FIG. 1 is a schematic diagram of a seawater flue gas desulphurization apparatus according to an embodiment. FIG. 2A is a plan view of aeration nozzles. FIG. 2B is a front view of the aeration nozzles. FIG. 3 is a schematic diagram of the inner structure of an aeration nozzle. FIG. 4 is a schematic diagram of an aeration apparatus according to an embodiment. FIG. 5 is a schematic, diagram of another aeration apparatus according to the embodiment. FIG. 6 is a graph showing a change in pressure loss of a diffuser membrane over time when supply of air is temporarily stopped. FIG. 7 is a graph showing a change in pressure loss of the diffuser membrane over time when supply of air is temporarily increased. FIG. 8 is a schematic diagram of the inner structure of an aeration nozzle according to the embodiment. FIG. 9 is a schematic diagram of the inner structure of another aeration nozzle according to the embodiment. FIG. 10 is a schematic diagram of a disk-type aeration nozzle according to the embodiment. FIG. 11A diagram illustrating I ho states of the outflow ol till (humid air at low saturation), the inflow of seawater, timl uoniioiil.rated seawater In a HI.II: of a diffuser membrano. FIG. 11B is a diagram illustrating the states of the outflow of air, the inflow of seawater, and concentrated seawater in the slit of the diffuser membrane. FIG. 11C is a diagram illustrating the states of the outflow of air, the inflow of seawater, concentrated seawater, and precipitates in the slit of the diffuser membrane. Description of Embodiments [0020] Hereinafter, the present invention will be described in detail with reference to the drawings. However, the present invention is not limited to embodiments described below. The components in the following embodiments include those readily apparent to persons skilled in the art and those substantially similar thereto. [Embodiments] [0021] An aeration apparatus and a seawater flue gas desulphurization apparatus according to embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic diagram of the seawater flue gas desulphurization apparatus according to one embodiment. As shown in FIG. 1, a seawater flue gas desulphurization apparatus 100 includes: a flue gas desulphurization absorber 102 in which flue gas 101 and seawater 103 comes in gas-liquid contact to desulphurize S02 into sulfurous acid (H2S03) ; a dilution-mixing basin 105 disposed below the flue gas desulphurization absorber 102 to dilute and mix used seawater 103A containing sulfur compounds with dilution seawater 103; and an oxidation basin 106 disposed on the downstream side of the dilution-mixing ban1n 105 to subject diluted used seawater 103B to water qiki I I. !:y rocovory treatment. [0022] In the seawater flue gas desulphurization apparatus 100, the seawater 103 is supplied through a seawater supply line Li, and part of the seawater 103 is used for absorption, i.e., is brought into gas-liquid contact with the flue gas 101 in the flue gas desulphurization absorber 102 to absorb SO2 contained in the flue gas 101 into the seawater 103. The used seawater 103A that has absorbed the sulfur components in the flue gas desulphurization absorber 102 is mixed with the dilution seawater 103 supplied to the dilution-mixing basin 105 disposed below the flue gas desulphurization absorber 102. The diluted used seawater 103B diluted and mixed with the dilution seawater 103 is supplied to the oxidation basin 106 disposed on the downstream side of the dilution-mixing basin 105. Air 122 supplied from an oxidation air blower 121 is supplied to the oxidation basin 106 from aeration nozzles 123 to recover the quality of the seawater, and the resultant water is discharged to the sea as treated water 124. In FIG. 1, reference numeral 102a represents spray nozzles for injecting seawater upward as liquid columns; 120 represents an aeration apparatus; 122a represents air bubbles; Li represents a seawater supply line; L2 represents a dilution seawater supply line; L3 represents a desulphurization seawater supply line; L4 represents a flue gas supply line; and L5 represents an air supply line. [0023] The structure of the aeration nozzles 123 is described with reference to FIGs. 2A, 2B, and 3. FIG. 2A is a plan view of the aeration nozzles; FIG. 2B is a front view of the aeration nozzles; and FIG. 3 is a schematic diagram of the inner structure of an aeration nozzle. AM ulmwii In KUIH. 2A and 2B, each acini Inn nozzle 123 has a lanjn iiuiiihur d wmall slits 12 hnmml In a rubber-made diffiiMdt: membrane 11 that covers l:ho circumference of a base and ;j.a ijoneral.Ly referred to am a "ill t (user nozzle." In such an aeration nozzle 123, when the diffuser membrane 11 is expanded by the pressure of the air 122 supplied from the air supply line L5, the slits 12 open to allow a large number of fine air bubbles of substantially equal size to be ejected. [0024] As shown in FIGs. 2A and 2B, the aeration nozzles 123 are attached through flanges 16 to headers 15 provided in a plurality of (eight in the present embodiment) branch pipes (not shown) branched from the air supply line L5. In consideration of corrosion resistance, resin-made pipes, for example, are used as the branch pipes and the headers 15 disposed in the diluted used seawater 103B. [0025] For example, as shown in FIG. 3, each aeration nozzle 123 is formed as follows. A substantially cylindrical support body 20 that is made of a resin in consideration of corrosion resistance to the diluted used seawater 103B is used, and a rubber-made diffuser membrane 11 having a large number of slits 12 formed therein is fitted on the support body 20 so as to cover its outer circumference. Then the left and right ends of the diffuser membrane 11 are fastened with fastening members 22 such as wires or bands. [0026] The slits 12 described above are closed in a normal state in which no pressure is applied thereto. In the seawater flue gas desulphurization apparatus 100, the air 122 is continuously supplied, so that the slits 12 are constantly in an open state. [0027] A first end 20a of the support body 20 is attached to a header 15 and allows the introduction of the air 122, and the support body 20 has an opening at its second end 20b that allows the introduction of the seawater 103. In tlm Hiipporl. body 20, the side chum I n t:he first end 20a in In itommun I nation with tho 1 na.1 d«i nf the header 15 through mi *» l..r Inlet port 20c thai pammn through the header lb iind I hu I hinge 16. The inn hie ul I ho support body 20 in \;n\ i I It I onud by a partition plnl.ii ,!0d disposed at some axial ponltion in the support body 20, and the flow of air is blocked by the partition plate 20d. Air outlet holes 20e and 20f are formed in the side surface of the support body 20 and disposed on the header 15 side of the partition plate 20d. The air outlet holes 20e and 20f allow the air 122 to flow between the inner circumferential surface of the diffuser membrane 11 and the outer circumferential surface of the support body, i.e., into a pressurization space 11a for pressurizing and expanding the diffuser membrane 11. Therefore, the air 122 flowing from the header 15 into the aeration nozzle 123 flows through the air inlet port 20c into the support body 20 and then flows through the air outlet holes 20e and 20f formed in the side surface into the pressurization space 11a, as shown by arrows in FIG. 3. The fastening members 22 fasten the diffuser membrane 11 to the support body 20 and prevent the air flowing through the air outlet holes 20e and 20f from leaking from the opposite ends. [0028] In the aeration nozzle 123 configured as above, the air 122 flowing from the header 15 through the air inlet port 20c flows through the air outlet holes 20e and 20f into the pressurization space 11a. Since the slits 12 are closed in the initial state, the air 122 is accumulated in the pressurization space 11a to increase the inner pressure. The increase in the inner pressure of the pressurization space 11a causes the diffuser membrane 11 to expand, and the slits 12 formed in the diffuser membrane 11 are thereby opened, so that fine bubbles of the air 122 are injected into the diluted used seawater 103H. Such fine air bubbles are generated in all the aeration nozzles 123 to which air is supplied through branch pipes L5A to LsH and the headeiH 1!> (Bee FIGS. 4 and 5), [0029] AornlIon apparatuses according In B pnrformed for a lorn] t.tmo, [0035] llnwnvor, In the state shown In KIM, TIC, since the concern! l pfn .it.ua for aerating polluted water in polluted water t.re.i I IIIBHI , plu<|rjLng caused by <.U$|>OBII I.mi of sludge component:fi on illflunor slits (membrtino HIIIH) can be preventod, ritiil I ho duration apparatUM r.au Im HUibly operated ioi: M IOIHJ I I me. [0057] In I ho pruHent embodiment, laibo I y|>«) aeration nozzles ani UHtid In the aeration apparatvuma, but the present invention is not limited thereto. For example, the invention is applicable to disk-type and flat-type aeration apparatuses having diffuser membranes and to diffusers including ceramic or metal diffuser membranes having slits that are open at all times. Industrial Applicability [0058] As described above, in the aeration apparatus according to the present invention, precipitates generated in the slits of the diffuser membranes of the aeration apparatus can be removed. For example, when applied to a seawater flue gas desulphurization apparatus, the aeration apparatus can be continuously operated in a stable manner for a long time. Reference Signs List [0059] 11 diffuser membrane 12 slit 100 seawater flue gas desulphurization apparatus 102 flue gas desulphurization absorber 103 seawater 103A used seawater 103B diluted used seawater 105 dilution-mixing basin 106 oxidation basin 120A, 120B aeration apparatus 123 aeration nozzle CLAIMS 1. An aeration apparatus that is immersed in water to be treated and generates fine air bubbles in the water to be treated, the aeration apparatus comprising: an air supply pipe for supplying air through discharge unit; an aeration nozzle including a diffuser membrane having a slit, the air being supplied to the aeration nozzle; and a control unit for performing control to temporarily stop supply of the air at predetermined intervals. 2. An aeration apparatus that is immersed in water to be treated and generates fine air bubbles in the water to be treated, the aeration apparatus comprising: an air supply pipe for supplying air through discharge unit; an an rat n nozzle including a membrane having a Milt, I: ha a I i: being supplied a deration nozzle; and a ml 11, for performing CM 11 In temporarily increase apply air at predetermine! Intervals. 3. The aeration apparatus according to claim 2, wherein the control unit performs control to temporarily increase the supply of the air and simultaneously feed water to the air supply pipe. 4. The aeration apparatus according to claim 1, wherein the control unit performs control to temporarily stop the supply of the air and simultaneously feed water to the air supply pipe. 5. The aeration apparatus according to any one of claims 1 to 4, wherein the aeration nozzle further includes: a cylindrical base support body into which the air is introduced; a hollow cylindrical body that has a diameter smaller than a diameter of the base support body and that is disposed at an axial position of the base support body via a partition plate; an end support body that is disposed at one end of the hollow cylindrical body and that has approximately the same diameter as the diameter of the base support body; a tubular diffuser membrane that covers the base support body and the end support body and of which both ends are fastened to the base support body and the end support body, respectively; a large number of the slits formed in the tubular diffuser membrane; and an air outlet hole formed in the side surface of the base support body for allowing introduced air to flow into a pressurization space between an inner circumferential surface of the diffuser membrane and outer circumferential of the support IUM.||MH in front of the part l I cm pinto. 6. The aeration on apparatus according to any one of claims 1 to 4, wherein the aeral Ion nozzle further includes a cylindrical base support body into which the air is introduced; an end support body that has approximately the same diameter as the base support body; a tubular diffuser membrane that covers the base support body and the end support body and of which both ends are fastened to the base support body and the end support body, respectively; and a large number of the slits formed in the tubular diffuser membrane. 7. A seawater flue gas desulphurization apparatus, comprising: a desulfurizer that uses seawater as an absorbent; a water passage for allowing used seawater discharged from the desulfurizer to flow therethrough and be discharged; and the aeration apparatus according to any of claims 1 to 6 that is disposed in the water passage, the aeration apparatus generating fine air bubbles in the used seawater to decarbonate the used seawater. 8. A method for operating an aeration apparatus, the method comprising: using an aeration apparatus that is immersed in water to be treated and used to generate fine air bubbles in the water to be treated; and temporarily stopping or increasing supply of air at predetermined intervals when supplying air through discharge unit, thereby preventing clogging. 9. The method according to claim 8, further comprising: feeding water to an air supply pipe, the feeding being performed independently or at the same time when temporarily stopping or increasing the supply of air.