Patent Publication Number: US-2012042783-A1

Title: Aeration apparatus, seawater flue gas desulphurization apparatus including the same, and method for operating aeration apparatus

Description:
FIELD 
     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 
     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 (SO x ) such as sulfur dioxide (SO 2 ) 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. 
     In a flue gas desulphurization apparatus that uses the seawater method (hereinafter referred to as a “seawater flue gas desulphurization apparatus”), its desulphurization method uses seawater as an absorbent. In this method, seawater and flue gas from a boiler 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 
     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 
     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. 
     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 surfaces of the slits of the diffuser membranes and around the openings of the slits, causing the gaps of the slits to be narrowed and the slits to be clogged. This results an increase in pressure loss of the diffuser membranes, and the discharge pressure of discharge unit, such as a blower or compressor, for supplying the air to the diffuser is thereby increased, so that disadvantageously the load on the blower or compressor increases. 
     The occurrence of the precipitates may be due to the following reason. Seawater present outside a diffuser membrane permeates inside the diffuser membrane through its slits and comes into continuous contact with air passing through the slits for a long time. Drying (concentration of the seawater) is thereby facilitated, and the precipitates are deposited. 
     In view of the above problem, it is an object of the present invention to provide an aeration apparatus that can remove precipitates generated in the slits of diffuser membranes, a seawater flue gas desulphurization apparatus including the aeration apparatus, and a method for operating the aeration apparatus. Solution to Problem 
     According to an aspect of the present invention, an aeration apparatus that is immersed in water to be treated and generates fine air bubbles in the water to be treated includes: 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. 
     According to another aspect of the present invention, an aeration apparatus that is immersed in water to be treated and generates fine air bubbles in the water to be treated includes: 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 increase supply of the air at predetermined intervals. 
     Advantageously, in the aeration apparatus, the control unit performs control to temporarily increase the supply of the air and simultaneously feed water to the air supply pipe. 
     Advantageously, in the aeration apparatus, the control unit performs control to temporarily stop the supply of the air and simultaneously feed water to the air supply pipe. 
     Advantageously, in the aeration apparatus, 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 surfaces of the support bodies in front of the partition plate. 
     Advantageously, in the aeration apparatus, the aeration 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. 
     According to still another aspect of the present invention, a seawater flue gas desulphurization apparatus includes: 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 described above that is disposed in the water passage, the aeration apparatus generating fine air bubbles in the used seawater to decarbonate the used seawater. 
     According to still another aspect of the present invention, a method for operating an aeration apparatus, includes: 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. 
     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 
     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 
         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  is a diagram illustrating the states of the outflow of air (humid air at low saturation), the inflow of seawater, and concentrated seawater in a slit of a diffuser membrane. 
         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 
     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 
     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 SO 2  into sulfurous acid (H 2 SO 3 ); a dilution-mixing basin  105  disposed below the flue gas desulphurization absorber  102  to dilute and mix used seawater  103 A containing sulfur compounds with dilution seawater  103 ; and an oxidation basin  106  disposed on the downstream side of the dilution-mixing basin  105  to subject diluted used seawater  103 B to water quality recovery treatment. 
     In the seawater flue gas desulphurization apparatus  100 , the seawater  103  is supplied through a seawater supply line L 1 , 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 SO 2  contained in the flue gas  101  into the seawater  103 . The used seawater  103 A 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  103 B 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  102   a  represents spray nozzles for injecting seawater upward as liquid columns;  120  represents an aeration apparatus;  122   a  represents air bubbles; L 1  represents a seawater supply line; L 2  represents a dilution seawater supply line; L 3  represents a desulphurization seawater supply line; L 4  represents a flue gas supply line; and L 5  represents an air supply line. 
     The structure of the aeration nozzles  123  is described with reference to  FIGS. 2A ,  2 B, 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. 
     As shown in  FIGS. 2A and 2B , each aeration nozzle  123  has a large number of small slits  12  formed in a rubber-made diffuser membrane  11  that covers the circumference of a base and is generally referred to as a “diffuser 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 L 5 , the slits  12  open to allow a large number of fine air bubbles of substantially equal size to be ejected. 
     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 L 5 . 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  103 B. 
     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  103 B 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. 
     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. 
     A first end  20   a  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  20   b  that allows the introduction of the seawater  103 . 
     In the support body  20 , the side close to the first end  20   a  is in communication with the inside of the header  15  through an air inlet port  20   c  that passes through the header  15  and the flange  16 . The inside of the support body  20  is partitioned by a partition plate  20   d  disposed at some axial position in the support body  20 , and the flow of air is blocked by the partition plate  20   d.  Air outlet holes  20   e  and  20   f  are formed in the side surface of the support body  20  and disposed on the header  15  side of the partition plate  20   d.  The air outlet holes  20   e  and  20   f  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  11   a  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  20   c  into the support body  20  and then flows through the air outlet holes  20   e  and  20   f  formed in the side surface into the pressurization space  11   a,  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  20   e  and  20   f  from leaking from the opposite ends. 
     In the aeration nozzle  123  configured as above, the air  122  flowing from the header  15  through the air inlet port  20   c  flows through the air outlet holes  20   e  and  20   f  into the pressurization space  11   a.  Since the slits  12  are closed in the initial state, the air  122  is accumulated in the pressurization space  11   a  to increase the inner pressure. The increase in the inner pressure of the pressurization space  11   a  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  103 B. Such fine air bubbles are generated in all the aeration nozzles  123  to which air is supplied through branch pipes L 5A  to L 5H  and the headers  15  (see  FIGS. 4 and 5 ). 
     Aeration apparatuses according to an embodiment will next be described. The present invention provides means for removing precipitates deposited in the slits  12  by causing change in the pressure of the air  122  supplied to the diffuser membrane  11 . 
       FIG. 4  and  FIG. 5  are schematic diagrams of the aeration apparatus according to the present embodiment. 
     As shown in  FIG. 4 , an aeration apparatus  120 A according to the present embodiment is immersed in diluted used seawater (not shown), which is water to be treated, and generates fine air bubbles in the diluted used seawater. This aeration apparatus includes: an air supply line L 5  for supplying air  122  through blowers  121 A to  121 D serving as discharge unit; aeration nozzles  123 , each of which includes a diffuser membrane  11  having slits and to which air containing moisture is supplied; and a control unit (not shown) for performing control to temporarily stop supply of the air  122  at predetermined time intervals. Two cooling units  131 A and  131 B and two filters  132 A and  132 B are provided in the air supply line L 5 . The air compressed by the blowers  121 A to  121 D is thereby cooled and then filtrated. 
     Normally, three of the four blowers are used for operation, and one of them is a reserve blower. Since the aeration apparatus must be continuously operated, only one of the two cooling units  131 A and  131 B and only one of the two filters  132 A and  132 B are normally used, and the others are used for maintenance. 
     In the present embodiment, the salt concentration in seawater is generally 3.4%, and 3.4% of salts are dissolved in 96.6% of water. The salts include 77.9% of sodium chloride, 9.6% of magnesium chloride, 6.1% of magnesium sulfate, 4.0% of calcium sulfate, 2.1% of potassium chloride, and 0.2% of other salts. 
     Of these salts, calcium sulfate is deposited first as seawater is concentrated (dried), and the deposition threshold value of the salt concentration in seawater is about 14%. 
     A mechanism of deposition of precipitates in the slits  12  will be described with reference to  FIG. 11A  to  FIG. 11C . 
       FIG. 11A  is a diagram illustrating the states of the outflow of air (humid air at low saturation), the inflow of seawater, and concentrated seawater in a slit of a diffuser membrane.  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. 
     In the present invention, the slits  12  are cuts formed in the diffuser membranes  11 , and the gap of each slit  12  serves as a discharge passage of air. 
     The seawater  103  is in contact with slit wall surfaces  12   a  that form the passage. The introduction of the air  122  causes the seawater to be dried and concentrated to form concentrated seawater  103   a.  Then a precipitate  103   b  is deposited on the slit wall surfaces and clogs the passage in the slits. 
     In the state shown in  FIG. 11A , seawater is dried and concentration of the seawater gradually proceeds because relative humidity of the air  122  is low, so that the concentrated seawater  103   a  is formed. However, even after the concentration of the seawater is started, if the salt concentration in the seawater is equal to or lower than 14%, calcium sulfate or the like is not deposited. 
     In the state shown in  FIG. 11B , the precipitate  103   b  is generated in portions of the concentrated seawater  103   a  in which the salt concentration in the seawater locally exceeds approximately 14%. In this state, the amount of the precipitate  103   b  is very small. Therefore, although the pressure loss when the air passes through the slit  12  increases slightly, the air  122  can pass through the slit  12 . 
     In this state, by changing the pressure as will be described below, precipitates are forcibly removed and operation can be performed for a long time. 
     However, in the state shown in  FIG. 11C , since the concentration of the concentrated seawater  103   a  has proceeded further, a clogged (plugged) state due to the precipitate  103   b  is formed, and the pressure loss is high. Even in this state, the passage of the air  122  remains present, but the load on the discharge unit is considerably large. Therefore, the pressure is changed as will be described below so that the precipitates can be removed before the above situation occurs. 
     Even in this state, it is possible to forcibly remove precipitates by changing the pressure as will be described below. 
     In the present embodiment, the control unit issues a command to temporarily stop supply of the air  122  at predetermined time intervals in order to avoid the above clogged state. 
       FIG. 6  is a graph showing a change in pressure loss of the diffuser membrane over time when supply of air is temporarily stopped. 
     As shown in  FIG. 6 , the supply of the air  122  is temporarily stopped at predetermined time intervals, so that the pressure changes (the pressure temporarily becomes 0). Accordingly, the expanded diffuser membrane  11  is contracted and precipitates such as calcium sulfate deposited on the slit  12  come off, so that the slit  12  returns to normal. 
     Therefore, it is possible to prevent clogging of the slits  12  and narrowing of the gaps of the slits  12 , which are caused by deposition of calcium sulfate through continuous operation. As a result, it is possible to prevent pressure loss of the diffuser membranes  11 . 
     The interval to stop the supply of the air  122  may be appropriately changed according to the deposition states of precipitates. Preferably, the supply of air is stopped once a day or once every two days. 
     By stopping the supply of air in order to change the pressure of the air passing through the slits  12  at the early stage of the deposition, it is possible to cause precipitates to come off easily. 
     The supply of the air  122  may be stopped by stopping the blowers  121 A to  121 D serving as discharge unit. A switching valve (not shown) may be disposed in the air supply line L 5  to stop the supply of the air  122  toward the aeration nozzles  123  side. The air  122  of which flow has been switched, which is compressed air, is stopped or relieved by a damper means or a relief valve. 
     As shown in  FIG. 5 , an aeration apparatus  120 B according to the present embodiment includes a water supply line L 6  for supplying fresh water  141  from a fresh-water tank  140  to the air supply line L 5 . In this case, precipitates are purged due to water pressure. The control unit (not shown) may perform the control to supply the fresh water  141  to the air supply line L 5  at the same time with the control to temporarily stop the supply of the air  122 . 
     As described above, the fresh water  141  is supplied and thereby introduced into the aeration nozzles  123 . Accordingly, the slits  12  of the diffuser membranes  11  are cleaned, so that precipitates such as calcium sulfate adhered to the slits  12  can be dissolved and removed. 
     As a result, it is possible to prevent clogging of the slits  12  or narrowing of the gaps of the slits  12 , which are caused by the deposition of calcium sulfate, making it possible to prevent pressure loss of the diffuser membranes  11 . 
     The cleaning is appropriately performed when the pressure loss of the slits is not recovered by stopping the supply of air. 
     It is possible to supply water at the same time when air is being introduced. 
     In the present embodiment, the fresh water  141  is used as water to be supplied. However, instead of the fresh water, seawater (such as seawater  103  from the dilution seawater supply line L 2 , used seawater  103 A in the dilution-mixing basin  105 , or the diluted used seawater  103 B in the oxidation basin  106 ) or water vapor may be used. When water vapor is used, water vapor is liquidized by a cold condensation means (not shown). 
       FIG. 7  is a graph showing a change in pressure loss of the diffuser membrane over time when supply of air is temporarily increased. As shown in  FIG. 7 , purge operation for increasing the amount of air is performed for a predetermined time after a lapse of a predetermined time during steady operation. 
     The supply of the air  122  is increased at predetermined time intervals as above, so that the pressure changes (the amount of air temporarily increases) and the speed of air passing through the slits increases. Therefore, precipitates of calcium sulfate deposited in the slits  12  are discharged to the outside, and the slits  12  returns to normal. 
     As a result, it is possible to prevent clogging of the slits  12  and narrowing of the slits  12 , which are caused by deposition of calcium sulfate through continuous operation. Consequently, it is possible to prevent pressure loss of the diffuser membranes  11 . 
     The interval of increase may be appropriately changed according to the deposition states of precipitates. Preferably, the supply is increased once a day or once every two days. 
     By temporarily increasing the supply of air in order to change the pressure of the air passing through the slits  12  at the early stage of the deposition, it is possible to easily discharge precipitates to the outside. 
     For temporarily increasing the supply of air, when, for example, three blowers  121 A to  121 C are normally operated in the aeration apparatus  120 A shown in  FIG. 4 , an increased amount of the air  122  can be supplied to the air supply line L 5  by additionally driving the reserve blower  121 D. 
     That is, by operating the blowers  121 A to  121 D, the amount of the air  122  introduced into the aeration nozzles  123  temporarily increases. Therefore, the speed of air passing through the slits increases and calcium sulfate can be removed to the seawater side. 
     Consequently, it is possible to prevent clogging of the slits  12  and narrowing of the gaps of the slits  12 , which are caused by deposition of calcium sulfate. As a result, it is possible to prevent pressure loss of the diffuser membranes  11 . 
     When the capacity of the blower is insufficient, a predetermined purge condition may be set so that precipitates in the slits  12  are pushed and flushed out by using an additional blower. 
     It is also possible to use the aeration apparatus  120 B shown in  FIG. 5  that includes the water supply line L 6  for supplying the fresh water  141  to the air supply line L 5 , and cause the control unit (not shown) to perform control for temporarily increasing the supply of the air  122  and simultaneously feeding the fresh water  141  to the air supply line L 5 . 
     Aeration nozzles according to the present embodiment will next be described. The present invention provides aeration nozzles that cause precipitates deposited in the diffuser membranes  11  to come off easily. 
       FIG. 8  is a schematic diagram of the inner structure of an aeration nozzle  123 A according to the present embodiment. 
     As shown in  FIG. 8 , the aeration nozzle  123 A according to the present embodiment includes: a cylindrical base support body  20 A into which air is introduced; a hollow cylindrical body  20   g  that has a diameter smaller than the diameter of the base support body  20 A and that is disposed at axial position via a partition plate  20   d;  an end support body  20 B that is disposed at one end of the hollow cylindrical body  20   g  and that has approximately the same diameter as the diameter of the base support body  20 A; a tubular diffuser membrane  11  that covers the base support body  20 A and the end support body  20 B and of which both ends are fastened to the base support body and the end support body, respectively, with fastening members  22 ; a large number of slits (not shown) formed in the diffuser membrane  11 ; and air outlet holes  20   e  and  20   f  formed in the side surface of the base support body  20 A for allowing the introduced air  122  to flow into the pressurization space  11   a  between the inner circumferential surface of the diffuser membrane  11  and the outer circumferential surfaces of the support bodies in front of the partition plate  20   d.  Therefore, as indicated by arrows in the figure, the air  122  flowing from the header to the aeration nozzle  123 A first flows into the base support body  20 A through the air inlet port  20   c  and then flows into the pressurization space  11   a  through the air outlet holes  20   e  and  20   f.    
     When the supply of the air  122  is stopped, as indicated by a dashed line in  FIG. 8 , the diffuser membrane  11  is contracted and portions corresponding to portions of the hollow cylindrical body  20   g  with a small diameter are deformed. Therefore, the slits  12  of the diffuser membrane  11  are deformed, which help precipitates to come off. 
       FIG. 9  is a schematic diagram of the inner structure of another aeration nozzle  123 B according to the present embodiment. The aeration nozzle  123 B according to the present embodiment includes: a cylindrical base support body  20 A into which air is introduced; an end support body  20 B that has approximately the same diameter as the diameter of the base support body  20 A; a tubular diffuser membrane  11  that covers the base support body  20 A and the end support body  20 B and of which both ends are fastened to the base support body and the end support body, respectively, with the fastening members  22 ; and a large number of slits  12  formed in the diffuser membrane  11 . 
     While the aeration nozzle  123  shown in  FIG. 3  is structured such that the diffuser membrane  11  covers the support body  20 , the diffuser membrane  11  of the aeration nozzle  123 B shown in  FIG. 9  stands by itself and is supported by the end support body  20 B only at the tips. Therefore, the diffuser membrane  11  expands while the air  122  is being supplied, and is contracted and deformed as indicated by a dashed line when the supply of the air  122  is stopped. Accordingly, precipitates adhered to the slits easily come off. 
     The precipitates that have come off are accumulated inside the diffuser membrane  11 . Therefore, it is not necessary to form slits at portions where the precipitates are accumulated. When forming slits, it is preferable to form extra slits in advance by taking into account clogging of slits that may occur, so that the supply amount of air is not reduced even when precipitates that have come off are accumulated in the slits. 
     In addition to the tube-type aeration nozzle, a disk-type aeration nozzle will be described. 
       FIG. 10  is a schematic diagram of a disk-type aeration nozzle according to the present embodiment. As shown in  FIG. 10 , a disk-type aeration nozzle  133  includes a precipitate housing unit  135  at the bottom portion of a cylindrical support body  134  of the diffuser membrane  11 . A partition such as punching metal  136  is disposed in the housing unit  135  so as not to block flow of introduced air  122 . Because precipitates are caused to fall down under the punching metal  136 , they are not blown upward even when the air  122  is supplied. 
     In the description in the present embodiment, seawater is exemplified as water to be treated, but the invention is not limited thereto. For example, in an aeration apparatus for aerating polluted water in polluted water treatment, plugging caused by deposition of sludge components on diffuser slits (membrane slits) can be prevented, and the aeration apparatus can be stably operated for a long time. 
     In the present embodiment, tube-type aeration nozzles are used in the aeration apparatuses, 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 
     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 
       11  diffuser membrane 
       12  slit 
       100  seawater flue gas desulphurization apparatus 
       102  flue gas desulphurization absorber 
       103  seawater 
       103 A used seawater 
       103 B diluted used seawater 
       105  dilution-mixing basin 
       106  oxidation basin 
       120 A,  120 B aeration apparatus 
       123  aeration nozzle